An op amp can be used to add or subtract two or more voltages. An electronic circuit that adds voltages is called a summing amplifier. A summing amplifier has two inputs and an output whose voltage is the sum of the two input voltages but with the opposite polarity. If one of the inputs is +1.5 V and the other is +1.0 V, for example, the output voltage will be –2.5 V.

For the summing amplifier to work, resistors R1, R2, and R3 should all be the same value.

If all the resistors in a summing amplifier are the same, the output voltage will be the sum of the input voltages. This is the usual way to configure a summing amplifier, though you can vary the resistor values if you want.

If the resistors have different values, each of the input voltages is weighted according to the value of the resistor on its input circuit, which has the effect of multiplying each input voltage by a certain value before the voltages are summed. The exact value by which each input is multiplied depends on the mix of resistors you use.

If R1 is 1 k and R2 is 10 k, for example, the input voltage applied through the 1 k resistor will be multiplied by 10 before being added to the voltage applied through the 10 k resistor. Thus, if the input at R1 is +1 V, and the input at R2 is +2 V, the output voltage will be –12 V. (For this formula to work, R3 must also be 10 k.)

The actual formula for calculating the output voltage based on the input voltages and the resistor values is:

Here are a few examples that should give you an idea of how the circuit will behave when R1 is 1 k and both R2 and R3 are 10 k:

VIN (1) VIN (2) VOUT
+1 V +1 V –11 V
+1 V +5 V –15 V
0 V +5 V –5 V
+2 V –5 V –15 V
–1 V –5 V +15 V

One drawback of the summing amplifier is that it inverts the polarity of the input, but you can easily feed the output of a summing amplifier into the input of a unity gain inverter. Here, the second op amp inverts the polarity of the output from the summing amplifier, which has the effect of returning the output voltage polarity to the polarity of the original inputs.

One common use for a summing amplifier circuit is as an audio mixer. When this type of circuit is used as an audio mixer, each input is connected to a microphone. The summing amplifier combines all the microphone inputs by adding the voltages from each microphone, and the resulting output is sent on to another amplifier stage.

The resistors in each input circuit are often potentiometers, which allows you to vary the signal level from each input source. When you increase the resistance on one of the input circuits, less of that input is represented in the output mix — especially useful if one of your singers is a bit off key.

A summing amplifier circuit can be extended with additional inputs. Here is a circuit with four inputs that uses potentiometers to control the level of each input. You can add as many inputs as you want, but you need to ensure that the total combined voltage from all inputs doesn’t exceed the power supply voltage (minus a volt or two).

One final variation of the summing amplifier circuit is used in conjunction with a second op amp configured as an inverter. This configuration preserves the polarity of the input voltages.