active components

8/2/2019 Active Components

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Active Components

Digital Electronics


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Creating a DiodeWhen you connect electrically N-type and P-type silicon asshown below, you find that the following happens and thisproduces a useful effect.

p type n typeNegative Positive

The electrons in the N-typewill move across the

junction to recombine withthe holes in the P-type. Thiswould continue until theregions were depleted of

carries (holes or electrons).There are though twofactors which limit thenumber which cross the

junction.


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The value of potential at which the system stabilises isabout 0.6 volts for Silicon.

For Germanium (which was the first semiconductormaterial used for electronic devices) the value is about0.2 volts.

Let us look at what happens if we now place a voltageacross the PN junction.


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Forward Bias.

V

p nThe voltage source willattempt to drive electronsaround the circuitanticlockwise (oppositedirection to conventional

current flow).

If V is less than the junction potential (Vj) for Si,electrons flowing into the N-type will see the negative

barrier and therefore current flow will be extremelysmall.


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Diode equationThere exists a relationship between the voltage applied to

the diode and the current flowing through it. It has thefollowing form:

1KT

Vq

o eII

whereI is the current flowing through the diodeIo is the reverse leakage current (typically 1 x 10-10 A)

V is the applied voltageq is the charge on an electron 1.602 x 10-19 CK is Boltzmanns Constant 1.38 x 10-23 JK-1

T is the absolute temperature (C + 273)

Semiconductor Theory


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If Io = 1 x 10-10A then determine the how the currentvaries with applied voltage.

V I

0.05 739pA

0.1 5.46nA

0.2 298nA0.3 16.3A

0.4 889A

0.5 48.5mA

0.6 2.65A0.7 145A

The graph shows this as aplot of Current I against

Applied Voltage V.

0

1

2

34

5

6

7

8

9

10

0.5 0.55 0.6 0.65

Voltage

Current

You can see that up to about 0.6volts the current is relatively

small whilst above 0.6 thecurrent increases rapidly.


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Terminal voltage vis negative

vis negative and a few times larger than

Current in the reverse direction is constant andequal to and called saturation current.

Reverse-Bias PN Junction

0Ii

V

0I


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Thepnjunction excited by a constant-current sourceI in the reverse direction. To avoid breakdown, I iskept smaller than I0. Note that the depletion layerwidens and the barrier voltage increases by Vr volts,

which appears between the terminals as a reversevoltage.

Reverse-Bias PN Junction

Reverse Bias:

Drift current I0, indep.

of voltage,

III D 0


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Rectification.This is the process of converting an A.C. input into a D.C.

output.A.C. This is a signal that periodically changes polarity.Examples include the mains voltage and signals generatedfrom acoustic sources.

AC mains signal

-400

-200

0

200

400

time

volta

ge

D.C. This typeof signal neverchanges polarityand so it will be

either positiveor negative.Batteries willgenerate D.C.

outputs.Semiconductor Theory


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There are three basic rectifier circuits:Half Wave Rectification

VpLOAD

The diode will remove thenegative half cycle leavingonly the positive. Thoughthe voltage is fluctuating

it is D.C.

Half Wave Rextified

time

voltage

Vp


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Full Wave Rectification

VpLOAD

The diodes will remove the

negative half cycles fromthe two waveforms, whichare 180 out of phase leavingonly the positive half cycles.These are then summed to

give the output.Full Wave Rectified

time

voltage

Vp/2

Thisproduces a

smootheroutput butwith areduced

voltage.


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Bridge Rectification

Vp

L

OAD

The diodes directthe flow to the

load differentlydepending upon thepolarity of theinput A.C.

If the input is + on the top,diodes 2 and 3 conduct.

+

-

+

-

If + on the bottom diodes1 and 4 conduct.

This ensures that the polarity at the load never changes.



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Bridge Rectified

time

voltage

Vp

Note that at any moment in time two diodes areconducting which means that the output peak isactually Vp 1.2v (2 x 0.6v)



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DIODE TYPES

Zener diodes are used with reverse bias, making useof the breakdown that occurs across a silicon junctionwhen the reverse voltage causes a large electrostaticfield to develop across the junction.

Zener diodes are used for voltage reference purposesor for voltage stabilisation

Varactor diodes are used for electronic tuningapplications


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DIODE TYPES

Schottky diodes A Schottky diode consists of a metal-semiconductor junction, in

which the semiconductor is usually silicon, and the metal can be,

typically, silver, aluminium, gold, chromium, nickel, platinum or

tungsten, or alloys of exotic metals.

The forward drop is small, only about 0.2 V compared to the 0.6 V

of a silicon diode.

They have very fast switching times, meaning used in RF applications such as RF demodulation and in high-

frequency switch-mode power supplies.

Because of the low voltage drop, the diodes also make

excellent power rectifiers, particularly for high-frequency supplies


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DIODE TYPES

Schottky diodes


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DIODE TYPES

LEDs

Light-emitting diodes (LEDs) use compound Semiconductormaterials such as gallium arsenide or indium phosphide.

When forward current passes, light is emitted from the junction.

The colour of the light depends on the semiconductor material usedfor the diode and the brightness is approximately proportionalto the size of forward current.


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DIODE TYPES

LEDs

LEDs have higher forward voltages when conducting; around 1.6 Vto 2.2 V as compared to the 0.5 V to 0.8 V of a silicon junction.The maximum permitted reverse voltages are very low, typicallyonly 3V.

A silicon diode must be connected across the LED if there is anylikelihood of reverse voltage (or an AC signal) being applied to thediode.A series resistor must always be used to limit the forward currentunless pulsed operation is used.

D

LED


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DIODE TYPES

LEDs

Typical applications include remote controls and short-rangesignalling

Note that the infra-red types emit little or no visible light.

D

LED


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DIODE TYPES

Photodiodes

A pn diode the reverse current of which is dependent on theamount of light falling on the junction

Photodiodes are constructed like any other diodes, using silicon,but without the opaque coating that is normally used on signal and

rectifier diodes.


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TransistorsModern active components are all based on transistors,

which were invented in 1948 by Brittain, Bardeen, andShockley.

A transistor is a semiconductor component with three

terminals. An input between two of the terminals canalter the amount of current flowing to or from the thirdterminal.


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There are two main types of transistors, calledbipolar transistors and field-effect transistors (FETs).

Even though the bipolar type was the first to be used,most modern electronics systems now, however, arebased on the FET, usually in integrated circuit (IC)form.


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The BJTThis is a device that makes use of two junctions in a crystal with avery thin layer between the junctions.

The thin layer is called the base, and the type of BJT depends onwhether this base layer is made from P-type or from N-typematerialThe symbol that is used on circuit diagrams for a bipolar transistoris a useful reminder.

This shows three connections, labelled as emitter, collector, andbase, with an arrowhead on the emitter lead that points in thedirection of current.


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If the base layer is of N-type material, the transistoris a P-N-P type, and if the base layer is of P-type

material, the transistor is an N-P-N type.The N-P-N type of transistor, is more widely used.


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Note that, in the circuit symbol, the arrow on theemitter lead points in the direction of positive currentflow. You can tell whether a transistor in a schematicdiagram is PNP or NPN by the direction of the arrow.The differences lie in the polarity of power suppliesand signals rather than in the way that the transistorsact.


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It is customary to reverse bias the base-collectorjunction of a bipolar junction transistor

as shown below.Note that this increases the width of the depletionregion.The reverse bias voltage could be a few volts to tens ofvolts for most transistors. There is no currentflow, except leakage current, in the collector circuit.


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If the base region were thick, as in a pair of back-to-back diodes, allthe current entering the base would flow out the base lead.

In our NPN transistor example, electrons leaving the emitter for thebase would combine with holes in the base, making room for moreholes to be created at the (+) battery terminal on the base aselectrons exit.


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CollectorP N PEmitter

Base+

+ -

FB RB

Bipolar Transistor Biasing (PNP)

90% of the current carriers passes through the reversebiased base - collector PN junction and enter the collector ofthe transistor.

10% of the current carriers exit transistor through the base.

T i Ch i i C


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Transistor Characteristic Curve

IC

VCE

Q-PointIB

0 uA

10 uA

20 uA30 uA

40 uA

50 uA

60 uA

70 uA80 uA90 uA

Saturation

Cutoff


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Analyzing The Transistor Operation Region:Conduction, Saturation or Cutt-Off

Conduction For anytransistor to conduct, two things must occur.

The emitter - base PN junction mustbe forward biased. The base - collector PN junction mustbe reverse biased.

In this region the transistor presents an amplification in current (gain)or , which relates collector current (Ic) and base current (Ib)according to the following equation:

The ratio of collector current (Ic) to base current (Ib) is called

(or hfe)

Ic = Ib (Equation 1)

It is also known that Ie = Ic + Ib (Equation 2)

Then: Ie = ( Ib) + Ib => Ie = ( + 1) Ib (Equation 3)


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Amplifier Electric Switch OperationWhen the input signal is large enough, the transistor can

be driven into saturation & cutoff which will make thetransistor act as an electronic switch.

Saturation-The region of transistor operation where a

further increase in the input signal causes no furtherincrease in the output signal.

Cutoff- Region of transistor operation where the input

signal is reduced to a point where minimum transistorbiasing cannot be maintained => the transistor is no longerbiased to conduct. (no current flows)


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Amplifier Electric Switch Operation

Transistor Q-pointQuiescent point: region of transistor operationwhere the biasing on the transistor causes operation/ output with no input signal applied.

The biasing on the transistor determines the amount

of time an output signal is developed.

Transistor Characteristic CurveThis curve displays all values of IC and VCE for agiven circuit.

This curve is based on the level of DC biasingthat is provided to the transistor prior to theapplication of an input signal.

The values of the circuit resistors, and VCC will

determine the location of the Q-point.

A lifi O i


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The transistor below is biased such that there is a

degree of forward bias on the base-emitter PN junction.

Any input received will change the magnitude of forwardbias & the amount of current flow through the

transistor.

Amplifier Operation

RBRC

Q

1

+0

+VCC

Input Signal

+

0

Output Signal


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TRANSISTOR AMPLIFIERS


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In small-signal amplifiers the main factors are:

Amplification Linearity Gain

Since large-signal, or power, amplifiers handle relatively large

voltage signals and current levels, the main factors are:

Efficiency Maximum power capability Impedance matching to the output device

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Generally, an amplifier or simply amp, is a device for increasing thepower of a signal.

An appliance or circuit that increases the strength of a weakelectrical signal without changing the other characteristics of thesignal.


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Amplifier Types

Class A

The amplifier conducts through the full 360 of the input. TheQ-point is set near the middle of the load line.

Class BThe amplifier conducts through 180 of the input. The Q-point

is set at the cutoff point.

Class ABThis is a compromise between the class A and B amplifiers.The amplifier conducts somewhere between 180 and 360 .

The Q-point is located between the mid-point and cutoff.

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Amplifier Types

Class CThe amplifier conducts less than 180 of the input. The Q-point is located below the cutoff level.

Class DThis is an amplifier that is biased especially for digital signals.

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Class B Amplifier

A class B amplifier output

only conducts for 180 or

one-half of the AC input

signal.

The Q-point is at 0V on the

load line, so that the ACsignal can only swing for

one-half cycle.

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Class AB Amplifier

This amplifier is a compromise between the

class A and class B amplifierthe Q-point

is above that of the Class B but below the

class A.

The output conducts between 180 and

360 of the AC input signal.

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Class C

The output of the class C

conducts for less than 180 of the

AC cycle. The Q-point is belowcutoff.

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Field-Effect Transistors

FETs have been available for almost as long a time as thebipolar type, but they were not extensively used untillater.

Nowadays, the field-effect type is used to a much

greater extent (in ICs) than the bipolar type,

The most important field-effect type is the metal-oxidesemiconductor field-effect transistor, abbreviated to

MOSFET.


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Metal Oxide Field-Effect Transistors(MOSFET)

The MOSFET is a form of field-effect transistor whichhas become the most commonly used type of transistor.

The MOSFET uses quite different principles.

There are three terminals, called source, gate, and drain,with the voltage on the gate controlling the Currentbetween the source and the drain.


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The current flowing in the gate is almost immeasurablysmall.

Current can pass between two terminals called thesource and drain, and this current is controlled by thevoltage (not current) on a third terminal, the gate.


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The MOSFET uses quite different principles.Current can pass between two terminals called thesource and drain, and this current is controlled by thevoltage (not current) on a third terminal, the gate.

The important point about it is that there is no currentpassing to or from the gate for either positive ornegative bias.


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The power needed at the input of FET is very muchsmaller than is needed for a bipolar transistor

Depending on the design of the MOSFET, a bias voltagecan be used or the MOSFET can be operated without bias.

MOSFETs do not provide as much amplification (voltagegain) as bipolar transistors, and their uses were at onetime predominantly for digital IC circuits.

Specialized types of MOSFETs are now used in audioamplifiers and in tuning circuits for FM radios.


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There are various varieties of MOSFET, such as

PMOS P-type metal-oxide-semiconductor

NMOS N-channel enhancement-mode MOSFET

CMOS - Complementary Metal Oxide Semiconductor

All of these are used extensively in digital circuits.

The MOSFET is sometimes known as IGFET, with IGmeaning insulated gate.


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A typical metal-oxide-semiconductor field-effect transistor (MOSFET)amplifier circuit


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Transistor Switching Issues

The analog uses of transistors, BJT or MOSFET, foramplifier circuits require the designer to set thecorrect bias, and use circuits that will provide thenearest approximation to a straight-line graph of outputplotted against input.

These amplifier circuits all cause transistor to dissipatepower, because the optimum bias is usually with thecollector or drain voltage set to about half of thesteady voltage supply, and passing a current that isabout the amount that needs to be supplied.


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Transistor Switching IssuesThe principle of using a transistor and a load is at

the heart of most of the electronic circuits (calledanalog or linear circuits) that existed beforedigital circuits appeared.

Transistors are better suited to digital circuits.

The snag about using transistors for amplificationis that the output is never a perfect copy of the

input (though the imperfections can be made to bevery small).


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Transistor Switching Issues

But air cooling, with or without a fan, is much morecommon.

There are ways of improving this situation.

That is to use for working with digital signals(pulses).

The power dissipation will be very low, particularlyif the transistor is switched on for only a shorttime in each cycle.


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Transistor Switching IssuesTransistors that are used for analog designs are

not as linear as we would like, and they dissipatepower, risking damage to the transistors.

The alternative way of using transistors is as

switches

This switching use of transistors is not suited tolinear amplifiers, but it is ideal for digital circuits

In this sense dissipation can be very low.


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Integrated Circuits

Came about as manufacturers attenpted to manufactureresistors, capacitors, and connections on thesame piece of semiconductor material, making it possible toproduce a complete circuit in one set of operations.

It is possible to make individual electronic componentsreasonably reliable, but the weak point in large andelaborate circuits is the number of connections that haveto be made between components.


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Integrated Circuits

If you can replace 20 individual components by a singlecomponent then, other things being equal, you have madethe circuit connections 20 times more reliable, and this iswhat creating a complete circuit in one set of operationsamounts to.

Now circuits are created with several million componentson one small piece (chip) of silicon.

This device is the integrated circuit or IC.We can now buy complete circuits on a single tiny chip ofsilicon that are vastly more reliable than anything we hadever imagined.


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Integrated Circuits

ICs are mainly for digital circuits, though there are alsomany types of linear (amplifier) ICs.

In addition to reliability the advantages of ICs include: low cost,

small size,low dissipation, andpredictable performance.

The size of a complete circuit is little more than that of asingle transistor


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Linear Integrated Circuits

Though digital circuits have always accounted for amajority of the ICs that have been manufactured, therehas always been one particularly important type of linearIC, called the operational amplifier(or opamp).

The circuit of an opamp is unimportant to the user, andonly the designer is likely to know exactly what goes oninside the chip, so the user of an opamp works from a setof figures that describe its performance.

The usual symbol is


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Linear Integrated Circuits

A typical opamp circuit in which the gain (amount ofamplification) is determined by the values of resistors R1and R 2


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Digital Integrated Circuits

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