Summing amplifier using op amp have two or more inputs connected to the inverting terminal and non inverting terminal is grounded (**Inverting amplifier configuration**). Its output will be negative of the algebraic sum of inputs. Hence the name summing amplifier. It have a negative feedback.

Summing amplifier have two variations – summing amplifier with unity gain and summing amplifier with gain greater than unity . Input resistance of each inputs are same. If this input resistance is less than feedback resistance, then that circuit have gain greater than unity. If both input resistance and feedback resistance are same, then circuit will have unity gain. According to this gain output will change.

With slight variations in summing amplifier we can generate two other amplifiers called Averaging amplifier and Scaling amplifier. Average amplifier’s output is negative average of input signal. In scaling amplifier output is sum of the input signals, where each input signal is multiplied with different factors.

## Averaging Amplifier using op amp

Consider above mentioned summing amplifier with gain greater than unity, If the ratio in the equation of output voltage is replaced with reciprocal of number of inputs. then in total output voltage will be negative of average of inputs.

Suppose if there are five inputs to an averaging amplifier then each input resistance should be five times greater than feedback resistance. thus the ratio in the output become \(\frac{1}{5}\). And in overall, output is negative of the average of input signals. This negative signal is due to connection of input signals to inverting terminal (inverting configuration of op amp).

## Scaling Amplifier using op amp

Resistance of each input of summing amplifier is same. If we make it different for each inputs in summing amplifier with gain greater than unity, we will get scaling amplifier. Scaling amplifier’s main application is its usage in digital to analog conversion.