Transistor Amplifiers Topics Covered in Chapter 29 29-1: AC Resistance of a Diode 29-2: Small-Signal...

Transistor AmplifiersTransistor 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

ChapterChapter2929

© 2007 The McGraw-Hill Companies, Inc. All rights reserved.

Topics Covered in Chapter 29Topics 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

McGraw-Hill © 2007 The McGraw-Hill Companies, Inc. All rights reserved.

29-1: AC Resistance of a Diode29-1: 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 Diode29-1: AC Resistance of a Diode

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Fig. 29-1

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.

29-2: Small-Signal Amplifier 29-2: Small-Signal Amplifier OperationOperation

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 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.



Fig. 29-3 (a)

Common Emitter Amplifier



Fig. 29-3 (b)

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.

29-3: AC Equivalent Circuit 29-3: AC Equivalent Circuit of a CE Amplifierof 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.



Fig. 29-5 (a)

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 (b)

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.

29-4: Calculating the 29-4: Calculating the Voltage Gain, Voltage Gain, AAVV

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 29-4: Calculating the Voltage Gain, Voltage Gain, AAVV

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.Fig. 29-7 (a)

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).

29-5: Calculating the Input and 29-5: Calculating the Input and Output Impedances in a CE AmplifierOutput 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

in in (base) 1 2z = z R R

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 Amplifier29-6: 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.



Fig. 29-8 (a)

Fig. 29-8 (a) shows a CC amplifier circuit which is also called an emitter follower.



Fig. 29-8(b)

Fig. 29-8 (b) illustrates a dc load line showing IC (sat), VCE (off), IC, and VCE.

29-7: AC Analysis of 29-7: AC Analysis of an Emitter Followeran 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 29-7: AC Analysis of an Emitter Followeran Emitter Follower


Fig. 29-9

Fig. 29-9 shows the ac equivalent circuit for the emitter follower circuit in Fig. 29-8 (a).

29-8: Emitter Follower 29-8: Emitter Follower ApplicationsApplications

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 29-8: Emitter Follower ApplicationsApplications


Fig. 29-12

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.

29-9: Common Base Amplifier29-9: 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.

29-9: Common Base Amplifier29-9: Common Base Amplifier


Fig. 29-14(a)

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.

29-10: AC Analysis of a 29-10: AC Analysis of a Common-Base AmplifierCommon-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 Common-29-10: AC Analysis of a Common-Base AmplifierBase Amplifier


Fig. 29-15

Fig. 29-15 illustrates the ac equivalent circuit of the common-base amplifier in Fig. 29-14.

Transistor Amplifiers Topics Covered in Chapter 29 29-1: AC Resistance of a Diode 29-2: Small-Signal...

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