CHAPTER 3: BIPOLAR JUNCTION TRANSISTOR

Two main categories of transistors: ◦ bipolar junction transistors (BJTs) and ◦ field effect transistors (FETs).

Transistors have 3 terminals where the application of current (BJT) or voltage (FET) to the input terminal increases the amount of charge in the active region.

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.

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.

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.

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.

There are three different currents in transistors : collector current IC, base current IB and emitter current IE.

Since the emitter is the source of the electrons, it has the largest of the three currents.

The collector current is slightly lower than the emitter current, while the base current is usually very small.

The relationship is expressed as follows:

IE = IB + IC

The relationship between the collector current and the base current is expressed in the following formula

β = IC / IB

Current gain(HFE)

The ratio between collector current and emitter current is referred as α (alpha) and is shown in the following relationship :

α = IC/IE

DC GainDC Gain

Emitter voltage,VE

Base voltage,VB

Collector voltage,VC

Collector emitter voltage,VCE

Base emitter voltage,VBE

Collector Base voltage,VCB

DC supply voltage,VCC

VCCVCE

VBEVC

VB

VE

RB

RC

B

C

E

Voltages in Transistor Circuit Voltages in Transistor Circuit

I - V characteristics are used to explain the operation of transistors.

Output I-V characteristics curvesInput I-V characteristics

I-V Characteristics I-V Characteristics

A load line is a line drawn over the collector curves to show each and every possible operating point of a transistor.

Load line

Load Line Load Line

The point at which the base current intersects the load line is referred to as the operating point or the quiescent point.

Q

VCC = VRB + VBE

VRB = VCC - VBE

IB = VCC -VBE

RB

IC = βIB

VCC = VCE + VRC

VCE = VCC - ICRC

VCE

RB RC

VCC

IB

IC

IE

• It consists only of a fixed bias resistor and load resistor.

VBE

Fixed Base Bias Circuit (Simple Bias Circuit) and Formula

VCE

R2

R1

VCC

RC

RE

VBE

IC

IB

IE

It offers the best resilience against changes in temperature and device characteristics and by far more stabilized.

R1 and R2 form a potential divider, which will fix the base potential of the transistor. The current is usually set at 10 times greater than the base current required by the transistor

The base emitter voltage drop is approximated as 0.7volt. There will also be a voltage drop across the emitter resistor, RE, this is generally set to about 10% of the supply voltage.

If IC increases, then so will IE.

Potential Divider Bias CircuitPotential Divider Bias Circuit

VCC = IR1 + IR2

VB = R2

R1 + R2VCC

VRE = VB - VBE

IE = VRE/ RE

IB can be ignored if the value is too small

IE = IB + IC

VCE = VCC –VRC - VRE

VC = VCE + VRE

FormulaFormula

One factor can affect the IB is AC input signal. AC input signal must not too big so that can

control the IB.

1284

0V

20A

40A

60A

80A

100A

8

10 20155

10

6

4

2

VC (V)

IC(mA)

Q point is at IB = 60µA, Ic = 6mA, Vc = 8V.

If IB is between 40µA and 80µA, while Ic is between 4mA and 8mA

So Vc is between 4V and 12V.

Vc is output voltage for the amplifier circuit, Vo.

If input signal is too big, It will affect the IB.

IB will become big, followed by Ic and Vc. Ic and Vc will over the saturation point and

will cut the signal. When this happen, there is a distortion to

the output signal. If there is a distortion to the output signal,

the amplifier is not efficient.

From figure above, Load resistance,

rL : R3 // R6

Will have a new AC loadline (new value of cut off and saturation point).

Ic(sat)new = ICQ + (VCQ/RL)

Vc(sat)new = VCQ + ICQ . RL

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1

0.5

VC (V)

Av = Vo/Vi = rL / re

Where re = 25mV/ IE

AC loadline exists when the load for AC voltage output is different with the load for DC voltageoutput.

3

15Vp 12Vp

VCQ 27V

VC(alih)

0

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VC (V)

Bahagian 1 :

VC(ALIH)(AU) - VCQ = 27V – 15V

= 12Vp

Bahagian 2 :

VCQ – 0 = 15V – 0 = 15Vp

24Vp-p

30Vp-p

Is output voltage that oscillated within the operation point symmetrically and no distortion.

Can be determine from equation Av = rL / re

Use the Av to determine maximum input voltage, Vi.

Av = Vo/Vi, so Vi = Vo/Av.

It can define as undistorted maximum inputsignal because it has undistorted maximum output signal.

Frequency response is characterized by the response magnitude (measure in dB) versus frequency.

From frequency response graph above : Y – axis (shows response in decibel (dB)) X – axis (shows frequency (Hz)) Important parameters of frequency

response curve :◦ Maximum voltage gain, Av(max).◦ Maximum voltage gain, Av (max) in dB◦ Cut off frequency (-3dB)◦ Frequency bandwidth.

Maximum voltage gain , Av(max) = maximum ratio of output voltage, Vo over input voltage, Vi.

Av in dB = 20 log Av(max) Cut off frequency – or corner frequency is the frequency either above or below power output of a circuit.

fc1 – lower 3dB cut off frequency

fc2 – upper 3dB cut off frequency

Frequency Bandwidth – range of frequency between fc1 and fc2.

Use Av(max)dB instead of Av(max)because to scale a large measurement down to a much smaller and more useable range.

Eg : Vo = 100, Vi = 1 Av(max) = Vo/Vi = 100/1 = 100 In dB, Av(max)dB = 20 log Av(max)

= 20 log 100 = 40.

This frequency is where a device will quit working or operating in an efficient manner resulting in it shutting down or being cut off.

Refer to the 3dB point since a fall of 3dB corresponds approximately to half power of the output circuit.

There is lower cut off frequency and upper cut off frequency.

a band of a given width can carry the same amount of information.

So to relate bandwidth to the performance of amplifier,

◦ wide bandwidth, the amplifier is more efficient.

◦BW = fc2 – fc1

fB = 1 / (2π RCB) where :

R = Rs + Rin ; Rin = R1 ║R2 ║hie

CB = value of the Base coupling capacitor, C1

fC = 1 / [2π (RC + RL) CC]

RC + RL = sum of the resistance in the collector circuit

CC = value of the Base coupling capacitor, C2

Rout = RE ║ ( re + Rin / hfe ); re = VT / IE

Rin = R1 ║ R2 ║ Rs

fE = 1 / (2π Rout CE )CE = value of the Base coupling capacitor,C3

There are 4 classes of amplifier.

◦Class A◦Class B◦Class AB◦Class C

Class A Amplifier The most common and simplest form of

power amplifier that uses the switching transistor in the standard common emitter circuit configuration as seen previously.

The transistor is always biased "ON" so that it conducts during one complete cycle of the input signal waveform producing minimum distortion and maximum amplitude to the output.

Is the ideal operating mode, because there can be no crossover or switch-off distortion to the output waveform even during the negative half of the cycle.

Class A power amplifier output stages may use a single power transistor or pairs of transistors connected together to share the high load current.

Class-B amplifiers only amplify half of the input wave cycle, thus creating a large amount of distortion, but their efficiency is greatly improved and is much better than class A.

Class B has a maximum theoretical efficiency of π/4. (i.e. 78.5%) This is because the amplifying element is switched off altogether half of the time, and so cannot dissipate power.

A single class-B element is rarely found in practice, though it has been used for driving the loudspeaker in the early IBM Personal Computers with beeps, and it can be used in RF power amplifier where the distortion levels are less important. However, class C is more commonly used for this.

A practical circuit using class-B elements is the push–pull stage, such as the very simplified complementary pair arrangement .

Complementary or quasi-complementary

devices are each used for amplifying the opposite halves of the input signal, which is then recombined at the output.

This arrangement gives excellent efficiency, but can suffer from the drawback that there is a small mismatch in the cross-over region – at the "joins" between the two halves of the signal, as one output device has to take over supplying power exactly as the other finishes. This is called crossover distortion

In class-AB operation, each device operates the same way as in class B over half the waveform, but also conducts a small amount on the other half.

As a result, the region where both devices simultaneously are nearly off (the "dead zone") is reduced. The result is that when the waveforms from the two devices are combined, the crossover is greatly minimised or eliminated altogether. 

Collector current flows for less than one half cycle of the input signal.

By reverse biasing the emitter-base junction, which sets the dc operating point below cut off and allows only the portion of the input signal that overcomes the reverse bias to cause collector current flow.

Class A – amps sound the best, cost the most, and are the least practical. They waste power and return very clean signals.

Class B - operated amplifier is used extensively for audio amplifiers that require high-power outputs. It is also used as the driver- and power-amplifier stages of transmitters.

Class C - operated amplifier is used as a radio-frequency amplifier in transmitters.

Class AB - operated amplifier is commonly used as a push-pull amplifier to overcome a side effect of class B operation called crossover distortion

Bipolar transistor amplifiers must be properly biased to operate correctly.

There are : - base biased with emitter feedback

technique. - biased voltage divider technique

Vin

Fig 1 Fig 2

Circuit in Fig 1 is unstable

Circuit in Fig 2 is stable

Adding resistance (RE) to the emitter of transistor.

RE improves performance by adding negative feedback.

IC begins to increase as temperature rises. The increase in IC increases IE and as a result VE

rises. VBE (or VB - VE) is fixed, but the rise in VE reduces

VBE. The overall effect of reducing VBE is to reduce IC which in turn makes the circuit stable. Negative feedback.

RE greatly reduces error (variation in gain and consequent distortion).

Emitter resistor RE also solves other problems such as temperature instability and distortion.

The voltage divider is formed using external resistors R1 and R2.

The voltage across R2 forward biases the emitter junction.

By proper selection of resistors R1 and R2, the operating point of the transistor can be made independent of β.

RTH = R1//R2 RTH = R1R2 R1 + R2 VTH = R2 x VCC R1 + R2 IE = (β+1)IB

KVL from input loop :

- VTH + IBRTH + VBE + IERE = 0

- VTH + IBRTH + VBE + (β+1)IBRE = 0

- VTH + IB(RTH + (β+1)RE) + VBE = 0

IB = VTH – VBE (RTH + (β+1)RE)

KVL from output loop:

- VCC – ICRC – VCE – IERE = 0

IC = βIB

IE = (β+1)IB

Transistors are commonly used for :

1. Amplifier circuit 2. Switching circuit

ApplicationApplication