Op-amp gain resistor selection and tolerance effects
The op-amp gain equation gives the resistor ratio. The engineering task is choosing values that also suit bias current, noise, bandwidth, swing, and tolerance.
How should you choose op-amp gain resistors?
Start from the ideal gain ratio, choose practical resistor values, then check tolerance, input bias current error, noise, op-amp bandwidth, slew rate, output swing, and stability.
Model summary
- Non-inverting gain: Av = 1 + Rf / Rg.
- Inverting gain: Av = -Rf / Rin.
- Gain in dB: 20 * log10(|Av|).
Worked example
For a non-inverting stage with Rf = 30 kOhm and Rg = 10 kOhm, gain is 1 + 30 kOhm / 10 kOhm = 4 V/V.
With a 0.5 V input, the ideal output is 2.0 V.
The gain is 12.04 dB, but real output swing and bandwidth still need checking.
Practical sequence
Solve the ideal ratio, choose values that suit impedance and noise, then review tolerance and op-amp limits before committing the schematic.
Common mistakes
- Choosing very high resistor values and ignoring input bias current or noise.
- Checking ideal gain but forgetting output swing and common-mode range.
- Assuming resistor tolerance has no effect because the op-amp is precise.
When the approximation breaks down
- The ideal gain equation does not prove stability, bandwidth, slew rate, input range, or output current capability.
- Precision gain may need matched resistor networks or ratio-tolerance specifications.