3-34
(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 12-kilohm resistor, the values of the other resistors would be:
