3-32
Figure 3-22.—Five-input 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 three-input 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 3-20 to a summing amplifier with a gain of -10, you would replace the
feedback resistor (R3) with a 10-kilohm resistor. This new circuit is shown in figure 3-23.
