E1

IR1 =

R1

+ 3V

IR1 =

1k Ω

IR1 = + 3 mA

NOTE: The + sign indicates a current flow from right to left.

7-84. Using the same type of calculation (with +4 volts at E2 and 0 volts at point A) the

current through R2 must be 4 milliamps. This means that a total of 7 milliamps is flowing

from point A through R1 and R2. If 7 milliamps is flowing from point A, then 7 milliamps

must be flowing into point A. The 7 milliamps flowing into point A flows through R3

causing 7 volts to be developed across R3. With point A at 0 volts and 7 volts developed

across R3, the voltage potential at EOUT must be -7 volts. Figure 7-20 shows these voltages

and currents.

7-85. 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 7-21 shows a five input adder circuit with voltages and currents indicated.

7-86. Besides adders, 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. You can compute (for a

three-input circuit) as follows:

EOUT = gain (E1 + E2 + E3)

If the circuit gain is -10:

EOUT = -10 (E1 + E2 + E3)

7-87. The gain of the circuit is determined by the ratio between the feedback resistor and

the input resistors. To change Figure 7-19 to a summing amplifier with a gain of -10, you

would replace the feedback resistor (R3) with a 10-kilohm resistor. Figure 7-22 shows this

new circuit. 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:

EOUT = gain (E1 + E2)

EOUT = 10 ([+2 V] + [+3 V])

EOUT = 10 (+5 V)

EOUT = -50 V

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