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/
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