A Summing Amplifier is an operational amplifier (op-amp) that   produces an output voltage that is equivalent to an amplified sum of two  or more voltages. It allows the circuit to effectively add or sum  together several different input voltages or signals.  It is typically  an inverting amplifier, meaning that the voltage from the output when  measured to ground should be a negative value. The typical circuit  comprises of two or more resistors in parallel from different voltage  sources that all lead into the negative terminal of the op-amp. The spot  at which all input signals connect is called the virtual earth node,  and because of this point all of the signals are effectively isolated  from each other. If the values of the input resistors are all the same,  the circuit becomes a unity gain inverting adder, while if the values  are different from each other the circuit becomes a scaling summing  amplifier. 

To calculate the Gain(Av) for this circuit, you would use the formula 

. 

There is another option for a summing amplifier, and that is the  non-inverting style. The difference between the two is that the  non-inverting amplifier has the signal sent to the positive terminal of  the op-amp and does also not have the virtual earth node but instead a  summing junction. Because of these differences the Non-inverting  variation has a substantial increase of input impedance. Also, the input summing part of the circuit is unaffected if the op-amps closed-loop voltage gain is changed.

To calculate the gain for this type of summing amplifier the equation slightly changes to 

Storr, W. (2022, August 4). The Summing Amplifier. Basic Electronics Tutorials. https://www.electronics- 

 Links to an external site.tutorials.ws/opamp/opamp_4.html

Summing Amplifier – Operational Amplifiers – Basics Electronics. (n.d.). https://ecstudiosystems.com/discover/textbooks/basic-electronics/operational-amplifiers/summing-amplifier/ 

 Links to an external site.

Summing Amplifier: Inversion Properties and Op-Amp Varieties. (2020, July 21). https://resources.pcb.cadence.com/blog/2020-summing-amplifier-inversion-properties-and-op-amp-varieties