Op-amp gain resistor calculator
Target gain in, standard resistor pairs out — with the exact gain each pair gives, its error, and the input impedance it forces on whatever drives the stage.
Best standard pairs
Input impedance: in the inverting configuration it is Rin; in the non-inverting one it is the op-amp’s own input impedance.
The formula
Inverting: G = −Rf / Rin Non-inverting: G = 1 + Rf / Rg
When to use it
Scaling a sensor into an ADC range, buffering and amplifying a small signal, or setting the gain of a stage on a legacy board you are re-drawing. Inverting stages set the input impedance with Rin itself; non-inverting stages present the op-amp’s own input impedance, usually very high, so the resistor values only need to suit the op-amp’s bias current and the noise budget.
Pitfalls
- Inverting stage: the source sees Rin as its load. A 1 kΩ Rin on a 10 kΩ source divides the signal by eleven before the amplifier does anything.
- Very high feedback values (megohms) turn bias current into offset and the feedback node into an antenna; very low ones load the op-amp output. Tens of kilohms suits most stages.
- Gain error here is the resistor ratio only. Add the op-amp’s open-loop gain and bandwidth limits, and the resistor tolerance — 1 % parts for anything that is measured rather than merely amplified.
- A non-inverting gain of exactly 1 needs no resistors: use a voltage follower.
Related note
Reverse engineering a board with no schematicNeed the whole board designed, not just the gain stage?