Module 8  Introduction to Amplifiers
Pages i,
11,
111,
121,
131,
21,
211,
221,
231,
31,
311,
321,
331,
341,
351,
361,
AI1,
Index
 
Matter, Energy,
and Direct Current 
 
Alternating Current and Transformers 
 
Circuit Protection, Control, and Measurement 
 
Electrical Conductors, Wiring Techniques,
and Schematic Reading 
 
Generators and Motors 
 
Electronic Emission, Tubes, and Power Supplies 
 
SolidState Devices and Power Supplies 
 
Amplifiers 
 
WaveGeneration and WaveShaping Circuits 
 
Wave Propagation, Transmission Lines, and
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Microwave Principles 
 
Modulation Principles 
 
Introduction to Number Systems and Logic Circuits 
 
 Introduction to Microelectronics 
 
Principles of Synchros, Servos, and Gyros 
 
Introduction to Test Equipment 
 
RadioFrequency Communications Principles 
 
Radar Principles 
 
The Technician's Handbook, Master Glossary 
 
Test Methods and Practices 
 
Introduction to Digital Computers 
 
Magnetic Recording 
 
Introduction to Fiber Optics 
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An adder circuit is not restricted to two inputs.
By adding resistors in parallel to the input terminals, any number of inputs can
be used. The adder circuit will always produce an output that is equal to the sum
of the input signals but opposite in polarity. Figure 322 shows a fiveinput adder
circuit with voltages and currents indicated.
Figure 321.  Current and voltage in a twoinput adder.
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Figure 322.  Fiveinput adder.
The previous circuits have been adders, but there are other types of summing
amplifiers. a summing amplifier can be designed to amplify the results of adding
the input signals. This type of circuit actually multiplies the sum of the inputs
by the gain of the circuit.
Mathematically (for a threeinput circuit):
If the circuit gain is 10:
The gain of the circuit is determined by the ratio between the feedback resistor
and the input resistors. To change figure 320 to a summing amplifier with a gain
of 10, you would replace the feedback resistor (R3) with a 10kilohm resistor.
This new circuit is shown in figure 323.
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Figure 323.  Summing amplifier.
If this circuit is designed correctly and the input voltages (E1 and E2) are
+2 volts and +3 volts, respectively, the output voltage (Eout) should be:
To see if this output (50 V) is what the circuit will produce with the inputs
given above, start by calculating the currents through the input resistors, R1 and
R2 (remember that point a is at virtual ground):
Next, calculate the current through the feedback resistor (R3):
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(The minus sign indicates current flow from left to right.)
Finally, calculate the voltage dropped across R3 (which must equal the output
voltage):
As you can see, this circuit performs the function of adding the inputs together
and multiplying the result by the gain of the circuit.
One final type of summing amplifier is the SCALING Amplifier. This circuit multiplies
each input by a factor (the factor is determined by circuit design) and then adds
these values together. The factor that is used to multiply each input is determined
by the ratio of the feedback resistor to the input resistor. For example, you could
design a circuit that would produce the following output from three inputs (E1,
E2, E3):
Using input resistors R1 for input number one (E1), R2 for input number two (E2),
R3 for input number three (E3), and R4 for the feedback resistor, you could calculate
the values for the resistors:
Any resistors that will provide the ratios shown above could be used. If the
feedback resistor (R4) is a 12kilohm resistor, the values of the other resistors
would be:
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Figure 324 is the schematic diagram of a scaling amplifier with the values calculated
above.
Figure 324.  Scaling amplifier.
To see if the circuit will produce the desired output, calculate the currents
and voltages as done for the previous circuits.
With:
the output should be:
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Calculate the current for each input:
Calculate the output voltage:
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You have now seen how an operational amplifier can be used in a circuit as an
adder, a summing amplifier, and a scaling amplifier.
Difference Amplifier (Subtractor)
A difference amplifier will produce an output based on the difference between
the input signals. The subtractor circuit shown in figure 325 will produce the
following output:
Figure 325.  Subtractor circuit.
Normally, difference amplifier circuits have the ratio of the inverting input
resistor to the feedback resistor equal to the ratio of the noninverting input resistors.
In other words, for figure 325:
and, by inverting both sides:
For ease of explanation, in the circuit shown in figure 325 all the resistors
have a value of 1 kilohm, but any value could be used as long as the above ratio
is true. For a subtractor circuit, the values of R1 and R3 must also be equal, and
therefore, the values of R2 and R4 must be equal. It is NOT necessary that the value
of R1 equal the value of R2.
Using figure 325, assume that the input signals are:
The output signal should be:
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Eout = E2  E1
Eout = (+12V)  (+3V)
Eout = +9V
To check this output, first compute the value of R2 plus R4:
With this value, compute the current through R2 (IR2):
(indicating current flow from left to right)
Next, compute the voltage drop across R2 (ER2):
Then compute the voltage at point B:
Since point B and point a will be at the same potential in an operational amplifier:
Now compute the voltage developed by R1 (ER1):
Compute the current through R1 (IR1):
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Compute the voltage developed by R3 (ER3):
Add this to the voltage at point a to compute the output voltage (Eout):
As you can see, the circuit shown in figure 325 functions as a subtractor. But
just as an adder is only one kind of summing amplifier, a subtractor is only one
kind of difference amplifier. a difference amplifier can amplify the difference
between two signals. For example, with two inputs (E1 and E2) and a gain of five,
a difference amplifier will produce an output signal which is:
The difference amplifier that will produce that output is shown in figure 326.
Notice that this circuit is the same as the subtractor shown in figure 325 except
for the values of R3 and R4. The gain of this difference amplifier is:
Figure 326.  Difference amplifier.
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Then, for a difference amplifier:
With the same inputs that were used for the subtractor, (E1 = + 3 V; E2 = + 12
V) the output of the difference amplifier should be five times the output of the
subtractor (Eout = + 45 V).
Following the same steps used for the subtractor: First compute the value of
R2 plus R4:
With this value, compute the current through R2 (IR2):
Next, compute the voltage drop across R2 (ER2):
Then, compute the voltage at point B:
Since point a and point B will be at the same potential in an operational amplifier:
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