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
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3-41 to 3-50
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3-61 to 3-70
, AI-1 to AI-3
Figure 3-6.—Differential amplifier.
Even though this circuit is designed to have two inputs and two outputs, it is not necessary to use both
inputs and both outputs. (Remember, a differential amplifier was defined as having two possible inputs and two
possible outputs.) A differential amplifier can be connected as a single-input, single-output device; a
single-input, differential-output device; or a differential-input, differential-output device.
many inputs and outputs are possible with a differential amplifier?
Q-2. What two transistor amplifier
configurations are combined in the single-transistor, two-input, single-output difference amplifier?
Q-3. If the two input signals of a difference amplifier are in phase and equal in amplitude, what will the
output signal be?
Q-4. If the two input signals to a difference amplifier are equal in amplitude and 180
degrees out of phase, what will the output signal be?
Q-5. If only one input signal is used with a
difference amplifier, what will the output signal be?
Q-6. If the two input signals to a difference
amplifier are equal in amplitude but neither in phase nor 180 degrees out of phase, what will the output signal
SINGLE-INPUT, SINGLE-OUTPUT, DIFFERENTIAL AMPLIFIER
Figure 3-7 shows a differential amplifier with one input (the base of Q1) and one output (the collector of Q2).
The second input (the base of Q2) is grounded and the second output (the collector of Q1) is not used.
Figure 3-7.—Single-input, single-output differential amplifier.
When the input signal developed by R1 goes positive, the current through Q1 increases. This increased
current causes a positive-going signal at the top of R3. This signal is felt on the emitter of Q2. Since the base
of Q2 is grounded, the current through Q2 decreases with a positive-going signal on the emitter. This decreased
current causes less voltage drop across R4. Therefore, the voltage at the bottom of R4 increases and a
positive-going signal is felt at the output.
When the input signal developed by R1 goes negative, the
current through Q1 decreases. This decreased current causes a negative-going signal at the top of R3. This signal
is felt on the emitter of Q2. When the emitter of Q2 goes negative, the current through Q2 increases. This
increased current causes more of a voltage drop across R4. Therefore, the voltage at the bottom of R4 decreases
and a negative- going signal is felt at the output.
This single-input, single-output, differential amplifier is very similar to a single-transistor amplifier as far
as input and output signals are concerned. This use of a differential amplifier does provide amplification of a.c.
or d.c. signals but does not take full advantage of the characteristics of a differential amplifier.
SINGLE-INPUT, DIFFERENTIAL-OUTPUT, DIFFERENTIAL AMPLIFIER
In chapter one of this module
you were shown several phase splitters. You should remember that a phase splitter provides two outputs from a
single input. These two outputs are 180 degrees out of phase with each other. The single-input,
differential-output, differential amplifier will do the same thing.
Figure 3-8 shows a differential
amplifier with one input (the base of Q1) and two outputs (the collectors of Q1 and Q2). One output is in phase
with the input signal, and the other output is 180 degrees out of phase with the input signal. The outputs are
Figure 3-8.—Single-input, differential-output differential amplifier.
This circuit’s operation is the same as for the single-input, single-output differential amplifier just
described. However, another output is obtained from the bottom of R2. As the input signal goes positive, thus
causing increased current through Q1, R2 has a greater voltage drop. The output signal at the bottom of R2
therefore is negative going. A negative-going input signal will decrease current and reverse the polarities of
both output signals.
Now you see how a differential amplifier can produce two amplified, differential output signals from a
single-input signal. One further point of interest about this configuration is that if a combined output signal is
taken between outputs number one and two, this single output will be twice the amplitude of the individual
outputs. In other words, you can double the gain of the differential amplifier (single output) by taking the
output signal between the two output terminals. This single-output signal will be in phase with the input signal.
This is shown by the phantom signal above R5 (the phantom resistor connected between outputs number one and two
would be used to develop this signal).
DIFFERENTIAL-INPUT, DIFFERENTIAL-OUTPUT, DIFFERENTIAL AMPLIFIER
When a differential
amplifier is connected with a differential input and a differential output, the full potential of the circuit is
used. Figure 3-9 shows a differential amplifier with this type of configuration (differential-input,
Figure 3-9.—Differential-input, differential-output differential amplifier.
Normally, this configuration uses two input signals that are 180 degrees out of phase. This causes the
difference (differential) signal to be twice as large as either input alone. (This is just like the two-input,
single-output difference amplifier with input signals that are 180 degrees out of phase.)
one is a signal that is in phase with input number two, and output number two is a signal that is in phase with
input number one. The amplitude of each output signal is the input signal multiplied by the gain of the amplifier.
With 180-degree-out-of-phase input signals, each output signal is greater in amplitude than either input signal by
a factor of the gain of the amplifier.
When an output signal is taken between the two output terminals of the amplifier (as shown by the phantom
connections, resistor, and signal), the combined output signal is twice as great in amplitude as either signal at
output number one or output number two. (This is because output number one and output number two are 180 degrees
out of phase with each other.) When the input signals are 180 degrees out of phase, the amplitude of the combined
output signal is equal to the amplitude of one input signal multiplied by two times the gain of the amplifier.
When the input signals are not 180 degrees out of phase, the combined output signal taken across output one
and output two is similar to the output that you were shown for the two-input, single-output, difference
amplifier. The differential amplifier can have two outputs (180 degrees out of phase with each other), or the
outputs can be combined as shown in figure 3-9.
In answering Q7 through Q9 use the following information:
All input signals are sine waves with a peak-to-peak amplitude of 10 millivolts. The gain of the differential
amplifier is 10.
Q-7. If the differential amplifier is configured with a single input and a single
output, what will the peak-to-peak amplitude of the output signal be?
Q-8. If the differential amplifier is configured with a single input and differential outputs, what
will the output signals be?
Q-9. If the single-input, differential-output, differential amplifier has an
output signal taken between the two output terminals, what will the peak-to-peak amplitude of this combined output
In answering Q10 through Q14 use the following information: A differential amplifier is configured
with a differential input and a differential output. All input signals are sine waves with a peak-to-peak
amplitude of 10 millivolts. The gain of the differential amplifier is 10.
Q-10. If the input signals are
in phase, what will be the peak-to-peak amplitude of the output signals?
Q-11. If the input signals are
180 degrees out of phase with each other, what will be the peak-to-peak amplitude of the output signals?
Q-12. If the input signals are 180 degrees out of phase with each other, what will the phase relationship be
between (a) the output signals and (b) the input and output signals?
Q-13. If the input signals are 180 degrees out of phase with each other and a combined output is taken between
the two output terminals, what will the amplitude of the combined output signal be?
Q-14. If the input
signals are 90 degrees out of phase with each other and a combined output is taken between the two output
terminals, (a) what will the peak-to-peak amplitude of the combined output signal be, and (b) will the combined
output signal be a sine wave?
An OPERATIONAL AMPLIFIER (OP AMP) is an amplifier which is designed to be used with other circuit
components to perform either computing functions (addition, subtraction) or some type of transfer operation, such
as filtering. Operational amplifiers are usually high-gain amplifiers with the amount of gain determined by
Operational amplifiers have been in use for some time. They were originally developed for analog (non-digital)
computers and used to perform mathematical functions. Operational amplifiers were not used in other devices very
much because they were expensive and more complicated than other circuits.
Today many devices use
operational amplifiers. Operational amplifiers are used as d.c. amplifiers, a.c. amplifiers, comparators,
oscillators (which are covered in NEETS, Module 9), filter circuits, and many other applications. The reason for
this widespread use of the operational amplifier is that it is a very versatile and efficient device. As an
integrated circuit (chip) the operational amplifier has become an inexpensive and readily available "building
block" for many devices. In fact, an operational amplifier in integrated circuit form is no more expensive than a
CHARACTERISTICS OF AN OPERATIONAL AMPLIFIER
symbols for an operational amplifier are shown in figure 3-10. View (A) shows the power supply requirements while
view (B) shows only the input and output terminals. An operational amplifier is a special type of high-gain, d.c.
amplifier. To be classified as an operational amplifier, the circuit must have certain characteristics. The three
most important characteristics of an operational amplifier are:
1. Very high gain
2. Very high input impedance
3. Very low output impedance
Figure 3-10A.—Schematic symbols of an operational amplifier.
Figure 3-10B.—Schematic symbols of an operational amplifier.
Since no single amplifier stage can provide all these characteristics well enough to be considered
an operational amplifier, various amplifier stages are connected together. The total circuit made up of these
individual stages is called an operational amplifier. This circuit (the operational amplifier) can be made up of
individual components (transistors, resistors, capacitors, etc.), but the most common form of the operational
amplifier is an integrated circuit. The integrated circuit (chip) will contain the various stages
operational amplifier and can be treated and used as if it were a single stage.
BLOCK DIAGRAM OF
AN OPERATIONAL AMPLIFIER
Figure 3-11 is a block diagram of an operational amplifier. Notice that there are three stages within the
Figure 3-11.—Block diagram of an operational amplifier.
The input stage is a differential amplifier. The differential amplifier used as an input stage provides
differential inputs and a frequency response down to d.c. Special techniques are used to provide the high input
impedance necessary for the operational amplifier.
The second stage is a high-gain voltage amplifier. This
stage may be made from several transistors to provide high gain. A typical operational amplifier could have a
voltage gain of 200,000. Most of this gain comes from the voltage amplifier stage.
The final stage of the
OP AMP is an output amplifier. The output amplifier provides low output impedance. The actual circuit used could
be an emitter follower. The output stage should allow the operational amplifier to deliver several milliamperes to
Notice that the operational amplifier has a positive power supply (+VCC) and a negative power supply (-V EE).
This arrangement enables the operational amplifier to produce either a positive or a negative output.
The two input terminals are labeled "inverting input" (-) and "noninverting input" (+). The operational amplifier
can be used with three different input conditions (modes). With differential inputs (first mode), both input
terminals are used and two input signals which are 180 degrees out of phase with each other are used. This
produces an output signal that is in phase with the signal on the noninverting input. If the noninverting input is
grounded and a signal is applied to the inverting input (second mode), the output signal will be 180 degrees out
of phase with the input signal (and one-half the amplitude of the first mode output). If the inverting input is
grounded and a signal is applied to the noninverting input (third mode), the output signal will be in phase with
the input signal (and one-half the amplitude of the first mode output).
Q-15. What are the three
requirements for an operational amplifier?
Q-16. What is the most commonly used form of the operational amplifier?
Q-17. Draw the
schematic symbol for an operational amplifier.
Q-18. Label the parts of the operational amplifier shown
in figure 3-12.
Figure 3-12.—Operational amplifier.
CLOSED-LOOP OPERATION OF AN OPERATIONAL AMPLIFIER
Operational amplifiers can
have either a closed-loop operation or an open-loop operation. The operation (closed-loop or open-loop) is
determined by whether or not feedback is used. Without feedback the operational amplifier has an open-loop
operation. This open-loop operation is practical only when the operational amplifier is used as a comparator (a
circuit which compares two input signals or compares an input signal to some fixed level of voltage). As an
amplifier, the open-loop operation is not practical because the very high gain of the operational amplifier
creates poor stability. (Noise and other unwanted signals are amplified so much in open-loop operation that the
operational amplifier is usually not used in this way.) Therefore, most operational amplifiers are used with
feedback (closed-loop operation).
Operational amplifiers are used with degenerative (or negative) feedback
which reduces the gain of the operational amplifier but greatly increases the stability of the circuit. In the
closed-loop configuration, the output signal is applied back to one of the input terminals. This feedback is
always degenerative (negative). In other words, the feedback signal always opposes the effects of the original
input signal. One result of degenerative feedback is that the inverting and noninverting inputs to the operational
amplifier will be kept at the same potential.
Closed-loop circuits can be of the inverting configuration
or noninverting configuration. Since the inverting configuration is used more often than the noninverting
configuration, the inverting configuration will be shown first.
Figure 3-13 shows an operational amplifier in a closed-loop, inverting configuration. Resistor R2 is used to feed
part of the output signal back to the input of the operational amplifier.
Figure 3-13.—Inverting configuration.
At this point it is important to keep in mind the difference between the entire circuit (or operational
circuit) and the operational amplifier. The operational amplifier is represented by the triangle-like symbol while
the operational circuit includes the resistors and any other components as well as the operational amplifier. In
other words, the input to the circuit is shown in figure 3-13, but the signal at the inverting input of the
operational amplifier is determined by the feedback signal as well as by the circuit input signal.
can see in figure 3-13, the output signal is 180 degrees out of phase with the input signal. The feedback signal
is a portion of the output signal and, therefore, also 180 degrees out of phase with the
input signal. Whenever
the input signal goes positive, the output signal and the feedback signal go negative. The result of this is that
the inverting input to the operational amplifier is always very close to 0 volts with this configuration. In fact,
with the noninverting input grounded, the voltage at the inverting input to the operational amplifier is so small
compared to other voltages in the circuit that it is considered to be VIRTUAL GROUND. (Remember, in a closed-loop
operation the inverting and noninverting inputs are at the same potential.)
Virtual ground is a point in a
circuit which is at ground potential (0 volts) but is NOT connected to ground. Figure 3-14, (view A) (view B) and
(view C), shows an example of several circuits with points at virtual ground.
Figure 3-14A.—Virtual ground circuits.
Figure 3-14B.—Virtual ground circuits.
Figure 3-14C.—Virtual ground circuits.
In view (A), V1 (the left-hand battery) supplies +10 volts to the circuit while V2 (the right-hand
battery) supplies -10 volts to the circuit. The total difference in potential in the circuit is 20 volts.
The total resistance of the circuit can be calculated:
Now that the total resistance is known, the circuit current can be calculated:
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Introduction to Amplifiers, Introduction to
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