The two basic gain configurations
An op-amp's closed-loop gain is set almost entirely by two resistors:
- Inverting: input through Rin to the − pin, feedback Rf from output back to it, + pin grounded. Gain = −Rf / Rin (note the sign — the output is flipped).
- Non-inverting: input straight into the + pin, with Rg from the − pin to ground and Rf from output to the − pin. Gain = 1 + Rf / Rg (always ≥ 1, same polarity as the input).
Only the resistor ratio sets the gain, so the reverse solver picks standard values that hit your target — then you check input impedance and bandwidth.
Worked examples
×10 inverting: Rin = 1 kΩ, Rf = 10 kΩ → gain −10 (20 dB).
×11 non-inverting: Rg = 1 kΩ, Rf = 10 kΩ → gain 1 + 10 = 11.
Two things the ratio doesn't tell you
- Input impedance. The inverting amp's input impedance is just Rin (the − pin is a virtual ground), so a low Rin for high gain loads the source. The non-inverting amp's input is the op-amp's own very high impedance — often the deciding factor between the two.
- Bandwidth. Real op-amps have a fixed gain-bandwidth product (GBW): more gain means less bandwidth. A 1 MHz GBW part set to ×100 only reaches ~10 kHz. Check the datasheet's GBW against your gain.
Frequently asked questions
- Why is the inverting gain negative?
- The signal enters the inverting input, so the output moves opposite to the input — a 180° phase shift. The magnitude is Rf/Rin; the sign is just polarity.
- Can a non-inverting amp have gain less than 1?
- No — its minimum is 1 (a buffer, with Rf = 0 or Rg = ∞). For attenuation use a voltage divider ahead of a buffer, or an inverting stage with Rf < Rin.
- What resistor values should I use?
- Keep them in the ~1 k–100 kΩ range: too low loads the output and burns current, too high lets bias currents and noise matter. The reverse solver's impedance selector sets the ballpark.
- How do I express the gain in dB?
- Voltage gain in dB is 20·log₁₀|G| — shown above, and explained on the decibel calculator.