Entering a value for Gain will find the optimum values for R1 and R2. If you specify the values for R1 and R2, the gain is found. If you enter a resistor values (R1 or R2) along with the gain, the other value will be found.

The circuit configuration shown is an inverting amplifier. It take the input voltage and provides an amplified version of it at the output. When the input is positive the output will be negative. This results in an AC waveform appearing inverted, or 180° out of phase, with the input.

The use of R3 is optional and can be replaced with a short to ground (R3 is replaced with a conductor). It is used to reduce the offset voltage error of the op-amp. A full discussion is given in “Op-amp Errors (Article)”.

The input resistance of the amplifier is equal to the resistance of R1. This is due to the feedback loop of the amplifier which keeps the inverting and non-inverting input of the amplifier at the same potential. Because the non-inverting input is at ground potential (there is negligible current flow through R3 and therefore a negligible voltage appears across it), the inverting input is also at ground which results in the input resistance being the same as R1.

The output resistance of the amplifier is 0Ωas long as the current rating of the op-amp is not exceeded by the load current in combination with the current through R2.

If you don’t like the magnitude of the resistor values found, you can divide or multiply the values by a multiple of ten and the resultant values will still be a standard value.

This solver uses the following equation to compute its values.