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Op-amp gain calculator

An op-amp with two resistors around it makes a signal bigger. Wired non-inverting, the output is the input times 1 + Rf ÷ Rg, the right way up. Wired inverting, it's −Rf ÷ Rin, flipped. Type your resistors, input and supply to get the gain, the output and whether it clips.

From the output to the − input.

From the − input to ground.

Into the + input. 50m = 50 mV.

12 for ±12 V. On one 9 V battery, biased at half, type 4.5.

About 1.5 V for a classic op-amp, nearly 0 for a rail-to-rail one.

Output peak, Vout = (1 + Rf ÷ Rg) × Vin

4.9 V
Gain, G
4.9
In decibels
13.8 dB
Phase
In phase, 0°
Output limit
±10.5 V
Biggest input before clipping
2.14 V
Input resistance
Very high, often over 1 GΩ

The formulas

A non-inverting amplifier takes the signal into the + input. Rf runs from the output to the − input, and Rg from the − input to ground:

G = 1 + Rf over Rg

An inverting amplifier grounds the + input and takes the signal in through Rin to the − input:

G = − Rf over Rin

Either way, Vout = G × Vin, and the gain in decibels is 20 × log10|G|.

A worked example

A non-inverting amplifier with Rf = 39 kΩ and Rg = 10 kΩ:

G = 1 + 39 kΩ over 10 kΩ = 1 + 3.9 = 4.9

A 1 V peak input comes out 4.9 V peak, in phase, about 13.8 dB. On ±12 V rails the output stops at ±10.5 V, so anything up to 10.5 ÷ 4.9 ≈ 2.1 V peak fits. Feed it more and the tops are clipped flat.

Picking resistors

For a non-inverting gain G, choose Rf ÷ Rg = G − 1 from standard values: 22 kΩ and 2.2 kΩ give ×11. Keep both in the kilohms. The output can supply only about 25 mA, and it drives the feedback resistors as well as the load.

On one battery, rest the + input at half the supply with two equal resistors and bring the signal in through a capacitor. The output then swings around the middle, so type half the battery as the supply: 4.5 for 9 V.

Free lesson

Now build it yourself

Each lesson comes in three short parts: see the idea, try it on a live board, then build it yourself, with quick checks as you go.

Try it yourself: free lesson

“Non-inverting amplifiers” is lesson 4 of 6 in Amplifier Peaks, Electronics World 6. Free with an account. Lessons unlock in order, so you’ll do the 38 before it first.

Or start from the very first lesson, no account needed

The ideas behind it

Questions people ask

What is the gain of a non-inverting amplifier?

G = 1 + Rf ÷ Rg, where Rf runs from the output to the − input and Rg from the − input to ground. With Rf = 47 kΩ and Rg = 10 kΩ, G = 1 + 4.7 = 5.7. The output is in phase with the input.

What is the gain of an inverting amplifier?

G = −Rf ÷ Rin. The minus sign means the output is flipped upside down, 180° out of phase. With Rf = 22 kΩ and Rin = 10 kΩ, G = −2.2: 1 V in gives −2.2 V out.

Why is my op-amp's output clipping?

The output can't go past its supply rails, and a classic op-amp stops about 1.5 V short of each one. On ±12 V that's ±10.5 V, so a 2 V peak at a gain of 5.7, which asks for 11.4 V, is cut flat at 10.5 V. Lower the gain, shrink the input, or use a rail-to-rail op-amp.

What gain does a voltage follower have?

Exactly 1. It's a non-inverting amplifier with Rf a plain wire and no Rg, so G = 1 + 0 = 1. The output copies the input, but it can drive a load the source couldn't.

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