element 2 - bipolar junction transistor (bjt)
TRANSCRIPT
Bipolar Junction Transistors (BJT)
LECTURE 6
Transistors Transistor is a 3 terminals semiconductor device that controls
current between two terminals based on the current (BJT) or voltage (FET) at the third terminal
Is used for the amplification or switching of electrical signals Two main categories of transistors:
Bipolar Junction Transistors (BJTs) and Field Effect Transistors (FETs)
The physics of "transistor action" is quite different for the BJT and FET
In analog circuits, transistors are used in amplifiers and linear regulated power supplies
In digital circuits, they function as electrical switches, including logic gates, Random Access Memory (RAM), and microprocessors.
Bipolar Junction Transistors (BJT) A bipolar transistor
essentially consists of a pair of PN Junction diodes that are joined back-to-back.
There are therefore two kinds of BJT, the NPN and PNP varieties.
The three layers of the sandwich are conventionally called the Collector, Base, and Emitter.
Modern Transistors
How the BJT works Figure shows the energy levels
in an NPN transistor under no externally applying voltages
In each of the N-type layers
conduction can take place by the free movement of electrons in the conduction band
In the P-type (filling) layer
conduction can take place by the movement of the free holes in the valence band
However, in the absence of any
externally applied electric field, we find that depletion zones form at both PN-Junctions, so no charge wants to move from one layer to another.
NPN Bipolar Transistor
How the BJT works What happens when we apply
a moderate voltage between the collector and base parts
The polarity of the applied
voltage is chosen to increase the force pulling the N-type electrons and P-type holes apart
This widens the depletion
zone between the collector and base and so no current will flow
In effect we have reverse-
biased the Base-Collector diode junction.
Apply a Collector-Base voltage
Charge Flow What happens when we apply a
relatively small Emitter-Base voltage whose polarity is designed to forward-bias the Emitter-Base junction.
This 'pushes' electrons from the Emitter into the Base region and sets up a current flow across the Emitter-Base boundary.
Once the electrons have managed to get into the Base region they can respond to the attractive force from the positively-biased Collector region.
As a result the electrons which get into the Base move swiftly towards the Collector and cross into the Collector region.
Hence a Emitter-Collector current magnitude is set by the chosen Emitter-Base voltage applied.
Hence an external current flowing in the circuit.
Apply an Emitter-Base voltage
Charge Flow Some of free electrons crossing the Base encounter a hole and 'drop into it'.
As a result, the Base region loses one of its positive charges (holes).
The Base potential would become more negative (because of the removal of the holes) until it was negative enough to repel any more electrons from crossing the Emitter-Base junction.
The current flow would then stop.Some electron fall into a hole
Charge Flow To prevent this happening we use the applied E-B voltage to remove the captured electrons from the base and maintain the number of holes.
The effect, some of the electrons which enter the transistor via the Emitter emerging again from the Base rather than the Collector.
For most practical BJT, only about 1% of the free electrons which try to cross Base region get caught in this way.
Hence a Base current, IB, which is typically around one hundred times smaller than the Emitter current, IE.
Some electron fall into a hole
Characteristics Maximum collector current (IC) – the maximum continuous current
that can flow in the collector leg of the transistor without damage to the transistor (50 mA to 50 A)
Maximum power dissipation (PD) – the maximum power the transistor can dissipate without being damaged (0.2 W to 250 W)
Small signal beta (β) or (hfe) – the signal current gain of the transistor in the common-emitter configuration (β = ic/ib)
DC beta (β) or (hFE) – the DC current gain of the transistor in the common-emitter configuration (10 to 1000). β = IC/IB
Maximum base current (IB) – the maximum current that can flow in the base leg of the transistor without damage to the transistor
Collector to base breakdown voltage (VCBO) – the maximum reverse-biased voltage that can be applied across the collector to base junction (20 V to 1500 V)
Characteristics Collector to emitter breakdown voltage (VCEO) – the maximum
voltage that can be applied across the transistor from collector to emitter (20 V to 800 V)
Emitter to base breakdown voltage (VEBO) – the maximum reverse-biased voltage that can be applied across the emitter to base junction (4 V to 20 V)
Gain-bandwidth product (fT) – the frequency at which the gain of the transistor drops to unity (1 MHz to 5000 MHz)
Output Characteristics Curve
VBE
IB
5 uA
10uA
15 uA
20 uA
0.7 V
Terminals & Operations
Three terminals: Base (B): very thin and lightly doped central region (little
recombination). Emitter (E) and collector (C) are two outer regions
sandwiching B. Normal operation (linear or active region):
B-E junction forward biased; B-C junction reverse biased. The emitter emits (injects) majority charge into base region
and because the base very thin, most will ultimately reach the collector.
The emitter is highly doped while the collector is lightly doped.
The collector is usually at higher voltage than the emitter.
Terminals & Operations
The NPN Transistor
Operation Forward bias of EBJ injects electrons from emitter into base
(small number of holes injected from base into emitter) Most electrons shoot through the base into the collector across
the reverse bias junction (think about band diagram) Some electrons recombine with majority carrier in (P-type) base
region
Current flow in a pnp transistor biased to operate in the active mode.
The PNP Transistor
Operation Mode
Operation Mode
Active: Most importance mode, e.g. for amplifier operation. The region where current curves are practically flat.
Saturation: Barrier potential of the junctions cancel each other out
causing a virtual short. Ideal transistor behaves like a closed switch.
Cutoff: Current reduced to zero Ideal transistor behaves like an open switch.
Operation Mode
Circuit Symbols
For a transistor to function properly as an amplifier, an external dc supply voltage (or voltages) must be applied to produce the desired collector current, Ic.
Several biasing techniques exist that include: Base Biasing Voltage Divider Emitter Biasing
Transistor Biasing Techniques
Base Bias, VBB
The simplest way to bias a transistor VBB is the base supply voltage – used to forward –bias the
base-emitter junction RB is used to provide the desired value of base current VCC is the collector supply voltage which provides the
reverse-bias voltage required for the collector-base junction of the transistor
RC provides the desired voltage in the collector circuit
Transistor Biasing Techniques
+vcc
+vBB
Dual supply Single supply
DC Load Line
Transistor Biasing Techniques
VCC = IC RC
IC(sat) = VCC / RC
VCE(off) = VCC
Q pointICQ
VCEQ
VCE
IC
IC(sat) = VCC / RC
Saturation region
Cut-off
VCE(off) = VCC
Active region
IC = β x IB
Example 1: If VBB = 5 V, VCC = 15 V, RB = 56 k Ω, RC = 1 kΩ, β = 100
Transistor Biasing Techniques
+vcc
+vBB
Vcc = IBRB + Vbe
IB = (VBB - Vbe)/ RB = (5 V- 0.7 V)/ 56 kΩ = 76.78 μA
IC = β x IB = 100 x 76.78 μA = 7.68 mA
VCE = VCC - ICRC = 15 V - 7.68 mA x 1 kΩ = 15 V – 7.68 V = 7.32 V
IC(sat) = 15 mA
Q pointICQ = 7.68 mA
VCEQ = 7.32 V
VCE
IC
IC(sat) = Vcc / RC = 15 V / 1 kΩ = 15 mA
VCE(off) = Vcc = 15 V
VCE(off) = 15 V
Example 2: If VCC = 12 V, RB = 390 k Ω, RC = 1.5 kΩ, β = 150
Transistor Biasing Techniques
Vcc = IBRB + Vbe
IB = (Vcc - Vbe)/ RB = (12 V – 0.7 V)/ 56 kΩ = 28.97 μA
IC = β x IB = 150 x 28.97 μA = 4.35 mA
VCE = VCC – ICRC = 12 V – 4.35 mA x 1.5 kΩ = 12 V – 6.52 V = 5.48 V
IC(sat) = Vcc / RC = 12 V / 1.5 kΩ = 8 mA
VCE(off) = Vcc = 12 V
IC(sat) = 8 mA
Q pointICQ = 4.35 mA
VCEQ = 5..48 V
VCE
IC
VCE(off) = 12 V
Voltage Divider Bias, VBB
The most popular way to bias a transistor The advantage lies in its stability As shown in the figure, two resistor R1 and R2 set up a
voltage divider on the base
Transistor Biasing Techniques
voltage across
provided
DC Load Line
Transistor Biasing Techniques
VCC = IC RC
IC(sat) = VCC / (RE + RC)
VCE(off) = VCC
Q pointICQ
VCEQ
VCE
IC
IC(sat) = VCC / (RE + RC)
Saturation region
Cut-off
VCE(off) = VCC
Active region
IC = β x IB
Emitter Bias, VEE
This type can only be used when dual (split) power supply is available
The advantage lies in its stability similar to voltage divider VEE forward-biases the emitter-base junction through the
emitter resistor, RE
Transistor Biasing Techniques
VB - VE = VBE
IE = (VEE - VBE)/RE
If RB is small enough, base voltage will be approximately zero. Therefore emitter current is,
BJT as a Switch Operation
In the circuits, transistor works as a switch, the biasing of the transistor, either NPN or PNP is arranged to operate it at the both sides of the I-V characteristics curves.
The areas of operation for a transistor switch are:
Cut-off region Saturation region
Cut-off RegionThe operating conditions of the transistor are zero input base current (IB), zero output collector current (IC) and maximum collector voltage (VCE) which results in a large depletion layer and current flowing through the device
BJT as a Switch Cut-off Region
In the circuits, transistor works as a switch, the biasing of the transistor, either NPN or PNP is arranged to operate it at the both sides of the I-V characteristics curves.
The input and Base are grounded (0v) Base-Emitter voltage VBE < 0.7v
Base-Emitter junction is reverse biased Base-Collector junction is reverse biased Transistor is "fully-OFF" (Cut-off region) No Collector current flows ( IC = 0 ) VOUT = VCE = VCC Transistor operates as an "open switch”
“Cut-off Region” can be referred to as “OFF mode”.
BJT as a Switch Saturation Region
The transistor is biased so that the maximum amount of base current is applied, resulting in maximum collector current resulting in the minimum collector emitter voltage drop which results in the depletion layer being as small as possible and maximum current flowing through the transistor. Therefore the transistor is switched "Fully-ON“.
BJT as a Switch Saturation Region
The input and Base are connected to VCC
Base-Emitter voltage VBE > 0.7v
Base-Emitter junction is forward biased Base-Collector junction is forward biased Transistor is "fully-ON" (saturation region) Max Collector current flows (IC = Vcc/RL) VCE = 0 (ideal saturation) VOUT = VCE Transistor operates as a "closed switch"
“Saturation Region” can be referred to as “ON mode”.
BJT as a Switch Basic NPN Transistor Switching Circuit
Digital Logic NPN Transistor Switching Circuit
PNP BJT as a Switch Digital Logic NPN Transistor Switching Circuit
Bipolar Transistor Configurations
There are three possible ways to connect BJT within an electronic circuit with one terminal being common to both the input and output. Common Base Configuration
has Voltage Gain but no Current Gain. Common Emitter Configuration
has both Current and Voltage Gain. Common Collector Configuration
has Current Gain but no Voltage Gain.
The Common Base Configuration
Schematic symbols
Dramatic symbol
The input is applied to the emitter The output is taken from the collector Low input impedance High output impedance Current gain less than unity Very high voltage gain Vin and Vout are in-phase.
The common base circuit is mainly used in single stage amplifier circuits such as microphone or radio frequency (RF) amplifiers due to its very good high frequency response
The Common Base Transistor Circuit
The Common Emitter Amplifier Circuit
Schematic symbols
Dramatic symbol
The Common Emitter Amplifier Circuit
The input is applied to the base The output is from the collector High voltage and current gain, Vout > Vin
High input impedance Low output impedance Phase shift between input and output is
180˚
The Common Collector (CC) Configuration
This type of configuration is known as a Voltage Follower or Emitter Follower
It is very useful for impedance matching applications because of its very high input impedance, hundreds to thousands of Ohms and a relatively low output impedance
The current gain approximately equal to the β value of the transistor itself
The load resistance is connected to the emitter so its current is equal to that of the emitter current
The Common Collector (CC) Configuration
Summary of Transistor ConfigurationsCharacteristic
sCommon
BaseCommon Emitter
Common Collector
Input Impedance
Low Medium High
Output Impedance
Very High High Low
Phase Angle 0o 180o 0o
Voltage Gain High Medium Low
Current Gain Low Medium High
Power Gain Low Very High Medium