A summing amplifier is one that takes in more than one input and sums them all together to give the resulting output. The amplifier might also add a gain to the voltage. In an audio system, this would be called a mixer because it mixes the signals (different audio sources) together to give one resulting signal.

This video describes two configurations, one that is inverting (the inputs are phase shifted by 180 degrees to give the outputs) and one that is non-inverting. For both configurations, the equations relating input voltage to output voltage are derived and a couple of simulation examples are presented.

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