Module 8—Introduction to AmplifiersPages i  ix,
11 to 110, 111 to 120,
121 to 130, 131 to 140,
21 to 210, 211 to 220,
221 to 230, 231 to 235,
31 to 310, 311 to 320,
321 to 330, 331 to 340,
341 to 350, 351 to 360,
361 to 370, AI1 to AI3,
Index
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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Introduction to Matter, Energy, and Direct Current, Introduction
to Alternating Current and Transformers, Introduction to Circuit Protection,
Control, and Measurement, Introduction to Electrical Conductors, Wiring Techniques,
and Schematic Reading, Introduction to Generators and Motors,
Introduction to Electronic Emission, Tubes, and Power Supplies,
Introduction to SolidState Devices and Power Supplies,
Introduction to Amplifiers, Introduction to
WaveGeneration and WaveShaping Circuits, Introduction to Wave Propagation, Transmission
Lines, and Antennas, 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
