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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
© 2007 The McGraw-Hill Companies, Inc. All rights reserved.
Topics Covered in Chapter 29
 29-5: Calculating the Input and Output Impedances in
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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
McGraw-Hill
© 2007 The McGraw-Hill Companies, Inc. All rights reserved.
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.
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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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. 29-3 (a) in the next slide, shows a commonemitter amplifier.
 Cin is an input coupling capacitor that couples ac but
blocks dc.
 CE is an emitter bypass capacitor that provides a lowimpedance path for ac signals between the emitter
terminal and ground.
29-2: Small-Signal Amplifier
Operation
Common Emitter Amplifier
Fig. 29-3 (a)
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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. 29-3 (b)
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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.
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. 29-5 (a)
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29-3: AC Equivalent Circuit
of a CE Amplifier
 A simplified and more condensed version of the ac equivalent circuit is
shown in Fog. 29-5 (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. 29-5 (b)
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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.
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. 29-7 (a).
Fig. 29-7 (a)
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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
zin = zin (base) R1 R2
 The output impedance, zout of a CE amplifier equals the
value of the collector resistor, RC, but does not include the
load resistor, RL.
29-6: Common-Collector Amplifier
 The common-collector CC amplifier is used to
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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.
29-6: Common-Collector Amplifier
 Fig. 29-8 (a) shows a CC amplifier circuit which is also called an emitter
follower.
Fig. 29-8 (a)
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29-6: Common-Collector Amplifier
 Fig. 29-8 (b) illustrates a dc load line showing IC (sat), VCE (off), IC, and VCE.
Fig. 29-8(b)
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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.
29-7: AC Analysis of
an Emitter Follower
 Fig. 29-9 shows the ac equivalent circuit for the emitter follower circuit
in Fig. 29-8 (a).
Fig. 29-9
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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.
29-8: Emitter Follower Applications
The main purpose of the circuit in Fig. 29-12 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. 29-12
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29-9: Common Base Amplifier
 The common-base CB amplifier is used less often
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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.
29-9: Common Base Amplifier
 Fig. 29-14 (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. 29-14(a)
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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.
29-10: AC Analysis of a CommonBase Amplifier
Fig. 29-15 illustrates the ac equivalent circuit of the commonbase amplifier in Fig. 29-14.
Fig. 29-15
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