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Non-inverting amplifier explained

A non-inverting amplifier is an op-amp circuit that makes a signal bigger and keeps it the right way up. The signal goes into the + input, and two resistors on the output set the gain: G = 1 + Rf ÷ Rg. A 1 V wave through a gain of 4.9 comes out as a 4.9 V wave, the same shape and in step.

The feedback divider

An op-amp multiplies the difference between its inputs by about 100,000, so on its own it slams to a limit. Feedback tames it. A feedback divider sits on the output: Rf from the output to the − input, and Rg from the − input to ground. The divider hands the − input a fixed share of the output.

The op-amp raises its output until that share matches the input on the + pin. With a 1-in-11 divider, the output must be 11 times the input. A bigger Rf sends back a smaller share, so the gain goes up.

−+Rg​Rf​Vin​Vout​
Non-inverting amplifier: the feedback divider (amber) hands the − input a fixed fraction of the output

Live experiment

Turn up the gain

A 1 V peak wave goes into a non-inverting amplifier on ±12 V rails, with R_g = 10 kΩ. Slide R_f from a plain wire up to 220 kΩ and compare the two waves on the scope, until the output's tips clip.

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.

G = 1 + Rf ÷ Rg

The gain is how many times bigger the output is, Vout = G × Vin, and it has no unit:

G = 1 + Rf over Rg

Example: Rf = 39 kΩ and Rg = 10 kΩ.

G = 1 + 39 kΩ over 10 kΩ = 4.9

To design for a gain, choose standard values with Rf ÷ Rg = G − 1. In decibels, ×4.9 is about 14 dB.

The rails set the ceiling

Gain can't create headroom. The output peak, G × Vin, must stay inside the output's limits, which on a classic op-amp sit about 1.5 V inside each rail: ±10.5 V on ±12 V. A 1 V peak at a gain of 11 asks for 11 V and loses its tips; at 23, it's mostly flat tops. The output can also supply only about 25 mA, and it drives the feedback resistors too, so use kilohms for them, not ohms.

On a single 9 V battery, rest the + input at half the supply with two equal resistors and bring the signal in through a coupling capacitor, and put a capacitor in series with Rg, so only the signal is amplified. The output then rests at 4.5 V and the wave swings around it instead of being clipped in half at 0 V.

Where it's used

Its input goes straight into the + pin, which draws almost no current, so even a weak source can drive it. That's why microphone preamps, guitar pedals and many sensor amplifiers use this circuit.

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

Work it out

Practice questions

Three quick questions from the lessons. Pick an answer to check it.

  1. Question 1 of 3In a non-inverting amplifier, a bigger feedback resistor (the top of the divider) gives…
    Show the answer

    Answer: more gain. A bigger top resistor sends a smaller share of the output back, so the op-amp has to lift its output higher before that share matches the input. More gain.

    • Why not “less gain”? Less gain comes from a smaller feedback resistor, which sends a bigger share of the output back to the − input.
    • Why not “the same gain”? The gain is 1 + Rf ÷ Rg, so it changes whenever Rf does.
  2. Question 2 of 3Ask one op-amp for more gain, and the frequency where its gain starts to fall…
    Show the answer

    Answer: gets lower. Gain and bandwidth trade off, like a bike's gears. Their product, the gain-bandwidth product, stays about the same, so more gain means a lower corner.

    • Why not “stays the same”? The corner moves with the gain. On a typical op-amp, ten times the gain means roughly a tenth of the bandwidth.
    • Why not “gets higher”? Gain and bandwidth trade off: their product stays roughly fixed, so more gain can't buy a higher corner.
  3. Question 3 of 3In an inverting amplifier with + grounded, the − input sits at…
    Show the answer

    Answer: about 0 V. Negative feedback makes V− match V+, and + is grounded, so − sits at about 0 V. It isn't wired to ground: the op-amp holds it there. That's the virtual ground.

    • Why not “the input voltage”? The input reaches the − input only through the input resistor. Negative feedback moves the output until − matches +, and + is at 0 V.
    • Why not “the supply voltage”? Nothing connects the − input to the supply. The op-amp's feedback holds it level with the grounded + input, at about 0 V.
Keep practising: 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.

Questions people ask

What is the gain of a non-inverting amplifier?

G = 1 + R_f ÷ R_g, where R_f runs from the output to the − input and R_g from the − input to ground. With R_f = 39 kΩ and R_g = 10 kΩ, the gain is 4.9. The output is in step with the input, not flipped.

What is the difference between inverting and non-inverting amplifiers?

A non-inverting amplifier takes the signal on its + input, keeps it the right way up, and has a gain of 1 + R_f ÷ R_g, so never less than 1. An inverting amplifier takes it through a resistor to the − input, flips it upside down, and has a gain of size R_f ÷ R_in.

Why is my op-amp output clipped?

The output can't go past its limits, which sit near the supply rails: about ±10.5 V on ±12 V for a classic op-amp. If gain × input peak asks for more than that, the tops are cut flat. Lower the gain, shrink the input, or use bigger rails.

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