As in chapter 6, 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. Remember that the text is intended to teach certain
facts about amplifiers, and in order to simplify the illustrations used, complete operational
circuits are not always shown.
This chapter discusses the following three types of special 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.
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.
Differential amplifiers can 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.
produce amplification of an electronic signal. The magnetic amplifier uses a device called a
"saturable-core reactor" to control an AC 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 rest of this
chapter will provide you with more detailed information about 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 with the differential amplifier and that is that it operates on the
difference between two inputs. However, the difference amplifier has only one output
while the differential amplifier can have two outputs.
7-10. You should be familiar with some amplifier circuits, which should give you an
idea of what a difference amplifier is like. Figure 7-1 shows two of the basic configurations
for transistor amplifiers (the CE and the CB).
23 June 2005