1. Transistor Amplifiers
Chapter 29
Transistor Amplifiers
Topics Covered in Chapter 29
29-1: AC Resistance of a Diode
29-2: Small-Signal Amplifier Operation
29-3: AC Equivalent Circuit of a CE Amplifier
29-4: Calculating the Voltage Gain, AV
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2. Topics Covered in Chapter 29
29-5: Calculating the Input and Output Impedances in a CE Amplifier
29-6: Common-Collector Amplifier
29-7: AC Analysis of an Emitter Follower
29-8: Emitter Follower Applications
29-9: Common Base Amplifier
29-10: AC Analysis of a Common-Base Amplifier
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© 2007 The McGraw-Hill Companies, Inc. All rights reserved.

3. 29-1: AC Resistance of a Diode
For small ac signals, the diode acts like a resistance.
The ac resistance for a diode is calculated as:
rac = 25 mV/Id
When analyzing a common-emitter amplifier, it is common practice to represent the emitter diode as a small resistance.
By doing this, important characteristics of an amplifier, such as its voltage gain and input impedance, can be calculated.

4. 29-1: AC Resistance of a Diode
Figure 29-1 (a) shows a dc source in series with an ac source and both supply current to the diode.
The dc source provides the forward bias for D1, while the ac source produces fluctuations in the diode current.
The graph in 29-1 (b) illustrates how the diode current varies with the ac voltage.
Fig. 29-1
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5. 29-2: Small-Signal Amplifier Operation
In a common-emitter amplifier, the input signal is applied to the base and the output signal is taken from the collector.
Fig (a) in the next slide, shows a common-emitter amplifier.
Cin is an input coupling capacitor that couples ac but blocks dc.
CE is an emitter bypass capacitor that provides a low-impedance path for ac signals between the emitter terminal and ground.

6. 29-2: Small-Signal Amplifier Operation
Common Emitter Amplifier
Fig (a)
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7. 29-2: Small-Signal Amplifier Operation
Figure 29-3 (b) shows how the operating point moves up and down the dc load line with changes in IB and IC.
For small signal operation, only a small portion of the dc load line is used.
Fig (b)
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8. 29-3: AC Equivalent Circuit of a CE Amplifier
When analyzing transistor amplifier circuits, it is commonplace to draw the ac equivalent circuit.
In the ac equivalent circuit of a transistor amplifier, the emitter diode is replaced with its equivalent ac resistance, r’e.

9. 29-3: AC Equivalent Circuit of a CE Amplifier
Cin and CE appear as ac short circuits.
VCC has been reduced to zero.
The emitter diode has been replaced with its equivalent resistance, r’e.
The biasing resistors, R1 and R2, are shown in parallel.
Fig (a)
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10. 29-3: AC Equivalent Circuit of a CE Amplifier
A simplified and more condensed version of the ac equivalent circuit is shown in Fog (b).
The input voltage vin of 10 mVpp appears directly across the ac resistance (r’e) of the emitter diode.
The output is directly across the collector resistance, RC.
Fig (b)
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11. 29-4: Calculating the Voltage Gain, AV
The ac equivalent circuit is used to help understand the ac operation of the common-emitter amplifier circuit.
Voltage gain, AV, is expressed as
AV = vout/vin
The voltage gain of a common-emitter amplifier equals rL/r’e when the dc emitter resistance is completely bypassed.
The ac load resistance, designated rL, equals the equivalent resistance of RC and RL in parallel.

12. 29-4: Calculating the Voltage Gain, AV
One way to reduce greatly the variations in AV caused by changes in r’e is to add a swamping resistor in the emitter circuit, as shown in Fig (a).
Fig (a)
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13. 29-5: Calculating the Input and Output Impedances in a CE Amplifier
The input impedance of an amplifier is the input impedance seen by the ac source driving the amplifier.
Biasing resistors are included is the input impedance.
Input impedance is calculated as
The output impedance, zout of a CE amplifier equals the value of the collector resistor, RC, but does not include the load resistor, RL.

14. 29-6: Common-Collector Amplifier
The common-collector CC amplifier is used to provide current gain and power gain.
The voltage gain equals approximately one, or unity.
The collector is common to both the input and output sides of the amplifier.
The input signal is applied to the base, while the output is taken from the emitter.
The output signal is in phase with the input signal.
The CC amplifier is usually referred to as an emitter follower.

15. 29-6: Common-Collector Amplifier
Fig (a) shows a CC amplifier circuit which is also called an emitter follower.
Fig (a)
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16. 29-6: Common-Collector Amplifier
Fig (b) illustrates a dc load line showing IC (sat), VCE (off), IC, and VCE.
Fig. 29-8(b)
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17. 29-7: AC Analysis of an Emitter Follower
In an emitter follower circuit, the input is applied to the base while the output is taken from the emitter.
Because the collector is tied directly to the collector supply voltage, VCC, no ac signal appears there.
Because the emitter follower takes its output from the emitter, an emitter bypass capacitor is not used.
The emitter is typically unbypassed, therefore, the swamping is heavy and the distortion in the output signal is extremely small.

18. 29-7: AC Analysis of an Emitter Follower
Fig shows the ac equivalent circuit for the emitter follower circuit in Fig (a).
Fig. 29-9
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19. 29-8: Emitter Follower Applications
An emitter follower has high input impedance and low output impedance.
This makes the emitter follower ideal for impedance matching applications.

20. 29-8: Emitter Follower Applications
The main purpose of the circuit in Fig is to use the emitter follower as a buffer to isolate the relatively low value of load resistance, RL, from the high impedance collector Q1.
Fig
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21. 29-9: Common Base Amplifier
The common-base CB amplifier is used less often than the CE or CC amplifiers.
The common-base amplifier provides a high voltage and power gain but the current gain is less than one.
The common-base amplifier has an extremely low input impedance, zin.
The CB amplifier provides some desirable features for operation at higher frequencies.
The CB amplifier is also used in a differential amplifier which is used in linear integrated circuits known as op amps.

22. 29-9: Common Base Amplifier
Fig (a) shows a common-base amplifier.
The base is grounded.
The input signal, Vin, is applied to the emitter and the output is taken from the collector.
Fig (a)
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23. 29-10: AC Analysis of a Common-Base Amplifier
The main drawback of the common-base amplifier is its extremely low input impedance which is approximately equal to the low value of r’e.

24. 29-10: AC Analysis of a Common-Base Amplifier
Fig illustrates the ac equivalent circuit of the common-base amplifier in Fig
Fig
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