1 bipolar junction transistor (bjt) ( 08 h ): pnp and npn transistors–current components in bjt...

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Bipolar Junction Transistor (BJT) ( 08 H ):

PNP and NPN transistors–current components in BJT – BJT static characteristics (Input and Output) 1H– Early effect- CB, CC,CE configurations (cut off, active, and saturation regions) CE configuration as two port network 1 H – Alpha and Beta of a transistor ,Biasing and load line analysis – Fixed bias and self bias arrangement 1H. Transistor action, Transistor as an amplifier, Operating point, Load line 1 H, expressions for current gain, voltage gain, input impedance, output impedance and power gain 1 H. Power amplifier - power BJT - Thermal resistance - Maximum power 1 H- Class A, Class B,Class AB and Class C amplifiers1 H -Basic operational amplifier- Differential amplifier 1H.

Prof. Dr. Ali S. Hennache- Department of Physics - College of Sciences - Al-Imam

Muhammad Ibn Saud Islamic University - Riyadh Sept. 2012.

Prof. Dr. Ali S. Hennache- Department of Physics - College of Sciences - Al-Imam Muhammad Ibn Saud Islamic University - Riyadh Sept. 2012.

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The Bipolar Transistor

OBJECTIVESAfter studying the material in this chapter, student will be able to describe and/or analyze:transistor architecture,transistors functioning as switches,transistor characteristics,base biasing of amplifier circuits,signal parameters of the amplifier circuit,the procedure for measuring input and output impedance,transistor output characteristic curves


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INTRODUCTIONIn 1947. J. Bardeen and walker Brattain were the first to invent the “transistor” by adding another junction to the p-n junction diode and Schottkty diodes which could control the flow of majority carriers and to form electronic switching circuits. During the discovery, the device was nothing more than a special kind of resistor whose value changes and thus the inventors first called it “the transformative changing variable resistor “,because of such long name, they shorted the name to “the transforming resistor” but they were still believing the name was quite long and thus they finally

agreed with what is well known device in technology revolution the “Transistor” Thus, the name transistor comes from the phrase \transferring an electrical signal across aresistor".

Look around you, Everything you see is controlled either with computers or electronics of some sort. Your car today has more computational control systems and even energy, water, air-conditioning house are electronically controlled and all electronically controlled these days are solid state and based on the transistor so that means transistor and its derivatives are all extremely important and an absolutely basic to everyday life. In this part of the course we will find out how transistors work , how they are made and the physics behind them and what they do.

The “Transforming Resistor”


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INTRODUCTION

Transistors are thus, solid state devices used for:1- amplifying, and thus are capable of • Current gain• Voltage gain• Signal-power gain2- controlling and 3- generating electrical signals.They are used widely in electronic equipments such as pocket calculators, radios, communication satellites and in general transistors are used in information and communication technologies (ICT).


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Bipolar junction transistor fundamentals

A transistor is a three-layer device alternately doped semiconductor regions, whereas a diode has only two layers. Figure 1(a) shows how two layers of N-type material sandwich one layer of P-type material to make an NPN transistor. Figure 1(b) shows how two layers of P-type material sandwich one layer of N-type material to form a PNP transistor. Most transistors used today are NPN because this is the easiest type to make from silicon .In both cases, the layer in the center is the base, and the two outer layers are the emitter and collector. The arrow on the schematic symbol identifies the emitter and always points to the N-type material.

Figure 1: A three-layer device (Transistor),(a) shows how two layers of N-type material sandwich one layer of P-type ,(b) shows how two layers of N-type material sandwich one layer of P-type.

(a) (b)

E Emitter (n-type)

Base

(p-type)Collector (n-type)

B

C E Emitter (p-type)

Base

(n-type)Collector (p-type)

B

C


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The Bipolar TransistorBipolar transistors, also called BJTs (Bipolar Junction Transistors), are three-terminal devices that can function as electronic switches or as signal amplifiers. In this chapter, you will learn how transistors perform both of these important functions. Figure 2. shows the schematic symbols for the two types of bipolar transistors.The schematic symbols show the three terminals of the transistor. The leg with the arrow is called the emitter (E), the collector (C) is the other slanted leg, and the middle region the base(B) is the leg connected perpendicular to the line connecting the emitter and the collector and is very thin compared to the diffusion length of minority carriersThese terms refer to the internal operation of a transistor but they are not much help in understanding how a transistor is used.

Figure 2. Transistor symbols

Base Base

Collector Collector

Emitter EmitterNPN TYPE

PNP TYPE


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npn BJT Structure

The emitter (E) and is heavily doped (n-type).The collector (C) is also doped n-type.The base (B) is lightly doped with opposite type to the emitter and collector (i.e. p-type in the npn transistor).The base is physically very thin .


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1) NPN. If the base is at a higher voltage than the emitter, current flows from collector to emitter.

2) NPN. Small amount of current also flows from base to emitter.

3) NPN. Voltage at base controls amount of current flow through transistor (collector to emitter). 4) PNP. If the base is at a lower voltage than the emitter, current flows from emitter to collector.

5) PNP. Small amount of current also flows from emitter to base.

6) PNP. Voltage at base controls amount of current flow through transistor (emitter to collector). 7) The arrow to shows the direction of current flow.

NPN vs PNP


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TYPES OF TRANSISTORsDarlington pair is two transistors connected together to give a very high current gain. In addition to standard (bipolar junction) transistors, there are (Figure 3):The Field-Effect Transistor which are usually referred to as FETsThe Metal-Oxide Semiconductor Field-Effect Transistor (MOSFET) The Uni -Junction Transistor (UJT).All the three above types are beyond the scope of this chapter

http://www.mikroe.com/old/books/keu/04.htm

Figure. 3: Different transistors

Darlington pairThis is two transistors connected together so that the current amplified by the first is amplified further by the second transistor. The overall current gain is equal to the two individual gains multiplied together:


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Current flow convention

EEmitter (n-type)

Base (p-type)

Collector (n-type)

B

C

IE

IB

IC

- VCE

- VBE + VBC

+

+ -

We shall treat the transistor as a current node and write according to 1st and 2nd Law of Kirchhoff the following :

IE = IB + IC and VEB + VBC + VCE = 0 VCE = VEC


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Current flow convention

EEmitter (p-type)

Base (n-type)

Collector (p-type)

B

C

IE

IB

IC

+ VEB

-

+ - VCB

+ VEC

-

IE = IB + IC and VEB + VBC + VCE = 0 VCE = VEC


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TRANSISTOR CONFIGURATIONSMost electronic devices take the signal between two input terminals and deliver from it an output signal between two output terminals. The BJT has only three terminals so one of these is usually shared (i.e. made common) between input and output circuits. In this chapter all three methods that a transistor can be connected will be covered 1-common emitter (CE), 2-common base (CB) and 3-common collector (CC) configurations.

The CE configuration is the one most commonly encountered since it provides both good current and voltage gain for ac signals.In the CE configuration the input is between the base and the emitter. The output is between the collector and the emitter.


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TRANSISTOR CONFIGURATIONS

In normal operation for analogue (linear amplifier) circuits the emitter-base junction is forward biased and the collector-base junction is reverse biased.These ‘bias’ or ‘quiescent’ conditions are set by d.c. bias circuits. The a.c. (‘analogue’) signal to be amplified is superimposed on top of the d.c. bias voltages and currents. (Exactly as for dynamic resistance, small variations about a Q point, in our discussion of diodes.)

The forward bias between the base and emitter injects electrons from the emitter into the base and holes from the base into the emitter.

The p-n junction joining the base and emitter regions is called the base-emitter (B-E) junction. (or emitter-base)The p-n junction between the base and collector regions is called the collector-base (C-B) junction.(or base-collector)


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GENERAL CONNECTION CHARACTERISTICS

Comparison of the characteristic sizes (table below) of the 3 different transistor connections .

Common Base

Common Emitter

Common Collector

Input Impedance

Low (about 50 Ω)

Medium (1-5 KΩ)

High (300-500 KΩ)

Output Impedance

High (500KΩ-1MΩ)

Medium (about 50KΩ)

Low (up to 300 Ω)

Current Gain Low (<1) High (50 - 800) High (50-800)

Voltage Gain Low (about 20) High (about 200)

Low (<1)

Power Gain Low (about 20) High (up to 10000)

Medium (about 50)


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SWITCH FUNCTION

If Vin is high, T is ON, switch is closed and Vout is low. Digital “0”If Vin is low, T is OFF, switch is open and Vout is high. Digital “1”

Switch function occurs when high base voltage (>0.7 V)saturates the transistor and it fully conducts current in the C-E path resulting in Vout =0. or when the the base voltage is negative. Then it cuts off the current in the C-E path and Vout =Vcc.This is the means by which digital or on/off switching can be accomplished and forms the basis for all digital circuits (including computers)


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TRANSISTOR SWITCHESA transistor can function as an SPST (single-pole single-throw) switch, but rather than being mechanically controlled, it is controlled by an electronic signal driving the base terminal. Figure below shows a comparison between an open SPST switch and an NPN transistor.

Figure 4–8 Open switch equals off transistor

When the switch is open, as shown in Figure 4–8(a), there is no current flowing in the circuitand the bulb is off. When the control signal on the base terminal of the transistor turns thetransistor off, as shown in Figure 4–8(b), the transistor acts like an open switch. The resistance between the collector and the emitter terminals rises infinitely high and prevents current flow in the circuit. The bulb in series with the transistor is off.


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COMPARISON OF SPST SWITCH AND TRANSISTOR

Figure below shows a comparison between the SPST switch and the transistor when turned on. When the switch is closed, current flows in the circuit and lights the bulb. Likewise, when the control signal on the base terminal turns the transistor on, the resistance between the collector and emitter drops to zero, and the current flow lights the bulb. Actually, the transistor is not a perfect switch. When it is off, the resistance between the collector and emitter (RCE) does not go to infinity, and when it is on, the resistance between the collector and emitter (RCE) does not drop to zero. Even though the transistor is not a perfect switch, it is close enough to function well in most circuits.

“CLOSED” SWITCH = “ON” TRANSISTOR


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CONFIGURATIONS AS AMPLIFIEREach configuration, as you will see later, has particular characteristics that make it suitable for specific applications. An easy way to identify a specific transistor configuration is to follow three simple steps: 1- Identify the element (emitter, base, or collector) to which the input signal is applied. 2- Identify the element (emitter, base, or collector) from which the output signal is taken. 3- The remaining element is the common element, and gives the configuration its name. Therefore, by applying these three simple steps to the circuits in figure on the left , we can conclude that this circuit is more than just a basic transistor amplifier. 1st it is a common-emitter amplifier,2nd is a common base amplifier and 3rd is a common collector amplifier


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Transistors applications

•Silicon transistor (bipolar junction transistor)

high gain, bandwidth, analog amplifier•FET (field effect transistor)

high input impedance, analog amplifier•MOS FET (Metal Oxide Field Effect Transistor)

digital, fast switching (preferred in computers, microprocessors)

•CMOS (Complementary Metal Oxide Semiconductor) Transistor

low power, digital switching and analog (preferred in low power implanted devices)


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Current components

1 = hole current lost due to recombination in base, IBR

2 = hole current collected by collector, ~ IC

1 + 2 = hole part of emitter current, IEP

5 = electrons injected across forward biased E-B junction, (– IBE); same as electron part of emitter current, – IEN

4 = electron supplied by base contact for recombination with

holes lost, – IBR (= 1)

3 = thermally generated e & h making up reverse saturation current of reverse biased C-B junction. (generally neglected)


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TRANSISTOR CHARACTERISTICS AND DATA SHEETS

There are hundreds of transistors on the market. The differences in these transistors can be found by examining the electrical characteristics listed on the data sheets. The three most important characteristics to know are:1- the maximum collector current (IC),2- maximum power dissipation (PD), and3- small signal beta (β).

Maximum collector current (IC) is the maximum continuous current that can flow in the collector leg of the transistor without damage to the transistor. Bipolar transistors are available with maximum collector current ratings from 50 mA to 50 A.

Maximum power dissipation (PD) is the maximum power the transistor can dissipate without being damaged. An approximation of the power being dissipated by a transistor can be calculated by multiplying the voltage across the transistor from collector to emitter (VCE) times the collector current (IC). Bipolar transistors are available with maximum power dissipation ratings from 0.2 W to 250 W.


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BETA (hfe) OF THE TRANSISTOR

DC beta (β) or (hFE) is the DC current gain of the transistor in the common-emitterconfiguration. In most applications, the small signal beta and the DC beta are interchangeable. In this text, we will assume they are interchangeable unless otherwise noted. The formula for DC beta is β = IC/IB. Transistors are available with beta ratings from 10 to 1000.

Maximum base current (IB) is the maximum current that can flow in the base leg of the transistor without damage to the transistor.

Collector to base breakdown voltage (VCBO)is the maximum reverse-biased voltage that can be applied across the collector to base junction. Bipolar transistors are available with collector to base breakdown voltage ratings from 20 V to 1500 V.


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BETA OF THE TRANSISTORThe ratio of how the emitter current divides into base and collector current is a function of the particular transistor being used. Typically only about 1% of the emitter current will exit the base, and 99% will exit the collector. The ratio of collector current to base current is a parameter of the individual transistor and is called beta (β). Beta can be stated mathematically as β = IC/IB.Beta is sometimes referred to as hFE, which stands for forward current gain in the common emitter configuration. Beta is the current gain of the transistor where IC is output current and IB is input current (beta has no units, since it is a ratio of two currents). The important thing to remember about beta is that it is a fixed ratio between collector current and base current. Therefore, a small change in base current will cause a large change in collector current. The base current is the control current.

IE = IB + IC Transistor current divider

β = IC / IB Transistor current gain (beta)


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Performance parameters (pnp)

Neglect the reverse leakage (electron) current of C-B junction

Emitter efficiency:

E

EP

ENEP

EP

I

I

II

I

Base transport factor:

Ep

CT I

Iα

Common base dc current gain:EdcETEPTC IIII

Tdc

Fraction of emitter current carried by holes. close to 1.

Fraction of holes collected by the collector.T close to 1.

Note that is less than 1.0 but close to 1.0 (e.g. = 0.99)


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Performance parameters (pnp)Common emitter dc current gain, dc:

BdcC II

Bdc

dcC

BCdcEdcC

1

)(

II

IIII

T

T

dc

dcdc 11

Note that is large (e.g. = 100)

For NPN transistor, similar analysis can be carried out. However, the emitter current is mainly carried by electrons.


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EXAMPLES

A transistor is connected as shown in Figure 1. It has a base current of 8 μA and a collector current of 1.2 mA. What is the emitter current and the beta of the transistor?Step 1 Calculate emitter current.

IE = IB + ICIE = 8 μA + 1.2 mA

IE = 1208 μA

Step 2 Calculate beta.

β = IC/IBβ = 1200 mA/8 mA

β = 150

EXAMPLE 02

A transistor is connected as shown in Figure 1. It has an emitter current of 2.42 mA and a collector current of 2.4 mA. What is the base current and beta of the transistor?Step 1Calculate base current.

IE = IB + ICIB = IE – IC

IB = 2.42 mA – 2.40 mA = 0.02 mA or 20 μA

Step 2Calculate beta.

β = IC/IBβ = 2400 μA / 20 μA

β = 120Figure 1

EXAMPLE 01


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EXAMPLES

A transistor is connected as in Figure 1 and has a base current of 16 μA and a beta of 80. What is the collector current and emitter current of the transistor?

Step 1 Calculate the collector current.

β = IC/IBIC = β × IB

IC= 80 × 16μAIC= 1280 μA or 1.28 mA

Step 2Calculate the emitter current.IE = IB + ICIE = 16 μA + 1280 μA = 1296 μA or 1.296 mA

EXAMPLE 03


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examples

Design a circuit to control the on/off conditions of a 50 Ω load connected to 30 V. The control signal is a voltage switch between 0 V and 4 V. The load will be on when the control voltage is 4 V and off when the control voltage is 0 V. The transistor used in the circuit will have a beta of 50.

Step 1Draw a diagram of the switching circuit. (Figure 2 shows a diagram of the circuit.)

EXAMPLE 04


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ExamplesStep 2Calculate the collector current when the load is in the on state.The supply voltage is divided by the load resistance. The saturation voltage (0.2 V) of the transistor could be subtracted from the supply voltage, but it is not significant in this case.IC = IL = 30 V/50 Ω = 600mA

Step 3 Calculate the base current needed.

IB = IC/β = 600 mA/50 = 12 mA

Step 4Calculate the value of VRB.

VRB = Vcontrol – VBE = 4 V – 0.7 V = 3.3 VStep 5 Calculate the value of Rb.

Rb = VRB/IB = 3.3 V/12 mA = 275 Ω


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Electronics “ Phys. 324” Figure 4–10 shows a transistor switching circuit designed to control the on/off condition of a 12 V bulb.When the control signal equals 5 V, the bulb is on, and when the control signal equals 0 V, the bulb is off. The current flowing through the bulb is the collector current. The collector current is zero when the bulb is off and is limited by the bulb resistance when the bulb is on.Example 4.4 shows how to design the switching circuitry.

EXAMPLE 4.4

Design the transistor switching circuit shown in Figure 4–10.

Step 1. Calculate the collector current when the bulb is in the on state. The supplyvoltage is divided by the resistance of the load (bulb).IC = IL = VCC/RL = 12 V/10 Ω = 1.2 AStep 2. Using beta, calculate the needed base current.IB = IC/β = 1.2 A/100 = 12 mAStep 3. Calculate the value of VRB.VRB = Vcontrol – VBE = 5 V – 0.7 V = 4.3 VStep 4. Calculate the value of Rb.Rb = VRB/IB = 4.3 V/12 mA = 358 Ω (Use the next lower standard value, 330 Ω.)Step 5. Draw the switching circuit. (The switching circuit is shown in Figure 4–10.)


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THE TRANSISTOR AMPLIFIERGAIN AND AMPLIFICATIONTransistor amplifiers are circuits that provide signal gain. Gain is an important concept in electronics. Let us take a moment to make sure we have a common understanding of the word.Gain is the ratio of output to input. The general formula is Gain = Output/Input. Gain has no units because the output and the input must be in the same units, and the units cancel.

Figure below shows the symbol used for an amplifier. An amplifier is an electronic circuit used to obtain gain. In the figure , the amplifier has an input of 0.6 Vp-p and an output of 6 Vp-p. By using the gain formula, we can calculate the voltage gain to be:(6 Vp-p/0.6 Vp-p = 10). The word voltage preceding gain indicates that the gain ratio is comprised of the output voltage and the input voltage. It is also possible to have current and power gains.


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Expressions for gains

The general formula for the Gain (G) or Amplification (A) is:

GV (AV) = VOutput / VInput

Voltage Gain :

Current Gain :

Power Gain :

GV (AV) = Output / Input

GI (AI) = IOutput / IInput

Gp (Ap) = GV (AV) . GI (AI)


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Electronics “ Phys. 324” Darlington pairAs we already said that this is two transistors connected together so that the current amplified by the first is amplified further by the second transistor. The overall current gain is equal to the two individual gains multiplied together:

Darlington pair current gain,

hFE = hFE1 × hFE2

(hFE1 and hFE2 are the gains of the individual transistors)

This gives the Darlington pair a very high current gain, such as 10000, so that only a tiny base current is required to make the pair switch on. A Darlington pair behaves like a single transistor with a very high current gain. It has three leads (B, C and E) which are equivalent to the leads of a standard individual transistor. To turn on there must be 0.7V across both the base-emitter junctions which are connected in series inside the Darlington pair, therefore it requires 1.4V to turn on.


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TRANSISTOR OUTPUT CHARACTERISTIC CURVES

A graph showing collector current (IC) versus collector/emitter voltage (VCE) with base current (IB) held constant is called an output characteristic curve. Figure (a) below shows an experimental circuit used to obtain the needed data to draw an output characteristic curve. Figure (b) shows the output characteristic curve obtained by holding the base current constant at 5 μΑ and monitoring the collector current as the collector/emitter voltage is varied from 0 V to 20 V.

If the base current is adjusted to a second value and the process is repeated, data can be obtained for a second curve. This process can be repeated several times, and a family of output characteristic curves can be obtained as shown in Figure on the right.

Circuit for developing characteristic curves Family of output characteristic curves


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Inside transistor

The following drawing shows how the electrons and holes flow within the transistor

This is generally what happens inside a transistor when voltage is applied. The purpose of this theory is to explain how can someone use the transistor to design an amplifier or a switch.

In any transistor scheme the symbol VEE is for the emitter supply, VCC for the collector supply and VBB for the base supply.


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Hybrid parametersThe hybrid parameters [h]

The hybrid parameters are values that characterize the operation of a transistor, such as the amplification factor, the resistance and others. They are used to calculate and properly use the transistor in a circuit. Most of the the hybrid parameter values are given in the datasheet by the manufacturer.

The hybrid parameters for Common Emitter (CE) connection

Here is the first set of hybrid parameters for a transistor connected with Common Emitter.

hie - input impedanceThe first hybrid parameter that we will see is the hie. This parameter is defined by the result of the division of the VBE with the IB:

hie = VBE / IB

This parameters defines the input resistance of a transistor, when the output is short-circuited (VCE=0).


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Hybrid parametershfe - Current GainThis is the most important parameter and is extensively used when calculating a transistor amplifier. This is actually the only parameter you need to know to begin designing amplifiers. The equation for this parameter is the following:

hfe = IC / IB

When we have the output of the transistor short-circuited (VCE=0), hfe defines the current gain of the transistor in common emitter (CE) connection. Using this parameter we can calculate the output current (IC) from the input current (IB):

IC = IB x hfe

This explains why this parameter is so useful. A typical BJT transistor has typical current amplification from 30 to 800, while a Darlington pair transistor can have an amplification factor of 10.000 or more. Another symbol for the hfe is the Greek letter β (Beta).


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Hybrid parametershre - Dynamic transfer ratio reverse voltageThis parameter is calculated with this equation:

hre = VBE / VCE

If the input of the transistor is open (IB=0) then this parameter gives the voltage gain when the transistor is connected with common emitter (CE).

hoe - Output ConductivityThis parameter is defined with the input open (IB=0) and the transistor connected in common emitter (CE) connection. The equation is:

hoe = IC / VCE

With the above conditions, this parameter defines the conductivity of the output. So, the impedance of the output can be defined as follows:ro = 1 / hoe = VCE / IC


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Output characteristic, Load Lines and Quiescence point The DC Load Line

One of the most important characteristics is the Common Emitter output characteristic or IC to VCE characteristic and looks like as below:

This is the IC to VCE characteristic of a BC547 transistor. The horizontal axis (x) has the VCE voltage in volts, and the vertical axis has the IC current, usually in milliamperes. Between them, there are several different curves. Each one of these lines corresponds to a different base current, usually measured in microamperes.

The IC to VCE characteristic of a transistor


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Dc load line (operation point) The DC Load Line is a line (red line ) on these characteristics, which eventually determines all the points that the transistor will operate at. In other words, the operation point (usually called Q from the word "Quiescence") will be somewhere on the DC load line. To draw this load line, the collector current and the collector-emitter voltage needed to be known. For example for the characteristics below the load line drawn with red color is for IC=40mA and VCE=12V.

Before one can draw the load line, we must first discuss and explain the 4 basic regions of the output characteristic or IC to VCE 1- the saturation area, 2- the cut-off area, 3- the linear area and4- the breakdown point.

How to draw the load line


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SATURATION AREARegion 1: The Saturation Area

The saturation area is the area where the load line intersects with the saturation point of the characteristics. In the following curves , the saturation area is marked with a red transparent filter.


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Cut- off areaRegion 2:The Cut-Off area

The cut-off area is the area in which the collector current becomes zero. In the following curves , the cut-off area is marked with a yellow transparent filter.


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Linear areaRegion 3: The Linear Area

The linear area is the area between the cutoff and the saturation area of the transistor, as shown bellow with a green mask.


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Breakdown pointRegion 4: The Breakdown Point

The breakdown point is the point above which the collector current increases rapidly and the transistor is destroyed. This area is marked with a purple mask in the following drawing.


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Test

Q.1- What are the three transistor configurations? Q.2- Which transistor configuration provides a phase reversal between the

input and output signals? Q.3- What is the input current in the common-emitter circuit? Q.4- What is the current gain in a common-base circuit called? Q.5- Which transistor configuration has a current gain of less than 1? Q.6- What is the output current in the common-collector circuit? Q.7- Which transistor configuration has the highest input resistance? Q.8- What is the formula for GAMMA


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AMPLIFIER PARAMETERS

Amplifier ParametersAny amplifier is said to have certain parameters. These are the particular properties that make the amplifier perform in a certain way, and so make it suitable for a given task. Typical amplifier parameters are described below.GainThe gain of an amplifier is a measure of the "Amplification" of an amplifier, i.e. how much it increases the amplitude of a signal. More precisely it is the ratio of the output signal amplitude to the input signal amplitude, and is given the symbol "A". It can be calculated for voltage (Av), current (Ai) or power (Ap).

Voltage gain Av = Amplitude of output voltage ÷ Amplitude of input voltage.

AmplificationThe Voltage Amplification (Av) or Gain of a voltage amplifier is given by:


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The gain of an amplifier is governed, not only by the components (transistors etc.) used, but also by the way they are interconnected within the amplifier circuit.

Current gain Ai = Amplitude of output current ÷ Amplitude of input

current.

Power gain Ap = Signal power out ÷ Signal power in.


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The dBs can be used to indicate the gain of amplifiers

Converting a power gain ratio to dBs is calculated by multiplying the log of the ratio by 10:

When voltage gain(Av) or current gain (Ai) is plotted against frequency the

−3dB points are where the gain falls to 0.707 of the maximum (mid band) gain.


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AMPLIFIERS CLASSES

The class of operation of an amplifier is determined by the amount of time (in relation to the input signal) that current flows in the output circuit

There are four classes of operation for an amplifier(Figure 1). These are: A, AB, B and C (figure 1 (A), 1(B) (1C) and (1D) respectively) .Each class of operation has certain uses and characteristics.

The selection of the "best" class of operation is determined by the use of the amplifying circuit.

Figure 1: A comparison of output signals for the different amplifier classes of operation


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CLASS A AMPLIFIERCLASS A

The output is a replica of the input. Figure 2 is an example of a class A amplifier. Although the output from this amplifier is 180 degrees out of phase with the input, the output current still flows for the complete duration of the input and thus, amplifier current flows for 100% of the input signal. The class A operated amplifier is used as an audio- and radio-frequency amplifier in radio, radar, and sound systems, just to mention a few examples.

The class A amplifier has the characteristics of good FIDELITY( the output signal is just like the input signal in all respects except amplitude) and low EFFICIENCY.

Figure 2: A simple class A transistor amplifier. Class A - 100% of the input signal is used (conduction angle a = 360° or 2π)


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CLASS B AMPLIFIERCLASS B

A class B amplifier operates for 50% of the input signal. A simple class B amplifier is shown in figure 3.

Figure 3: A simple class B transistor amplifier.

In the circuit shown in figure 3, the base-emitter bias will not allow the transistor to conduct whenever the input signal becomes positive. Therefore, only the negative portion of the input signal is reproduced in the output signal.

The efficiency of Class B amplifiers is twice the efficient of class A amplifiers since the amplifying device only conducts (and uses power) for half of the input signal. If less than 50% of the input signal is needed, a class C amplifier is used. Class B - 50% of the input signal is used (a = 180° or π)


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CLASS AB AMPLIFIER

CLASS AB

Figure 4: A simple class AB transistor amplifier.

If the amplifying device is biased in such a way that current flows in the device for 51% - 99% of the input signal, the amplifier is operating class AB.

Class AB amplifiers have better efficiency and poorer fidelity than class A amplifiers. They are used when the output signal need not be a complete reproduction of the input signal, but both positive and negative portions of the input signal must be available at the output.

Any amplifier operating between class A and class B limits is operating class AB.

Class AB - more than 50% but less than 100% is used. (181° to 359°, π < a < 2π)


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CLASS C AMPLIFIERCLASS C

Figure 5: A simple class C transistor amplifier.

Notice that only a small portion of the input signal is present in the output signal. Since the transistor does not conduct except during a small portion of the input signal, this is the most efficient amplifier. It also has the worst fidelity. The output signal bears very little resemblance to the input signal.

Class C amplifiers are used where the output signal need only be present during part of one-half of the input signal. Any amplifier that operates on less than 50% of the input signal is operated class C.

Class C - less than 50% is used (0° to 179°, a < π)


54

Collector current waveforms operating in for different classes

Collector current waveforms for transistors operating in (a) class A, (b) class B, (c) class AB, and (d) class C amplifier stages.


55

PHYS 324 - ELECTRONICS

Field Effect Transistor (FET) ( 07 H ):

Field-Effect Transistors (FET): Construction and classification 1H, Principle of operation 2H, Characteristic curves, Characteristic parameters of the FET 1H, Effect of temperature on FET, Common source amplifier, Common drain amplifier 1H. Application of FET as voltage variable resistor and MOSFET as a switch 1H – Advantages of FET over transistor 1H


56

FIELD EFFECT TRANSISTOR ( FET)

A field-effect transistor (FET) is a type of transistor commonly used for weak-signal amplification (for example, for amplifying wireless signals). The device �can amplify analog or digital signals. It can also switch DC or function as an �oscillator.

In the FET, current flows along a semiconductor path called the channel. At �one end of the channel, there is an electrode called the source. At the other �end of the channel, there is an electrode called the drain. The physical �diameter of the channel is fixed, but its effective electrical diameter can be varied by the application of a voltage to a control electrode called the gate. The conductivity of the FET depends, at any given instant in time, on the electrical diameter of the channel. A small change in gate voltage can cause a large �variation in the current from the source to the drain. This is how the FET �amplifies signals.Field-effect transistors are fabricated onto silicon integrated circuit (IC) chips. �A single IC can contain many thousands of FETs, along with other components such as resistors, capacitors, and diodes.

INTRODUCTION


57

INTRODUCTION

The junction FET has a channel consisting of N-type semiconductor (N-channel) or P-type semiconductor (P-channel) material; the gate is made of the opposite semiconductor type. In � P-type material, electric charges are carried mainly in the form of electron deficiencies called holes. In � N-type material, the charge carriers are primarily electrons. In a JFET, the junction is the boundary between the channel and the gate. Normally, this P-N junction is reverse-biased (a DC voltage is applied to it) so that no current flows between the channel and the gate. However, under some conditions there is a �small current through the junction during part of the input signal cycle.


58

FET CLASSIFICATIONS

Field-Effect Transistors

Junction FET (JFET) Metal-Oxide- Semiconductor FET (MOSFET).

D-MOSFET - Depletion

Mode MOSFET

E- MOSFET - Enhancemen

t Mode MOSFETN-CHANNEL P-CHANNEL


59

JFET SYMBOL–FETs are 3 terminal devices

Drain (D)

Source (S)

Gate (G)–the gate is the control input–diagram illustrates the notation used for labelling voltages and currents

P-CHANNEL

N-CHANNEL


60

N AND P-CHANNELS


61

N-CHANNEL

There are three terminals: Drain (D) and Source (S) are connected to n-channelGate (G) is connected to the P-type material


62

P-CHANNEL

P-Channel JFET operates in a similar manner as the N-channel JFET except the voltage polarities and current directions are reversed Drain (D) and Source (S) are connected to P-channel and Gate (G) is connected to the N-type material.


63

D- MOSFET SYMBOLS


64

BASIC OPERATION OF FETThe field-effect transistor is another type of solid-state device that is becoming increasingly popular in electronic circuits. These transistors derive their name from the fact that current flow in them is controlled by variation of an electric field established by application of a voltage to a control electrode, referred to as the gate. In contrast, current flow in bipolar transistors is controlled by variation of the current injected into the base terminal. Moreover, the performance of bipolar transistors depends on the interaction of two types of charge carriers (holes and electrons). Field-effect transistors, however, are unipolar devices; as a result, their operation is basically a function of only one type of charge carrier, holes in p-channel devices and electrons in n-channel devices.

A charge-control concept can be used to explain the basic operation of field-effect transistors. A charge on the gate (control electrode) induces an equal, but opposite, charge in a semiconductor layer, referred to as the channel, located directly beneath the gate. The charge induced in the channel controls the conduction of current through the channel and, therefore, between the source and drain terminals which are connected to opposite ends of the channel.


65

OPERATIONThe JFET is a voltage−controlled device in which current flows from the SOURCE terminal (equivalent to the emitter in a bipolar transistor) to the DRAIN (equivalent to the collector). A voltage applied between the source terminal and a GATE terminal (equivalent to the base) is used to control the source − drain current. The main difference between a JFET and a bipolar transistor is that in a JFET no gate current flows, the current through the device is controlled by an electric field, hence "Field effect transistor". The JFET construction and circuit symbols are shown in Figures 1, 2 and 3.

Figure on the right shows the simplest form of construction for a Junction FET (JFET) . It uses a small slab of N type semiconductor into which are infused two P type areas to form the Gate. Current (electrons) flows through the device from source to drain along the N type silicon channel. As only one type of charge carrier (electrons) carry current in N channel JFETs, these transistors are also called "Unipolar" devices.


66

JFET OPERATION

In the N channel device, the N channel is sandwiched between two P type regions (the gate and the substrate) that are connected together electrically to form the gate. The N type channel is connected to the source and drain terminals via more heavily doped N+ type regions. The drain is connected to a positive supply, and the source to zero volts. N+ type silicon has a lower resistivity than N type. This gives it a lower resistance, increasing conduction and reducing the effect of placing standard N type silicon next to the aluminium connector, which because aluminum is a tri−valent material, having three valence electrons whilst silicon has four, would tend to create an unwanted junction, similar in effect to a PN junction at this point.


67

OPERATION

When a voltage is applied between drain and source (VDS) current flows and the silicon channel acts rather like a conventional resistor. Now if VDS is increased (with VGS held at zero volts) towards what is called the pinch off value VP, the drain current ID also at first, increases. The transistor is working in the "ohmic region" as shown above.However as drain source voltage VDS increases, the depletion layers at the gate junctions are also becoming thicker and so narrowing the N type channel available for conduction.


68

OPERATION

Figure below is showing the cross section of a N channel planar Junction FET (JFET) The load current flows through the device from source to drain along a channel made of N type silicon. In the planar device the second part of the gate is formed by the P type substrate.

P channel JFETs are also available and the principle of operation is the same as the N channel type described here, but polarities of the voltages are of course reversed, and the charge carriers are holes.


69

The nonconductive depletion region becomes thicker with increased reverse bias. (Note: The two gate regions of each FET are connected to each other.)

N-CHANNEL JFET OPERATION


70

OPERATION

gate volt controls the thickness of the channel consider an n-channel device

- making the gate more positive attracts electrons to the gate and makes the gate region thicker – reducing the resistance of the channel. The channel is said to be enhanced- making the gate more negative repels electrons from the gate and makes the gate region thinner – increasing the resistance of the channel. The channel is said to be depleted


71

BJT VS FETCURRENT CONTROLLED VS VOLTAGE CONTROLLED DEVICES

Compared to BJTs, FETs have faster switching speeds (time switching On to Off) and generate less heat per switch, than the BJT. FETs can be designed to smaller geometries than BJTs, allowing greater number of individual transistors per square micron of semiconductor space. However, FETs, particularly the early MOSFETs are more liable to damage from Electro-Static Discharge (ESD) than were the BJT devices.


72

TRANSFER CHARACTERISTICSThe input-output transfer characteristic of the JFET is not as straight forward as it is for the BJT

In a BJT, (hFE) defined the relationship between IB (input current) and IC (output current).

In a JFET, the relationship (Shockley’s Equation) between VGS (input voltage) and ID (output current) is used to define the transfer characteristics, and a little more complicated (and not linear):

As a result, FET’s are often referred to a square law devices

2GS

D DSSP

V I = I 1 -

V


73

JFET OPERATING CHARACTERISTICS

There are three basic operating conditions for a JFET:JFET’s operate in the depletion mode onlyA. VGS = 0, VDS is a minimum value depending on IDSS and the drain and

source resistanceB. VGS < 0, VDS at some positive value andC. Device is operating as a Voltage-Controlled Resistor

For an n channel JFET, VGS may never be positive*For an p channel JFET, VGS may never be negative*


74

SATURATION

At the pinch-off point: • any further increase in VGS does not produce any increase in ID. VGS at pinch-off is denoted as Vp. • ID is at saturation or maximum. It is referred to as IDSS. • The ohmic value of the channel is at maximum.


75

FET OUTPUT CHARACTERISTICS

There are 3 main important regions in a field effect transistor:1- Ohmic region2- Pinch-off voltage saturation3- Saturation region


76

FET as a Voltage-Controlled Resistor

The region to the left of the pinch-off point is called the ohmic region.The JFET can be used as a variable resistor, where VGS controls the drain-source resistance (rd). As VGS becomes more negative, the resistance (rd) increases.

od 2

GS

P

r r =

V 1 -

V


77

JFET OUTPUT CHARACTERISTICS

As VGS becomes more negative:• the JFET will pinch-off at a lower voltage (Vp).• ID decreases (ID < IDSS) even though VDS is increased.• Eventually ID will reach 0A. VGS at this point is called Vp or VGS(off).• Also note that at high levels of VDS the JFET reaches a breakdown situation. ID will increases uncontrollably if VDS > VDSmax.


78

In the JFET output characteristics shown in above, the Drain current ID shows very little change, and the curves are very nearly horizontal at voltages greater than the pinch off voltage. Almost all of the expected increase in current, due to the increase in voltage between Source and Drain (VDS), is offset by the narrowing of the conducting channel due to the growing depletion layers.


79

P-CHANNEL JFET CHARACTERISTICS

As VGS increases more positively• the depletion zone increases• ID decreases (ID < IDSS)• eventually ID = 0AAlso note that at high levels of VDS the JFET reaches a breakdown situation. ID

increases uncontrollably if VDS > VDSmax.


80

TRANSFER (TRANSCONDUCTANCE) CURVE

From this graph it is easy to determine the value of ID for a given value of VGS

It is also possible to determine IDSS and VP by looking at the knee where VGS is 0


81

TRANSFER CHARACTERISTICThe transfer characteristic for a JFET, which shows the change in Drain current (ID) for a given change in Gate−Source voltage (VGS), is shown in Fig 2.4. Because

the JFET input (the Gate) is voltage operated, the gain of the transistor cannot be called current gain, as with bipolar transistors. The drain current is controlled by the Gate−Source voltage, so the graph shows milliamperes per volt (mA / V), and as I / V is CONDUCTANCE (the inverse of resistance V / I) the slope of this graph (the gain of the device) is called the FORWARD or MUTUAL TRANSCONDUCTANCE, which has the symbol gm. Therefore the higher the value

of gm the greater the amplification.

Notice that VGS is always shown as being negative; in reality it may be zero or slightly above zero, but the gate is always more negative than the N type channel between source and drain. Note also that the slope of the curve in the transfer characteristic is less steep than that of the transfer characteristic for a typical bipolar transistor. This means that a JFET will have a lower gain than that of a bipolar transistor.


82

TRANSFER CHARACTERISTICS

JFET D-MOSFET E-MOSFET

Similar shape for all forms of FET – but with a different offset not a linear response, but over a small region might be considered to approximate a linear response


83

Differences between Common Source amplifier Common Drain amplifier Common Gate amplifier

Both the common-gate and common source has voltage gain of greater that 1 compared with the voltage gain of source- follower which is less than or approximately equal to 1 . The input resistance of both common-source and source follower is high typically ranges from kilo ohms and above while common-gate has a low input impedance ranges from hundred ohms or below.

The output resistance of both common-gate and common-source are dominated by RD while source follower has low output impedance and is not

dominated by RD


84

COMPARISON BETWEEN CONFIGURATIONS

Configuration Voltage Gain Current Gain Input Resistance

Output Resistance

Common- Source

AV >1 - ∞Moderate to

high

common Drain

AV ≈ 1 - ∞ Low

Common- Gate

AV > 1 Ai ≈ 1 LowModerate to

high


85

APPLICATIONS OF FET

In a Field Effect Transistor (FET), as we already said, conduction in a channel is controlled by the effect of an electric field produced by a voltage applied to the Gate electrode. There are no forward-biased junctions so the Gate draws no current. The fact that the Gate in a FET does not draw any current is the most important characteristic of FETs.The nonexistent Gate current results in a very high input impedance (ZIN) which is essential in many applications .

For applications like analog switches and amplifiers of ultra-high input impedance, FETs have no equivalent. In fact, FETs are the basic building blocks for op-amps and digital components like switches, microprocessors and memories. Consider FETs when you want:Very high ZIN. A bidirectional analog switch. A simple current source (2 terminal). A voltage-controlled resistance.


86

SWITCH

The following figure shows the MOSFET equivalent of the BJT switch discussed in the previous lectures.

This circuit is even simpler because we do not have to concern ourselves with the inevitable compromise of providing adequate base current with squandering excessive power. In the MOSFET-equivalent circuit we just apply a full-swing DC voltage drive to the high-impedance Gate. As long as the switched-on FET behaves like a resistance that is small compared with the load, it will bring its Drain close to ground - typical power MOSFET have RDS(ON)

< 0.2 W which is fine for this application.


87

CURRENT SOURCESJFETs are used in current sources because JFETs need no Gate bias in order to get current. This makes the current source circuit much simpler. JFETs are used as current sources within ICs (especially in op-amps) and also sometimes in discrete circuits. The simplest JFET current source is shown below where R1 is the load and ID is the output current of the current source:


88

JFET LIKE A RESISTORGiven that VGS in the above circuit is zero, the following graph shows the Drain

current curve for various values of VDS. From this graph we can see that ID will be

relatively constant for VDS > 3V. Note that this current source will only work as

long as you keep VDS > 3V, out of the so-called linear region, where the JFET

behaves not like a current source, but like a resistor (due to linear relation between ID and VDS in that region):


89

ADJUSTABLE CURRENT SOURCEThe previous circuit can be modified to create an adjustable current source by adding a self-biasing resistor, RS as follows:

In the previous circuit, VGS = 0 and ID was close to the maximum current, IDSS. In this circuit however, VGS = VG - IDRS = - IDRS (since VG is tied to ground). Because VGS = - IDRS and not zero, the JFET is brought closer to the pinch-off voltage VP and thereby, reducing ID to a more stable value. The previous circuit was operating at IDSS, and as shown from the data sheets IDSS can be unpredictable. Adding RS makes the current source more predictable because it reduces the Drain current value ID away from the unpredictable IDSS.


90

ADJUSTABLE CURRENT SOURCE

The following curve illustrates how ID varies with VDS for the above circuit as the Drain voltage VDD was varied from 1 to 5V. Unlike the previous current source which was operating at IDSS where it was obvious that ID was not constant with increasing VDS, there is little variation in ID along the saturation region in this improved current source


91

The figure below is already given in the previous lectures and shows how the Drain current ID varies with the Drain-Source voltage VDS for a few values of the controlling Gate-Source voltage VGS:


92

FET VARIABLE RESISTORS

In the linear region, the curves are approximately straight as long as VDS < VGS - VT. Actually, these curves extend through both directions through the zero point, meaning that the device can be used as a voltage controlled resistor for small signals of both polarities. From the universal equation for ID:

For the linear region we can easily find the ratio ID / VDS by dividing the first

equation by VDS to give:


93

The previous equation describes the resistance being supplied by the FET when it is running in the linear region. In fact, RDS in the linear region is also equal to 1/gm. In other words, the channel resistance in the linear region is the inverse of the transconductance gm in the saturation region.Typically the value of resistance we can produce for a FET will range from 10s of Ohms all the way up to an open circuit. A typical application might be in a circuit to automatically control the gain of an amplifier to keep the output within the linear region. The range of VDS over which the FET behaves like a good resistor depends on the FET itself, but is roughly proportional to the amount by which VG exceeds VT.


94

The good-old voltage divider is a very useful circuit:

It is used to produce a fraction of the input voltage VIN at VOUT:


95

Value of R2 VOUT

R2 = R1 VOUT = 0.5VIN

R2 = 2R1 VOUT = (2/3)VIN

R2 very low (short circuit) VOUT = 0

R2 very high (open circuit)

VOUT = VIN

From the previous equation, we note the following:

Therefore, by varying R2 we can vary VOUT, in other words, by varying R2 we can determine how much to attenuate the input signal. Now, using FET as a variable resistor, we can replace R2 in the above voltage divider with a FET running in the linear region to produce a voltage-controlled attenuator (or volume control):


96

ADVANTAGES OF FET OVER TRANSISTOR

FET’s (Field – Effect Transistors) are much like BJT’s (Bipolar Junction Transistors).

Similarities: • Amplifiers• Switching devices

• Impedance matching circuits

Differences:• FET’s are voltage controlled devices whereas BJT’s are current

controlled devices.• FET’s also have a higher input impedance, but BJT’s have higher

gains.• FET’s are less sensitive to temperature variations and because of

there construction they are more easily integrated on IC’s. • FET’s are also generally more static sensitive than BJT’s.


97

ADVANTAGES OF FET OVER TRANSISTOR

Basic FET CircuitsThe following categories represent circuit situations that take advantage of the unique properties of FETs, and hence work better with FETs rather than with BJTs or even cannot be built with BJTs:

High-impedance / low currentBuffers or amplifiers for applications where the base current and finite input impedance of the BJT limit performance. Although you can build such circuits with discrete FETs, it is always better to use integrated circuits ICs built with FETs. These ICs often use FETs as a high-impedance front end for a BJT-based design!

Analog SwitchesMOSFETs are excellent voltage-controlled analog-switches. Once again, use dedicated analog switch ICs rather than building them with discrete FETs.


98

Digital LogicMOSFETs dominate microprocessors, memory and most high-performance digital logic.

Power SwitchingPower MOSFETs are preferable to power BJTs for switching loads. Here, we can use discrete power FETs.

Variable Resistors (current sources)FETs behave like a voltage-controlled resistor in the linear region of the Drain curves (the voltage controls the value of the resistor). FETs also behave like a voltage-controlled current-source in the saturation region of the Drain curves (the voltage controls the value of the current).


99

Electronics “ Phys. 324” by

Prof. Dr. Ali S. Hennache

Uni Junction Transistor (UJT) ( 04 H ):

Structure and working of UJT 2H- Characteristics. 1H Application of UJT as a relaxation oscillator 1H.


100

UNIJUNCTION TRANSISTOR (UJT)

• Is a special transistor that has two bases and one emitter• Has two states: completely on or completely off• Part of thyristor family which include SCR, triac, and diac

• UJT consists of a bar of N-type silicon material (lightly-doped) and a small amount of diffused P-type material (heavily-doped)• An emitter terminal E is connected to the P material to form the PN junction• Two paths for current flow: B2 to B1; E to B1• Normally current does not flow in either path until Emitter voltage is about 10 volts higher than B1 voltage

• UJT is a break over-type switching device• Useful in industrial circuits: timers, oscillators, waveform generators, gate control circuits for SCRs and triacs


101

CONSTRUCTION OF A UJTThe basic structure of a uni-junction transistor is shown in figure below. It essentially consists of a lightly-doped N-type silicon bar with a small piece of heavily doped P-type material alloyed to its one side to produce single P-N junction. The single P-N junction accounts for the terminology uni-junction. The silicon bar, at its ends, has two ohmic contacts designated as base-1 (B1) and base-2 (B2), as shown and the P-type region is termed the emitter (E). The emitter junction is usually located closer to base-2 (B2) than base-1 (B1) so that the device is not symmetrical, because symmetrical unit does not provide optimum electrical characteristics for most of the applications.

The UJT has one pn junction and is used mainly as a triggering device in thyristor circuits and can also be used in oscillator circuits. The symbol is similar to a JFET. Note the angle of the emitter. The other terminals are called base 1 and base 2. The characteristics are quite different than any other transistor.


102

Between base B1 and base B2, the resistance of the n-type bar called inter-base resistance (RB ) and is in the order of a few kilo ohm.

This inter-base resistance can be broken up into two resistances—the resistance from B1 to the emitter is RB1 and the resistance from B2 to the emitter is RB2.

Since the emitter is closer to B2 the value of RB1is greater than RB2.Total resistance is given by:

RB = RB1 + RB2


103

EQUIVALENT CIRCUIT OF A UJT

The resistive equivalent circuit of a UJT shown makes it easier to understand its operation. The emitter current controls the value of RB1 inversely. The total resistance or inter-base resistance (RBB) equals the sum of RB1 and RB2. The

standoff ratio () is the ratio RB1/ RBB.

E

B2

B1

D1RB2

RB1


104

EQUIVALENT CIRCUITThe VBB source is generally fixed and provides a constant voltage from B2 to B1.

The UJT is normally operated with both B2 and E positive biased relative to B1.

B1 is always the UJT reference terminal and all voltages are measured relative to B1 . VEE is a variable voltage source.


105

UJT OPERATION

• When a voltage, called standoff voltage VP, applied to the emitter is about 10 volts higher than the voltage applied to B1 , UJT turns on and current flows through the B2-B1 path and from the emitter-B1 path

• Current will continue to flow through the UJT until the voltage applied to the emitter drops to a point that is about 3 volts higher than the voltage applied to B1

• When emitter voltage drops to this point , the UJT will turn off and will remain off until the voltage applied to the emitter again reaches a level about 10 volts higher than the voltage applied to B1


106

UJT OPERATION

Uni-junction transistor can trigger larger thyristors with a pulse at base B1.With the emitter disconnected, the total resistance RBB, is the sum of RB1 and RB2 . RBBO ranges from 4-12kΩ for different device types. The intrinsic standoff ratio η is the ratio of RB1 to RBBO. It varies from 0.4 to 0.8 for different devices.


107

OPERATIONAs VE increases, current IE increases up IP at the peak point. Beyond the peak point, current increases as voltage decreases in the negative resistance region. The voltage reaches a minimum at the valley point. The resistance of RB1, the saturation resistance is lowest at the valley point.

IP and IV, are datasheet parameters. VP is the voltage drop across RB1 plus a 0.7V diode drop; VV is estimated to be approximately 10% of VBB. Peak voltage of UJT Vp

Vp=ηVbb +Vd


108

PROGRAMMABLE UNI-JUNCTION TRANSISTOR

External PUT (Programmable Uni-junction Transistor ) resistors R1 and R2 replace uni-junction transistor internal resistors RB1 and RB2, respectively. These resistors allow the calculation of the intrinsic standoff ratio η.


109

OPERATION

RBB is known as the interbase resistance, and is the sum of RB1 and RB2:

RBB = RB1 + RB2 (1)

N.B. This is only true when the emitter is open circuit. VRB1 is the voltage developed across RB1; this is given by the voltage divider

rule:

VRB1 = RB1 / RB1 + RB2 (2)


110

OPERATIONSince the denominator of equation 2 is equal to equation 1, the former can be rewritten as:

VRB1 = (RB1 /RBB ) X VBB (3)

The ratio RB1 / RBB is referred to as the intrinsic standoff ratio and is denoted by

(the Greek letter eta).

If an external voltage Ve is connected to the emitter, the equivalent circuit can be redrawn as shown in Fig.5.If Ve is less than VRB1, the diode is reverse biased and the circuit behaves as

though the emitter was open circuit. If however Ve is increased so that it exceeds

VRB1 by at least 0.7V, the diode becomes forward biased and emitter current Ie

flows into the base 1 region. Because of this, the value of RB1 decreases. It has

been suggested that this is due to the presence of additional charge carriers (holes) in the bar. Further increase in Ve causes the emitter current to increase

which in turn reduces RB1 and this causes a further increase in current. This

runaway effect is termed regeneration. The value of emitter voltage at which this occurs is known as the peak voltage VP and is given by:

VP = AVVBB + VD (4)


111

OPERATION

The characteristics of the UJT are illustrated by the graph of emitter voltage against emitter current (Fig.6).

As the emitter voltage is increased, the current is very small - just a few microamps. When the peak point is reached, the current rises rapidly, until at the valley point the device runs into saturation. At this point RB1 is at its lowest value, which is known as the saturation resistance.


112

ON STATE

As VEE increases, the UJT stays in the OFF state until VE approaches the peak point value V P. As VE approaches VP the p–n junction becomes forward-biased and begins to conduct in the opposite direction.

As a result IE becomes positive near the peak point P on the VE - IE curve. When VE exactly equals VP the emitter current equals IP .

At this point holes from the heavily doped emitter are injected into the n-type bar, especially into the B1 region. The bar, which is lightly doped, offers very little chance for these holes to recombine.

The lower half of the bar becomes replete with additional current carriers (holes) and its resistance RB is drastically reduced; the decrease in BB1 causes Vx to drop.

This drop, in turn, causes the diode to become more forward-biased and IE increases even further.


113

OFF STATE

When a voltage VBB is applied across the two base terminals B1 and B2, the potential of point p with respect to B1 is given by:

VP =[VBB/ (RB1 +RB2)]*RB1=η*RB1

η is called the intrinsic stand off ratio with its typical value lying between 0.5 and 0.8.

The VEE source is applied to the emitter which is the p-side. Thus, the emitter diode will be reverse-biased as long as VEE is less than Vx. This is OFF state and is shown on the VE - IE curve as being a very low current region.

In the OFF the UJT has a very high resistance between E and B1, and IE is usually a negligible reverse leakage current. With no IE, the drop across RE is zero and the emitter voltage equals the source voltage.


114

STATIC EMITTER CHARACTRERISTIC

The static emitter characteristic (a curve showing the relation between emitter voltage VE and emitter current IE) of a UJT at a given inter base voltage VBB is shown in figure. From figure it is noted that for emitter potentials to the left of peak point, emitter current IE never exceeds IEo . The current IEo corresponds very closely to the reverse leakage current ICo of the conventional BJT. This region, as shown in the figure, is called the cut-off region. Once con duction is established at VE = VP the emitter po tential VE starts decreasing with the increase in emitter current IE. This Corresponds exactly with the decrease in resistance RB for increasing current IE. This device, therefore, has a negative resistance region which is stable enough to be used with a great deal of reliability in the areas of applications listed earlier. Eventually, the valley point reaches, and any further increase in emitter current IE places the device in the saturation region, as shown in the figure. Three other important parameters for the UJT are IP, VV and IV and are defined below:


115

IMPORTANT PARAMETERS FOR THE UJT

Peak-Point Emitter Current. Ip. It is the emitter current at the peak point. It repre sents the rnimrnum current that is required to trigger the device (UJT). It is inversely proportional to the interbase voltage VBB.Valley Point Voltage VV The valley point voltage is the emitter voltage at the valley point. The valley voltage increases with the increase in interbase voltage VBB.Valley Point Current IV The valley point current is the emitter current at the valley point. It increases with the increase in inter-base voltage VBB.

Special Features of UJT. The special features of a UJT are :A stable triggering voltage (VP)— a fixed fraction of applied inter base voltage VBB.A very low value of triggering current.A high pulse current capability.A negative resistance characteristic.Low cost.


116

V-I CHARACTERISTIC

STATIC EMITTER CHARACTERISTIC FOR A UJT


117

UJT RATINGS

Maximum peak emitter current : This represents the maximum allowable value of a pulse of emitter current.

Maximum reverse emitter voltage :This is the maxi mum reverse-bias that the emitter base junction B2 can tolerate before breakdown occurs.

Maximum inter base voltage :This limit is caused by the maxi mum power that the n-type base bar can safely dissipate.

Emitter leakage current :This is the emitter current which flows when VE is less than Vp and the UJT is in the OFF state.


118

APPLICATIONS

The UJT is very popular today mainly due to its high switching speed.

A few select applications of the UJT are as follows: (i) It is used to trigger SCRs and TRIACs (ii) It is used in non-sinusoidal oscillators (iii) It is used in phase control and timing circuits (iv) It is used in saw tooth generators (v) It is used in oscillator circuit design


119

APPLICATIONSUni-junction transistor (abbreviated as UJT), also called the double-base diode is a 2-layer, 3-terminal solid-state (silicon) switching device. The device has-a unique characteristic that when it is triggered, its emitter current increases re generatively (due to negative resistance characteristic) until it is restricted by emitter power supply.

The low cost per unit, combined with its unique characteristic, have warranted its use in a wide variety of applications.

A few include oscillators, pulse generators, saw-tooth generators, triggering circuits, phase control, timing circuits, and voltage-or current-regulated supplies.

The device is in general, a low-power-absorbing device under normal operating conditions and provides tremendous aid in the continual effort to design relatively efficient systems.


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WORTH POINTS ABOUT UJTThe worth noting points about UJT are given below:The device has only one junction, so it is called the uni-junction device.The device, because of one P-N junction, is quite similar to a diode but it differs from an ordinary diode as it has three terminals.The structure of a UJT is quite similar to that of an N-channel JFET. The main difference is that P-type (gate) material surrounds the N-type (channel) material in case of JFET and the gate surface of the JFET is much larger than emitter junction of UJT.In a uni-junction transistor the emitter is heavily doped while the N-region is lightly doped, so the resistance between the base terminals is relatively high, typically 4 to 10 kilo Ohm when the emitter is open.The N-type silicon bar has a high resistance and the resistance between emitter and base-1 is larger than that between emitter and base-2. It is because emitter is closer to base-2 than base-1.UJT is operated with emitter junction forward- biased while the JFET is normally operated with the gate junction reverse-biased.UJT does not have ability to amplify but it has the ability to control a large ac power with a small signal. It exhibits a negative resistance characteristic and so it can be employed as an oscillator.


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FEATURES OF UJT

Special Features of UJT. The special features of a UJT are :A stable triggering voltage (VP)— a fixed fraction of applied inter base voltage VBB.A very low value of triggering current.A high pulse current capability.A negative resistance characteristic.Low cost.

1 bipolar junction transistor (bjt) ( 08 h ): pnp and npn transistors–current components in bjt...

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