Module 8 - Introduction to Amplifiers
Pages i - ix,
1-1 to 1-10,
1-11 to 1-20,
1-21 to 1-30,
1-31 to 1-40,
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,
3-31 to 3-40,
3-41 to 3-50,
3-51 to 3-60,
3-61 to 3-70,
AI-1 to AI-3,
Learning objectives are stated at the beginning of each chapter. These learning objectives serve as a preview
of the information you are expected to learn in the chapter. The comprehensive check questions are based on the
objectives. By successfully completing the OCC/ECC, you indicate that you have met the objectives and have learned
the information. The learning objectives are listed below.
Upon completion of this chapter, you will be able
1. Define amplification and list several common uses; state two ways in which amplifiers are
2. List the four classes of operation of, four methods of coupling for, and the impedance
characteristics of the three configurations of a transistor amplifier.
3. Define feedback and list the two
types of feedback.
4. Describe and state one use for a phase splitter.
5. State a common use for
and one advantage of a push-pull amplifier.
This chapter is a milestone in your study of electronics. Previous modules have been concerned more with
individual components of circuits than with the complete circuits as the subject. This chapter and the other
chapters of this module are concerned with the circuitry of amplifiers. While components are discussed, the
discussion of the components is not an explanation of the working of the component itself (these have been covered
in previous modules) but an explanation of the component as it relates to the circuit.
The circuits this
chapter is concerned with are AMPLIFIERS. Amplifiers are devices that provide AMPLIFICATION. That doesn't explain
much, but it does describe an amplifier if you know what amplification is and what it is used for.
WHAT IS AMPLIFICATION?
Just as an amplifier is a device that provides amplification, amplification is the process of providing an
increase in AMPLITUDE. Amplitude is a term that describes the size of a signal. In terms of a.c., amplitude
usually refers to the amount of voltage or current. A 5-volt peak-to-peak a.c.signal would be larger in amplitude
than a 4-volt peak-to-peak a.c. signal. "SIGNAL" is a general term used to refer to any a.c. or d.c. of interest
in a circuit; e.g., input signal and output signal. A signal can be large or small, ac. or d.c., a sine wave or
nonsinusoidal, or even nonelectrical such as sound or light. "Signal" is a very general term and, therefore, not
very descriptive by itself, but it does sound more technical than the word "thing". It is not very impressive to
refer to the "input thing" or the "thing that comes out of this circuit."
Perhaps the concept of the relationship of amplifier-amplification-amplitude will be clearer if you
consider a parallel situation (an analogy). A magnifying glass is a magnifier. As such, it provides magnification
which is an increase in the magnitude (size) of an object. This relationship of magnifier- magnification-magnitude
is the same as the relationship of amplifier-amplification-amplitude. The analogy is true in one other aspect as
well. The magnifier does not change the object that is being magnified; it is only the image that is larger, not
the object itself. With the amplifier, the output signal differs in amplitude from the input signal, but the input
signal still exists unchanged. So, the object (input signal) and the magnifier (amplifier) control the image
An amplifier can be defined as 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.
USES OF AMPLIFICATION
Most electronic devices use amplifiers to provide various amounts of signal amplification. Since most signals are
originally too small to control or drive the desired device, some amplification is needed.
For example, the
audio signal taken from a record is too small to drive a speaker, so amplification is needed. The signal will be
amplified several times between the needle of the record player and the speaker. Each time the signal is amplified
it is said to go through a STAGE of amplification. The audio amplifier shown connected between the turntable and
speaker system in figure 1-1 contains several stages of amplification.
Figure 1-1. - Amplifier as used with turntable and speaker.
Notice the triangle used in figure 1-1 to represent the amplifier. This triangle is the standard block
diagram symbol for an amplifier.
Another example of the use of an amplifier is shown in figure 1-2. In a radio
receiver, the signal picked up by the antenna is too weak (small) to be used as it is. This signal must be
amplified before it is sent to the detector. (The detector separates the audio signal from the frequency that was
sent by the transmitter. The way in which this is done will be discussed later in this training series.)
Figure 1-2. - Amplifiers as used in radio receiver.
The audio signal from the detector will then be amplified to make it large enough to drive the speaker
of the radio.
Almost every electronic device contains at least one stage of amplification, so you will be
seeing amplifiers in many devices that you work on. Amplifiers will also be used in most of the NEETS modules that
follow this one.
Q-1. What is amplification?
Q-2. Does an amplifier actually change an input
signal? Why or why not?
Q-3. Why do electronic devices use amplifiers?
CLASSIFICATION OF AMPLIFIERS
Most electronic devices use at least one amplifier, but there are many types of amplifiers. This module will
not try to describe all the different types of amplifiers. You will be shown the general principles of amplifiers
and some typical amplifier circuits.
Most amplifiers can be classified in two ways. The first
classification is by their function. This means they are basically voltage amplifiers or power amplifiers. The
second classification is by their frequency response. In other words what frequencies are they designed to
If you describe an amplifier by these two classifications (function and frequency response) you
will have a good working description of the amplifier. You may not know what the exact circuitry is, but you will
know what the amplifier does and the frequencies that it is designed to handle.
AND POWER AMPLIFIERS
All amplifiers are current-control devices. The input signal to an amplifier controls the current output of
the amplifier. The connections of the amplifying device (electron tube, transistor, magnetic amplifier,
etc.) and the circuitry of the amplifier determine the classification. Amplifiers are classified as voltage or
A VOLTAGE AMPLIFIER is an amplifier in which the output signal voltage is
larger than the input signal voltage. In other words, a voltage amplifier amplifies the voltage of the input
A POWER AMPLIFIER is an amplifier in which the output signal power is greater than the input
signal power. In other words, a power amplifier amplifies the power of the input signal. Most power amplifiers are
used as the final amplifier (stage of amplification) and control (or drive) the output device. The output device
could be a speaker, an indicating device, an antenna, or the heads on a tape recorder. Whatever the device, the
power to make it work (or drive it) comes from the final stage of amplification which is a power amplifier.
Figure 1-3 shows a simple block diagram of a voltage amplifier with its input and output signals and a power
amplifier with its input and output signals. Notice that in view (A) the output signal voltage is larger than the
input signal voltage. Since the current values for the input and output signals are not shown, you cannot tell if
there is a power gain in addition to the voltage gain.
Figure 1-3A. - Block diagram of voltage and power amplifiers.
Figure 1-3B. - Block diagram of voltage and power amplifiers.
In view (B) of the figure the output signal voltage is less than the input signal voltage. As a voltage
amplifier, this circuit has a gain of less than 1. The output power, however, is greater than the input power.
Therefore, this circuit is a power amplifier.
The classification of an amplifier as a voltage or power
amplifier is made by comparing the characteristics of the input and output signals. If the output signal is larger
in voltage amplitude than the input signal, the amplifier is a voltage amplifier. If there is no voltage gain, but
the output power is greater than the input power, the amplifier is a power amplifier.
FREQUENCY RESPONSE OF AMPLIFIERS
In addition to being classified by function,
amplifiers are classified by frequency response. The frequency response of an amplifier refers to the band of
frequencies or frequency range that the amplifier was designed to amplify.
You may wonder why the
frequency response is important. Why doesn't an amplifier designed to amplify a signal of 1000 Hz work just as
well at 1000 MHz? The answer is that the components of the amplifier respond differently at different frequencies.
The amplifying device (electron tube, transistor, magnetic amplifier, etc.) itself will have frequency limitations
and respond in different ways as the frequency changes. Capacitors and inductors in the circuit will change their
reactance as the frequency changes. Even the slight amounts of capacitance and inductance between the circuit
wiring and other components (interelectrode capacitance and self-inductance) can become significant at high
frequencies. Since the response of components varies with the frequency, the components of an amplifier are
selected to amplify a certain range or band of frequencies.
Note: For explanations of interelectrode capacitance and self-inductance see NEETS Modules 2 - Introduction
to Alternating Current and Transformers; 6 - Introduction to Electronic Emission, Tubes, and Power Supplies; and
7 - Introduction to Solid-State Devices and Power Supplies.
The three broad categories of frequency response for
amplifiers are AUDIO AMPLIFIER, RF AMPLIFIER, and VIDEO AMPLIFIER.
An audio amplifier is designed to
amplify frequencies between 15 Hz and 20 kHz. Any amplifier that is designed for this entire band of frequencies
or any band of frequencies contained in the audio range is considered to be an audio amplifier.
term RF amplifier, the "RF" stands for radio frequency. These amplifiers are designed to amplify frequencies
between 10 kHz and 100,000 MHz. A single amplifier will not amplify the entire RF range, but any amplifier whose
frequency band is included in the RF range is considered an RF amplifier.
A video amplifier is an
amplifier designed to amplify a band of frequencies from 10 Hz to 6 MHz. Because this is such a wide band of
frequencies, these amplifiers are sometimes called WIDE-BAND AMPLIFIERS. While a video amplifier will amplify a
very wide band of frequencies, it does not have the gain of narrower-band amplifiers. It also requires a great
many more components than a narrow-band amplifier to enable it to amplify a wide range of frequencies.
Q-4. In what two ways are amplifiers classified?
Q-5. What type of amplifier would be used to drive the
speaker system of a record player?
Q-6. What type of amplifier would be used to amplify the signal from
a radio antenna?
A transistor amplifier is a current-control device. The current in the base of the transistor (which is
dependent on the emitter-base bias) controls the current in the collector. A vacuum-tube amplifier is also a
current-control device. The grid bias controls the plate current. These facts are expanded upon in NEETS Module 6,
Introduction to Electronic Emission, Tubes and Power Supplies, and Module 7, Introduction to Solid-State Devices
and Power Supplies.
You might hear that a vacuum tube is a voltage-operated device (since the grid does not need to draw
current) while the transistor is a current-operated device. You might agree with this statement, but both the
vacuum tube and the transistor are still current-control devices. The whole secret to understanding amplifiers is
to remember that fact. Current control is the name of the game. Once current is controlled you can use it to give
you a voltage gain or a power gain.
This chapter will use transistor amplifiers to present the concepts
and principles of amplifiers. These concepts apply to vacuum-tube amplifiers and, in most cases, magnetic
amplifiers as well as transistor amplifiers. If you wish to study the vacuum-tube equivalent circuits of the
transistor circuits presented, an excellent source is the EIMB, NAVSEA 0967-LP-000-0120, Electronics Circuits.
The first amplifier concept that is discussed is the "class of operation" of an amplifier.
AMPLIFIER CLASSES OF OPERATION
The class of operation of an amplifier is determined by the amount of time (in relation to the input signal)
that current flows in the output circuit. This is a function of the operating point of the amplifying device. The
operating point of the amplifying device is determined by the bias applied to the device. There are four classes
of operation for an amplifier. These are: A, AB, B and C. Each class of operation has certain uses and
characteristics. No one class of operation is "better" than any other class. The selection of the "best" class of
operation is determined by the use of the amplifying circuit. The best class of operation for a phonograph is not
the best class for a radio transmitter.
Class A Operation
A simple transistor
amplifier that is operated class A is shown in figure 1-4. Since the output signal is a 100% (or 360º) copy of the
input signal, current in the output circuit must flow for 100% of the input signal time. This is the definition of
a class A amplifier. Amplifier current flows for 100% of the input signal.
Figure 1-4. - A simple class A transistor amplifier.
The class A amplifier has the characteristics of good FIDELITY and low EFFICIENCY. Fidelity means that
the output signal is just like the input signal in all respects except amplitude. It has the same
shape and frequency. In some cases, there may be a phase difference between the input and output
signal (usually 180º), but the signals are still considered to be "good copies." If the output signal is not like
the input signal in shape or frequency, the signal is said to be DISTORTED. DISTORTION is any undesired change in
a signal from input to output.
The efficiency of an amplifier refers to the amount of power delivered to the
output compared to the power supplied to the circuit. Since every device takes power to operate, if the amplifier
operates for 360º of input signal, it uses more power than if it only operates for 180º of input signal. If the
amplifier uses more power, less power is available for the output signal and efficiency is lower. Since class A
amplifiers operate (have current flow) for 360º of input signal, they are low in efficiency. This low efficiency
is acceptable in class A amplifiers because they are used where efficiency is not as important as fidelity.
Class AB Operation
If the amplifying device is biased in such a way that current flows in the device for 51% - 99% of the
input signal, the amplifier is operating class AB. A simple class AB amplifier is shown in figure 1-5.
Figure 1-5. - A simple class AB transistor amplifier.
Notice that the output signal is distorted. The output signal no longer has the same shape as the input
signal. The portion of the output signal that appears to be cut off is caused by the lack of current through the
transistor. When the emitter becomes positive enough, the transistor cannot conduct because the base- to-emitter
junction is no longer forward biased. Any further increase in input signal will not cause an increase in output
Class AB amplifiers have better efficiency and poorer fidelity than class A amplifiers. They are used when
the output signal need not be a complete reproduction of the input signal, but both positive and negative portions
of the input signal must be available at the output.
Class AB amplifiers are usually defined as amplifiers
operating between class A and class B because class A amplifiers operate on 100% of input signal and class B
amplifiers (discussed next) operate on 50% of the input signal. Any amplifier operating between these two limits
is operating class AB.
Class B Operation
As was stated above, a class B amplifier operates for 50%
of the input signal. A simple class B
amplifier is shown in figure 1-6.
Figure 1-6. - A simple class B transistor amplifier.
In the circuit shown in figure 1-6, the base-emitter bias will not allow the transistor to conduct
whenever the input signal becomes positive. Therefore, only the negative portion of the input signal is reproduced
in the output signal. You may wonder why a class B amplifier would be used instead of a simple rectifier if only
half the input signal is desired in the output. The answer to this is that the rectifier does not amplify. The
output signal of a rectifier cannot be higher in amplitude than the input signal. The class B amplifier not only
reproduces half the input signal, but amplifies it as well.
Class B amplifiers are twice as efficient as
class A amplifiers since the amplifying device only conducts (and uses power) for half of the input signal. A
class B amplifier is used in cases where exactly 50% of the input signal must be amplified. If less than 50% of
the input signal is needed, a class C amplifier is used.
Class C Operation
1-7 shows a simple class C amplifier. Notice that only a small portion of the input signal is present in the
output signal. Since the transistor does not conduct except during a small portion of the input signal, this is
the most efficient amplifier. It also has the worst fidelity. The output signal bears very little resemblance to
the input signal.
Figure 1-7. - A simple class C transistor amplifier.
Class C amplifiers are used where the output signal need only be present during part of one-half of the input
signal. Any amplifier that operates on less than 50% of the input signal is operated class C.
determines the class of operation of an amplifier?
Q-8. What are the four classes of operation of a
Q-9. If the output of a circuit needs to be a complete representation of one-half
of the input signal, what class of operation is indicated?
Q-10. Why is class C operation more efficient
than class A operation?
Q-11. What class of operation has the highest fidelity?
Earlier in this module it was stated that almost every electronic
device contains at least one stage of amplification. Many devices contain several stages of amplification and
therefore several amplifiers. Stages of amplification are added when a single stage will not provide the required
amount of amplification. For example, if a single stage of amplification will provide a maximum gain of 100 and
the desired gain from the device is 1000, two stages of amplification will be required. The two stages might have
gains of 10 and 100, 20 and 50, or 25 and 40. (The overall gain is the product of the individual stages-10 x 100 =
20 x 50 = 25 x 40 = 1000.)
Figure 1-8 shows the effect of adding stages of amplification. As stages of
amplification are added, the signal increases and the final output (from the speaker) is increased.
Figure 1-8. - Adding stages of amplification.
Whether an amplifier is one of a series in a device or a single stage connected between two other devices (top
view, figure 1-8), there must be some way for the signal to enter and leave the amplifier. The process of
transferring energy between circuits is known as COUPLING. There are various ways of coupling signals into and out
of amplifier circuits. The following is a description of some of the more common methods of amplifier coupling.
The method of coupling that uses the least number of circuit elements and
that is, perhaps, the easiest to understand is direct coupling. In direct coupling the output of one stage is
connected directly to the input of the following stage. Figure 1-9 shows two direct-coupled transistor amplifiers.
NEETS Table of Contents
- Introduction to Matter, Energy, and Direct
- Introduction to Alternating Current and Transformers
- Introduction to Circuit Protection, Control,
- 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
- Introduction to Amplifiers
- Introduction to Wave-Generation and Wave-Shaping
- 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