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Voltage follower explained

A voltage follower is an op-amp with its output wired straight to its − input and the signal on its + input. Its output copies the input voltage exactly, a gain of 1, but it can supply milliamps while drawing almost nothing from the source. That makes it a buffer: a weak source can drive a heavy load without sagging.

Negative feedback

An op-amp amplifies the difference between its inputs, V+ − V−, by about 100,000. Wire the output back to the − input and it corrects itself: if the output rises too far, − rises with it and pulls it back down. It settles where the two inputs match, so the output equals the input.

With 6 V at the output, the inputs differ by just 6 V ÷ 100,000 = 60 µV. Close enough to call equal, which gives the two golden rules for any op-amp with negative feedback:

  • Rule 1: V+ = V−
  • Rule 2: no current flows into either input
Vout = V− = V+ = Vin

Buffering a divider

Seen from its output, a divider is a voltage behind a resistor, Rth = R1 ∥ R2. Any load forms a second divider with Rth and drags the output down.

Example: on 9 V, a 4.7 kΩ / 4.7 kΩ divider gives 4.5 V behind 2.35 kΩ. Hang a 470 Ω load straight on it:

V = 4.5 V × 0.47 kΩ over 2.35 kΩ + 0.47 kΩ ≈ 0.75 V

Put a follower in between and the load gets 4.5 V. The divider sees no load at all, and the op-amp supplies the load's 4.5 ÷ 470 ≈ 9.6 mA.

Live experiment

Let the op-amp carry the load

A 10 kΩ / 10 kΩ divider makes 6 V from 12 V. Hang a load straight on it and it sags: 1 kΩ gets only 1 V. Put U1, a follower, in between and the load gets the full 6 V, until it asks for more than U1's 25 mA.

Try it yourself: free lesson

“Feedback and the follower” is lesson 3 of 6 in Amplifier Peaks, Electronics World 6. Free with an account. Lessons unlock in order, so you’ll do the 37 before it first.

Its limits

The follower's strength comes from its supply rails, not from nowhere, so:

  • The output can only supply so much current, about 25 mA for a classic op-amp. Ask for more and the output collapses.
  • The output can't reach the rails. A classic op-amp stops about 1.5 V short of each; rail-to-rail parts get much closer.

Pick the rails so the voltage you're copying sits comfortably inside them.

Where you'll meet it

Followers sit between weak sources and whatever they feed: sensors, voltage references and a microcontroller's analog input. Battery-powered audio gear uses a buffered divider to make a steady half-supply point, 4.5 V on 9 V, so a wave can swing both up and down around it.

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

“Feedback and the follower” is lesson 3 of 6 in Amplifier Peaks, Electronics World 6. Free with an account. Lessons unlock in order, so you’ll do the 37 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 3A comparator's output goes high when…
    Show the answer

    Answer: V+ is above V−. The op-amp amplifies V+ − V−. When V+ is higher, the difference is positive, so the output swings high. When V− is higher, it swings low.

    • Why not “V− is above V+”? That's backwards. The op-amp amplifies V+ − V−, so when V− is higher the output swings low.
    • Why not “the two inputs are equal”? With no feedback, the inputs are almost never exactly equal. Any difference you could measure slams the output to a limit.
  2. Question 2 of 3Why can't a plain voltage divider power a 270 Ω load at a steady voltage?
    Show the answer

    Answer: The load pulls its output down. A divider acts like a battery behind Rth. A heavy load forms a second divider with Rth, so the output sags. A Zener holds its voltage instead.

    • Why not “Dividers only work on AC”? A resistor divider works the same on DC and AC. The trouble is its Rth, which sits in series with the load.
    • Why not “It makes too much voltage”? A divider's output can't exceed its supply, and a load only ever pulls it down.
  3. Question 3 of 3Feeding an op-amp's output back to its + input gives…
    Show the answer

    Answer: positive feedback. Back to + is positive feedback: it helps the output along, so it snaps to a limit, as in a Schmitt trigger. Back to − is negative feedback, which holds it steady.

    • Why not “no feedback”? Any path from the output back to an input is feedback. To + it helps the change along.
    • Why not “negative feedback”? Negative feedback goes back to the − input, as in a follower. Back to + the output snaps to a limit instead.
Keep practising: free lesson

“Feedback and the follower” is lesson 3 of 6 in Amplifier Peaks, Electronics World 6. Free with an account. Lessons unlock in order, so you’ll do the 37 before it first.

Questions people ask

What does a voltage follower do?

Its output copies its input voltage, but it can supply far more current than the input draws. So a weak source, like a high-resistance divider or sensor, can drive a load without being pulled down. It's also called a unity-gain buffer.

Why use a voltage follower if the gain is 1?

Because the voltage gain is 1 but the current gain is huge. The + input draws almost nothing, while the output supplies milliamps from the op-amp's supply rails. The source sees no load at all.

How do you wire an op-amp as a voltage follower?

Connect the output straight to the − input with a wire, and feed the signal to the + input. No resistors are needed. Power the op-amp from rails that leave room above and below the voltage you need.

What are the op-amp golden rules?

With negative feedback, and while the output is inside its limits: the output does whatever it takes to make V+ equal V−, and no current flows into either input. They're how you work out almost any op-amp amplifier.

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