impedance, feedback, frequency response, and coupling. With all this information behind you, you might
ask yourself "what more can there be to know about amplifiers?"
There is a great deal more to learn about amplifiers. Even after you finish this chapter you will have
only "scratched the surface" of the study of amplifiers. But, you will have prepared yourself for the
remainder of the NEETS. This, in turn, should prepare you for further study and, perhaps, a career in
As in chapter 2, the circuits shown in this chapter are intended to present particular concepts to you.
Therefore, the circuits may be incomplete or not practical for use in an actual piece of electronic
equipment. You should keep in mind the fact that this text is intended to teach certain facts about
amplifiers, and in order to simplify the illustrations used, complete operational circuits are not always
In this chapter three types of special amplifiers are discussed. These are: DIFFERENTIAL
AMPLIFIERS, OPERATIONAL AMPLIFIERS, and MAGNETIC AMPLIFIERS. These are called
special amplifiers because they are used only in certain types of equipment.
The names of each of these special amplifiers describe the operation of the amplifier, NOT what is
amplified. For example, a magnetic amplifier does not amplify magnetism but uses magnetic effects to
produce amplification of an electronic signal.
A differential amplifier is an amplifier that can have two input signals and/or two output signals.
This amplifier can amplify the difference between two input signals. A differential amplifier will also
"cancel out" common signals at the two inputs.
One of the more interesting aspects of an operational amplifier is that it can be used to perform
mathematical operations electronically. Properly connected, an operational amplifier can add, subtract,
multiply, divide, and even perform the calculus operations of integration and differentiation. These
amplifiers were originally used in a type of computer known as the "analog computer" but are now used
in many electronic applications.
The magnetic amplifier uses a device called a "saturable core reactor" to control an a.c.output signal.
The primary use of magnetic amplifiers is in power control systems.
These brief descriptions of the three special amplifiers are intended to provide you with a general
idea of what these amplifiers are and how they can be used. The remaining sections of this chapter will
provide you with more detailed information on these special amplifiers.
A differential amplifier has two possible inputs and two possible outputs. This arrangement means
that the differential amplifier can be used in a variety of ways. Before examining the three basic
configurations that are possible with a differential amplifier, you need to be familiar with the basic
circuitry of a differential amplifier.
BASIC DIFFERENTIAL AMPLIFIER CIRCUIT
Before you are shown the operation of a differential amplifier, you will be shown how a simpler
circuit works. This simpler circuit, known as the DIFFERENCE AMPLIFIER, has one thing in common