Summing Op Amp

This is the AQA version closing after June 2019. Visit the the version for Eduqas instead .

To gain access to revision questions, please sign up and log in.

Vout = - Rf (V1/R1 + V2/R2 + V3/R3)
Rf provides negative feedback. The gain of each input depends on Rf and the input resistor.

The inverting input labelled "P" is a Virtual Earth. The virtual earth comes about as follows ...

The op' amp' has a very high open loop gain.
If the op' amp' is not saturated and the output is a few volts, it follows that the potential at P is a few microvolts.
This is a very small (close to zero) voltage.

Op Amp Summing.gif

Applications include ...

For the Falstad Circuit Simulation, CTRL+Click Summing Amplifier for adding two Signals
In options, check European Resistors and uncheck Conventional Current.

Alternatively view Summing_Amplifier.txt.
Save or copy the text on the web page. Import the saved or copied text into the Falstad simulator.

Here is the new HTML5 Simulator Site.