How to use
- Choose Inverting or Non-inverting.
- To analyse a circuit, keep Gain from resistors and enter Rf and Rin (or Rg). To design one, choose Resistor for a gain, enter the gain you need and the other resistor.
- Enter an input voltage to see the ideal output voltage.
Formulas
For an ideal op-amp (very high open-loop gain, no input current):
- inverting: G = −Rf / Rin
- non-inverting: G = 1 + Rf / Rg
- gain in dB = 20 × log10 |G|
- output voltage = G × input voltage
For a target gain the calculator solves for Rf: Rf = G × Rin (inverting) or Rf = (G − 1) × Rg (non-inverting).
Worked examples
Inverting, Rf = 100 kΩ, Rin = 10 kΩ, input 0.5 V: G = −100 / 10 = −10, which is 20 dB, and the output is −5 V.
Non-inverting, Rf = 47 kΩ, Rg = 10 kΩ: G = 1 + 47 / 10 = 5.7, or 15.12 dB; 0.5 V in gives 2.85 V out.
Design: an inverting gain of 22 with Rin = 4.7 kΩ needs Rf = 22 × 4.7 = 103.4 kΩ. The nearest E24 value is 100 kΩ, which gives a gain of −21.2766.
Tips
- Keep resistors roughly between 1 kΩ and 1 MΩ: very small ones load the source and the op-amp output, very large ones add noise and offset from input bias current.
- The input impedance of an inverting stage is Rin; a non-inverting stage has a very high input impedance.
- If the gain must be exact, use E96 (1 %) resistors or put a trimmer in series with Rf.