Module 8 − Introduction to Amplifiers
Upon completion of this chapter, you will be able to:
1. Describe the basic operation of a differential amplifier.
2. Describe the operation of a differential amplifier under the following
a. Single Input, Single Output
b. Single input, differential output
c. Differential input, differential output
3. List the characteristics of an operational amplifier.
4. Identify the symbol for an operational amplifier.
5. Label the blocks on a block diagram of an operational amplifier.
6. Describe the operation of an operational amplifier with inverting
and noninverting configurations.
7. Describe the bandwidth of a typical operational amplifier and
methods to modify the bandwidth.
8. Identify the following applications of operational amplifiers:
9. State the common usage for a magnetic amplifier.
10. Describe the basic operation of a magnetic amplifier.
11. Describe various methods of changing inductance.
12. Identify the purpose of components in a simple magnetic amplifier.
If you were to make a quick review of the subjects discussed in this
module up to this point, you would see that you have been given a considerable amount
of information about amplifiers. You have been shown what amplification is and how
the different classes of amplifiers affect amplification. You also have been shown
that many factors must be considered when working with amplifiers, such as
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 electronics.
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
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
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
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
with the differential amplifier: It operates on the difference between two inputs.
However, the difference amplifier has only one output while the differential amplifier
can have two outputs.
By now, you should be familiar with some amplifier circuits, which should give
you an idea of what a difference amplifier is like. In NEETS, Module 7, you were
shown the basic configurations for transistor amplifiers. Figure 3-1 shows two of
these configurations: the common emitter and the common base.
In view (A) of figure 3-1 a common-emitter amplifier is shown. The output signal
is an amplified version of the input signal and is 180 degrees out of phase with
the input signal. View (B) is a common- base amplifier. In this circuit the output
signal is an amplified version of the input signal and is in phase with the input
signal. In both of these circuits, the output signal is controlled by the base-to-emitter
bias. As this bias changes (because of the input signal) the current through the
transistor changes. This causes the output signal developed across the collector
load (R2) to change. None of this information is new, it is just a review of what
you have already been shown regarding transistor amplifiers.
Figure 3-1A. - Common-emitter and common-base amplifiers.
Figure 3-1B. - Common-emitter and common-base amplifiers.
Note: Bias arrangements for the following explanations will be termed base-to-emitter.
In other publications you will see the term emitter-to-base used to describe the
same bias arrangement.
The TWO-Input, Single-Output, Difference Amplifier
If you combine the common-base and common-emitter configurations into a single
transistor amplifier, you will have a circuit like the one shown in figure 3-2.
This circuit is the two-input, single-output, difference amplifier.
Figure 3-2. - Two-input, single-output, difference amplifier.
In figure 3-2, the transistor has two inputs (the emitter and the
base) and one output (the collector). Remember, the current through the transistor
(and therefore the output signal) is controlled by the base-to-emitter bias. In
the circuit shown in figure 3-2, the combination of the two input signals controls
the output signal. In fact, the Difference BETWEEN The Input SignalS determines
the base-to-emitter bias.
For the purpose of examining the operation of the circuit shown in figure 3-2,
assume that the circuit has a gain of -10. This means that for each 1-volt change
in the base-to-emitter bias, there would be a 10-volt change in the output signal.
Assume, also, that the input signals will peak at 1-volt levels (+1 volt for the
positive peak and -1 volt for the negative peak). The secret to understanding this
circuit (or any transistor amplifier circuit) is to realize that the collector current
is controlled by the base-to-emitter bias. In other words, in this circuit the output
signal (the voltage developed across R3) is determined by the difference between
the voltage on the base and the voltage on the emitter.
Figure 3-3 shows this two-input, single-output amplifier with input signals that
are equal in amplitude and 180 degrees out of phase. Input number one has a positive
alternation when input number two has a negative alternation and vice versa.
Figure 3-3. - Input signals 180º out of phase.
The circuit and the input and output signals are shown at the top
of the figure. The lower portion of the figure is a comparison of the input signals
and the output signal. Notice the vertical lines marked "T0" through "T8." These
represent "time zero" through "time eight." In other words, these lines provide
a way to examine the two input signals and the output signal at various instants
In figure 3-3 at time zero (T0) both input signals are at 0 volts. The output
signal is also at 0 volts. Between time zero (T0) and time one (T1), input signal
number one goes positive and input signal number two goes negative. Each of these
voltage changes causes an increase in the base-to-emitter bias which causes current
through Q1 to increase. Increased current through Q1 results in a greater voltage
drop across the collector load (R3) which causes the output signal to go negative.
By time one (T1), input signal number one has reached +1 volt and input signal
number two has reached -1 volt. This is an overall increase in base-to-emitter bias
of 2 volts. Since the gain of the circuit is -10, the output signal has decreased
by 20 volts. As you can see, the output signal has been determined by the difference
between the two input signals. In fact, the base-to-emitter bias can be found by
subtracting the value of input signal number two from the value of input signal
Between time one (T1) and time two (T2), input signal number one goes
from +1 volt to 0 volts and input signal number two goes from -1 volt to 0 volts.
At time two (T2) both input signals are at 0 volts and the base-to-emitter bias
has returned to 0 volts. The output signal is also 0 volts.
Between time two (T2) and time three (T3), input signal number one
goes negative and input signal number two goes positive. At time three (T3), the
value of the base-to-emitter bias is -2 volts.
This causes the output signal to be +20 volts at time three (T3).
Between time three (T3) and time four (T4), input signal #1 goes from -1 volt
to 0 volts and input signal #2 goes from +1 volt to 0 volts. At time four (T4) both
input signals are 0 volts, the bias is 0 volts, and the output is 0 volts.
During time four (T4) through time eight (T8), the circuit repeats the sequence
of events that took place from time zero (T0) through time four (T4).
You can see that when the input signals are equal in amplitude and 180 degrees
out of phase, the output signal is twice as large (40 volts peak to peak) as it
would be from either input signal alone (if the other input signal were held at
Figure 3-4 shows the two-input, single-output, difference amplifier with two
input signals that are equal in amplitude and in phase.
Figure 3-4. - Input signals in phase.
Notice, that the output signal remains at 0 volts for the entire time (T0 - T8).
Since the two input signals are equal in amplitude and in phase, the difference
between them (the base-to-emitter bias) is always 0 volts. This causes a 0-volt
If you compute the bias at any time period (T0 - T8), you will see that the output
of the circuit remains at a constant zero.
From the above example, you can see that when the input signals are
equal in amplitude and in phase, there is no output from the difference amplifier
because there is no difference between the two inputs. You also know that when the
input signals are equal in amplitude but 180 degrees out of phase, the output looks
just like the input except for amplitude and a 180-degree phase reversal with respect
to input signal number one. What happens if the input signals are equal in amplitude
but different in phase by something other than 180 degrees? This would mean that
sometimes one signal would be going negative while the other would be going positive;
sometimes both signals would be going positive; and sometimes both signals would
be going negative. Would the output signal still look like the input signals? The
answer is "no," because figure 3-5 shows a difference amplifier with two input signals
that are equal in amplitude but 90 degrees out of phase. From the figure you can
see that at time zero (T0) input number one is at 0 volts and input number two is
at -1 volt. The base-to-emitter bias is found to be +1 volt.
Figure 3-5. - Input signals 90º out of phase.
This +1-volt bias signal causes the output signal to be -10 volts
at time zero (T0). Between time zero (T0) and time one (T1), both input signals
go positive. The difference between the input signals stays constant. The effect
of this is to keep the bias at +1 volt for the entire time between T0 and T1. This,
in turn, keeps the output signal at -10 volts.
Between time one (T1) and time two (T2), input signal number one goes in a negative
direction but input signal number two continues to go positive. Now the difference
between the input signals decreases
rapidly from +1 volt. Halfway between T1 and T2 (the dotted vertical line), input
signal number one and input signal number two are equal in amplitude. The difference
between the input signals is 0 volts and this causes the output signal to be 0 volts.
From this point to T2 the difference between the input signals is a negative value.
From time two (T2) to time three (T3), input signal number one goes
negative and input signal number two goes to zero. The difference between them stays
constant at -1 volt. Therefore, the output signal stays at a +10-volt level for
the entire time period from T2 to T3. At T3 the bias condition will be:
Between T3 and T4 input signal number one goes to zero while input
signal number two goes negative. This, again, causes a rapid change in the difference
between the input signals. Halfway between T3 and T4 (the dotted vertical line)
the two input signals are equal in amplitude; therefore, the difference between
the input signals is 0 volts, and the output signal becomes 0 volts. From that point
to T4, the difference between the input signals becomes a positive voltage. At T4:
(The sequence of events from T4 to T8 are the same as those of T0
As you have seen, this amplifier amplifies the difference between two input signals.
But this is NOT a differential amplifier. a differential amplifier has two inputs
and two outputs. The circuit you have just been shown has only one output. Well
then, how does a differential amplifier schematic look?
Typical Differential Amplifier Circuit
Figure 3-6 is the schematic diagram of a typical differential amplifier.
Notice that there are two inputs and two outputs. This circuit requires two transistors
to provide the two inputs and two outputs. If you look at one input and the transistor
with which it is associated, you will see that each transistor is a common-emitter
amplifier for that input (input one and Q1; input two and Q2). R1 develops the signal
at input one for Q1, and R5 develops the signal at input two for Q2. R3 is the emitter
resistor for both Q1 and Q2. Notice that R3 is NOT bypassed. This means that when
a signal at input one affects the current through Q1, that signal is developed by
R3. (The current through Q1 must flow through R3; as this current changes, the voltage
developed across R3 changes.) When a signal is developed by R3, it is applied to
the emitter of Q2. In the same way, signals at input two affect the current of Q2,
are developed by R3, and are felt on the emitter of Q1. R2 develops the signal for
output one, and R4 develops the signal for output two.
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