Module 8  Introduction to Amplifiers
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11 to 110,
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311 to 320,
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331 to 340,
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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. 331
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. 332
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): 333
(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: 334
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: 335
Calculate the current for each input: Calculate the output voltage: 336
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: 337
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): 338
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. 339
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: 340
NEETS Table of Contents
 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
