Op Amp Voltage follower / Voltage Buffer

Oct 082019

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What is an op-amp voltage follower?

An op-amp voltage follower (voltage follower using an operational amplifier) is a circuit that has a voltage gain of 1. To better understand the operation of a voltage follower, we must remember the operation of an op-amp as a non-inverting amplifier. See the diagram below.

Op-Amp Non-inverting Amplifier

The op-amp non-inverting amplifier gain is given by the formula 1 + (R2/R1).

If we make the R2 resistor value equal to zero and make the R1 resistor value very large, we have an amplifier with gain G = 1. This means that the output signal has the same value as the input signal. See the following diagram.

Op Amp Voltage Follower (Voltage Buffer)

A buffer has an output that is exactly like the input. This feature is very useful for solving impedance matching problems.

The input impedance of a buffer using an operational amplifier is very high, close to infinity
The output impedance is very low, just a few ohms.

If the input impedance is very high, it does not load the circuit that is sending the signal, and if its output impedance is low, it can deliver a sufficient amount of current to the circuit receiving the signal.

In other words, a buffer requests very little current from the circuit that gives the signal and greatly increases the capacity to deliver current to the circuit that receives the signal.

Voltage follower advantages

Provides power gain and current gain. (voltage gain = 1).
Low output impedance. Loading effects can be avoided.
High input impedance. The op-amp takes no current from the input.

Voltage follower applications

Sample and hold circuits.
Buffers for logic circuits.
Active filters. Voltage followers can be used to isolate filter stages from each other when building multistage filters.

Op-Amp voltage follower example

We need to get 6 volts from a 12-volt source to power a 100-ohm load resistor (RL).

We use two 100K resistors in series as a voltage divider (R1, R2). Since the resistors have the same value, the voltage between them is exactly 6 volts (A).

If we connect the 100-ohm load to point A, the voltage will no longer be 6 volts, because the lower 100K resistor is in parallel with the load. The equivalent resistance of parallel resistors is 100,000 x 100 / (100,000 + 100) = 99.9 ohms. We can safely assume the approximate value of 100 ohms.

Using the voltage divider again, the voltage at point A is VA = 12 / (100K + 100) x 100 = 0.012 volts. Very different from the expected 6 volts.

To avoid this problem, we use an op-amp voltage follower. The non-inverting input of the op-amp is connected to A, and the output is connected to the 100-ohm resistor (load). The load will have exactly 6 volts between its terminals.