Module 8—Introduction to Amplifiers
Pages i - ix
1-1 to 1-10
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1-21 to 1-30
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2-1 to 2-10
, 2-11 to 2-20
2-21 to 2-30
, 2-31 to 2-35
3-1 to 3-10
,3-11 to 3-20
3-21 to 3-30
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, AI-1 to AI-3
The feedback network in this amplifier is made up of R2 and C2. The value of C2 should be large so that the
capacitive reactance (X C) will be low and the capacitor will couple the signal easily. (This is also the case
with the input and output coupling capacitors C1 and C3.) The resistive value of R2 should be large to limit the
amount of feedback signal and to ensure that the majority of the output signal goes on to the next stage through
Figure 1-18.—Positive feedback in a transistor amplifier.
A more common configuration for transistor amplifiers is the common-emitter configuration. Positive feedback
is a little more difficult with this configuration because the input and output signals are 180º out of phase.
Positive feedback can be accomplished by feeding a portion of the output signal of the second stage back to the
input of the first stage. This arrangement is shown in figure 1-19.
Figure 1-19.—Positive feedback in two stages of transistor amplification.
The figure shows that each stage of amplification has a 180º phase shift. This means that the output signal of
Q2 will be in phase with the input signal to Q1. A portion of the output signal of Q2 is coupled back to the input
of Q1 through the feedback network of C3 and R3. R3 should have a large resistance to limit the amount of signal
through the feedback network. C3 should have a large capacitance so the capacitive reactance is low and the
capacitor will couple the signal easily.
Sometimes positive feedback is used to eliminate the effects of
negative feedback that are caused by circuit components. One way in which a circuit component can cause negative
feedback is shown in figure 1-20.
In view (A) a common-emitter transistor amplifier is shown. An emitter
resistor (R2) has been placed in this circuit to provide proper biasing and temperature stability. An undesired
effect of this resistor is the development of a signal at the emitter in phase with the input signal on the base.
This signal is caused by the changing current through the emitter resistor (R2) as the current through the
transistor changes. You might think that this signal on the emitter is a form of positive feedback since it is in
phase with the input signal. But the emitter signal is really negative feedback. Current through the transistor is
controlled by the base-to-emitter bias. If both the base and emitter become more positive by the same amount at
the same time, current will not increase. It is the difference between the base and emitter voltages that controls
the current flow through the transistor.
To eliminate this negative feedback caused by the emitter resistor, some way must be found to remove the
signal from the emitter. If the signal could be coupled to ground (decoupled) the emitter of the transistor would
be unaffected. That is exactly what is done. A DECOUPLING CAPACITOR (C3 in view B) is placed between the emitter
of Q1 and ground (across the emitter resistor). This capacitor should have a high capacitance so that it will pass
the signal to ground easily. The decoupling capacitor (C3) should have the same qualities as the coupling
capacitors (C1 and C2) of the circuit. Decoupling capacitors are also called bypass capacitors.
Figure 1-20A.—Decoupling (bypass) capacitor in a transistor amplifier.
Figure 1-20B.—Decoupling (bypass) capacitor in a transistor amplifier.
Regardless of the method used to provide positive feedback in a circuit, the purpose is to increase the
output signal amplitude.
Negative feedback is accomplished by
adding part of the output signal out of phase with the input signal. You have seen that an emitter resistor in a
common-emitter transistor amplifier will develop a negative feedback signal. Other methods of providing negative
feedback are similar to those methods used to provide positive feedback. The phase relationship of the feedback
signal and the input signal is the only difference.
Figure 1-21 shows negative feedback in a
common-emitter transistor amplifier. The feedback network of C2 and R2 couples part of the output signal of Q1
back to the input. Since the output signal is 180º out of phase with the input signal, this causes negative
Figure 1-21.—Negative feedback in a transistor amplifier.
Negative feedback is used to improve fidelity of an amplifier by limiting the input signal. Negative
feedback can also be used to increase the frequency response of an amplifier. The gain of an amplifier decreases
when the limit of its frequency response is reached. When negative feedback is used, the feedback signal decreases
as the output signal decreases. At the limits of frequency response of the amplifier the smaller feedback signal
means that the effective gain (gain with feedback) is increased. This will improve the frequency response of the
Q-23. What is feedback?
Q-24. What are the two types of feedback?
Q-25. What type
feedback provides increased amplitude output signals?
Q-26. What type feedback provides the best
Q-27. If the feedback signal is out of phase with the input signal, what type feedback is
Q-28. What type feedback is provided by an unbypassed emitter resistor in a common-emitter transistor
An audio amplifier has been described as an amplifier with a frequency response from 15 Hz to 20 kHz. The
frequency response of an amplifier can be shown graphically with a frequency response curve. Figure 1-22 is the
ideal frequency response curve for an audio amplifier. This curve is practically "flat" from 15 Hz to 20 kHz. This
means that the gain of the amplifier is equal between 15 Hz and 20 kHz. Above 20 kHz or below 15 Hz the gain
decreases or "drops off" quite rapidly. The frequency response of an amplifier is determined by the components in
Figure 1-22.—Ideal frequency response curve for an audio amplifier.
The difference between an audio amplifier and other amplifiers is the frequency response of the amplifier. In
the next chapter of this module you will be shown the techniques and components used to change and extend the
frequency response of an amplifier.
The transistor itself will respond quite well to the audio frequency
range. No special components are needed to extend or modify the frequency response.
You have already been
shown the purpose of all the components in a transistor audio amplifier. In this portion of the chapter, schematic
diagrams of several audio amplifiers will be shown and the functions of each of the components will be discussed.
SINGLE-STAGE AUDIO AMPLIFIERS
The first single-stage audio amplifier is shown in figure 1-23.
This circuit is a class A, common-emitter, RC-coupled, transistor, audio amplifier. C1 is a coupling capacitor
that couples the input signal to the base of Q1. R1 is used to develop the input signal and provide bias for the
base of Q1. R2 is used to bias the emitter and provide temperature stability for Q1. C2 is used to provide
decoupling (positive feedback) of the signal that would be developed by R2. R3 is the collector load for Q1 and
develops the output signal. C3 is a coupling capacitor that couples the output signal to the next stage. VCC
represents the collector-supply voltage. Since the transistor is a common-emitter configuration, it provides
voltage amplification. The input and output signals are 180º out of phase. The input and output impedance are both
Figure 1-23.—Transistor audio amplifier.
There is nothing new presented in this circuit. You should understand all of the functions of the components
in this circuit. If you do not, look back at the various sections presented earlier in this chapter.
second single-stage audio amplifier is shown in figure 1-24. This circuit is a class A, common-source, RC-coupled,
FET, audio amplifier. C1 is a coupling capacitor which couples the input signal to the gate of Q1. R1 is used to
develop the input signal for the gate of Q1. R2 is used to bias the source of Q1. C2 is used to decouple the
signal developed by R2 (and keep it from affecting the source of Q1). R3 is the drain load for Q1 and develops the
output signal. C3 couples the output signal to the next stage. VDD is the supply voltage for the drain of Q1.
Since this is a common-source configuration, the input and output signals are 180º out of phase.
Figure 1-24.—FET audio amplifier.
If you do not remember how a FET works, refer to NEETS Module 7 Introduction to Solid-State Devices
and Power Supplies.
The third single-stage audio amplifier is shown in figure 1-25. This is a class A,
common-emitter, transformer-coupled, transistor, audio amplifier. The output device (speaker) is shown connected
to the secondary winding of the transformer. C1 is a coupling capacitor which couples the input signal to the base
of Q1. R1 develops the input signal. R2 is used to bias the emitter of Q1 and provides temperature stability. C2
is a decoupling capacitor for R2. R3 is used to bias the base of Q1. The primary of T1 is the collector load for
Q1 and develops the output signal. T1 couples the output signal to the speaker and provides impedance matching
between the output impedance of the transistor (medium) and the impedance of the speaker (low).
Figure 1-25.—Single-stage audio amplifier.
Sometimes it is necessary to provide two signals that are equal
in amplitude but 180º out of phase with each other. (You will see one use of these two signals a little later in
this chapter.) The two signals can be provided from a single input signal by the use of a PHASE SPLITTER. A phase
splitter is a device that produces two signals that differ in phase from each other from a single input signal.
Figure 1-26 is a block diagram of a phase splitter.
Figure 1-26.—Block diagram of a phase splitter.
One way in which a phase splitter can be made is to use a center-tapped transformer. As you may remember
from your study of transformers, when the transformer secondary winding is center-tapped, two equal amplitude
signals are produced. These signals will be 180º out of phase with each other. So a transformer with a
center-tapped secondary fulfills the definition of a phase splitter.
A transistor amplifier can be configured to act as a phase splitter. One method of doing this is shown
in figure 1-27.
Figure 1-27.—Single-stage transistor phase splitter.
C1 is the input signal coupling capacitor and couples the input signal to the base of Q1. R1 develops
the input signal. R2 and R3 develop the output signals. R2 and R3 are equal resistances to provide equal amplitude
output signals. C2 and C3 couple the output signals to the next stage. R4 is used to provide proper bias for the
base of Q1.
This phase splitter is actually a single transistor combining the qualities of the
common-emitter and common-collector configurations. The output signals are equal in amplitude of the input signal,
but are 180º out of phase from each other.
If the output signals must be larger in amplitude than the
input signal, a circuit such as that shown in figure 1-28 will be used.
Figure 1-28 shows a two-stage
phase splitter. C1 couples the input signal to the base of Q1. R1 develops the input signal and provides bias for
the base of Q1. R2 provides bias and temperature stability for Q1. C2 decouples signals from the emitter of Q1. R3
develops the output signal of Q1. Since Q1 is configured as a common-emitter amplifier, the output signal of Q1 is
180º out of phase with the input signal and larger in amplitude. C3 couples this output signal to the next stage
through R4. R4 allows only a small portion of this output signal to be applied to the base of Q2. R5 develops the
input signal and provides bias for the base of Q2. R6 is used for bias and temperature stability for Q2. C4
decouples signals from the emitter of Q2. R7 develops the output signal from Q2. Q2 is configured as a
common-emitter amplifier, so the output signal is 180º out of phase with the input signal to Q2 (output signal
from Q1). The input signal to Q2 is 180º out of phase with the original input signal, so the output from Q2 is in
phase with the original input signal. C5 couples this output signal to the next stage. So the circuitry shown
provides two output signals that are 180º out of phase with each other. The output signals are equal in amplitude
with each other but larger than the input signal.
Figure 1-28.—Two-stage transistor phase splitter.
Q-29. What is a phase splitter?
One use of phase
splitters is to provide input signals to a single-stage amplifier that uses two transistors. These transistors are
configured in such a way that the two outputs, 180º out of phase with each other, combine. This allows more gain
than one transistor could supply by itself. This "push-pull" amplifier is used where high power output and good
fidelity are needed: receiver output stages, public address amplifiers, and AM modulators, for example.
The circuit shown in figure 1-29 is a class A transistor push-pull amplifier, but class AB or class B
operations can be used. Class operations were discussed in an earlier topic. The phase splitter for this amplifier
is the transformer T1, although one of the phase splitters shown earlier in this topic could be used. R1 provides
the proper bias for Q1 and Q2. The tapped secondary of T1 develops the two input signals for the bases of Q1 and
Q2. Half of the original input signal will be amplified by Q-1, the other half by Q-2. T2 combines (couples) the
amplified output signal to the speaker and provides impedance matching.
Figure 1-29.—Class A transistor push-pull amplifier.
Q-30. What is one use for a splitter?
Q-31. What is a common use for a push-pull amplifier?
Q-32. What is the advantage of a push-pull amplifier?
Q-33. What class of operation can be used with a
push-pull amplifier to provide good fidelity output signals?
This chapter has presented some general information that applies to all amplifiers, as well as some specific
information about transistor and audio amplifiers. All of this information will be useful to you in the next
chapter of this module and in your future studies of electronics.
An AMPLIFIER is a
device that enables an input signal to control an output signal. The output signal will have some (or all) of the
characteristics of the input signal but will generally be larger than the input signal in terms of voltage,
current, or power. A basic line diagram of an amplifier is shown below.
Amplifiers are classified by FUNCTION and FREQUENCY RESPONSE. Function refers to an amplifier being a
VOLTAGE AMPLIFIER or a POWER AMPLIFIER. Voltage amplifiers provide voltage amplification and power amplifiers
provide power amplification. The frequency response of an amplifier can be described by classifying the amplifier
as an AUDIO AMPLIFIER, RF AMPLIFIER, or VIDEO (WIDE-BAND) AMPLIFIER. Audio amplifiers have frequency response in
the range of 15 Hz to 20 kHz. An RF amplifier has a frequency response in the range of 10 kHz to 100,000 MHz. A
video (wide-band) amplifier has a frequency response of 10 Hz to 6 MHz.
Introduction to Matter, Energy, and Direct Current, Introduction
to Alternating Current and Transformers, Introduction to Circuit Protection,
Control, and Measurement, Introduction to Electrical Conductors, Wiring Techniques,
and Schematic Reading, Introduction to Generators and Motors,
Introduction to Electronic Emission, Tubes, and Power Supplies,
Introduction to Solid-State Devices and Power Supplies,
Introduction to Amplifiers, Introduction to
Wave-Generation and Wave-Shaping Circuits, Introduction to Wave Propagation, Transmission
Lines, and Antennas, Microwave Principles,
Modulation Principles, Introduction to Number Systems and Logic Circuits, Introduction
to Microelectronics, Principles of Synchros, Servos, and Gyros,
Introduction to Test Equipment, Radio-Frequency
Communications Principles, Radar Principles, The Technician's Handbook,
Master Glossary, Test Methods and Practices, Introduction to Digital Computers,
Magnetic Recording, Introduction to Fiber Optics