A summing amplifier can be used to perform addition or subtraction of signals. A summing amplifier can also add amplification/gain to the signals. The summing amplifier combines the input voltages V1, V2 and V3. If V2 is a negative voltage, it will be subtracted from V1 and V3 but if it is positive it will be added to V1 and V3. It should, however, be noted that the final signal is inverted (negative). Additional input branches can be added to the summing amplifier so that more than 3 input voltages are summed. This following can be used to describe a summing amplifier:

It can be seen from the equation that each input voltage (V1, V2 and V3) is amplified by a factor that is the ratio between R_{f} and the resistor next to the input voltage. The signals are then added to each other and lastly inverted, which is represented by the negative sign in the beginning of the equation.

If all the resistors in the circuit are 10kΩ and V1 is 2 V, V2 is 5 V and V3 is 3 V then the output voltage of the summing amplifier will be -10 V.