Voltage Follower OP Amplifier: What is it? (Gain & Circuit Diagram)

What is a Voltage Follower?

A voltage follower (also called a buffer amplifier, a unity-gain amplifier or an isolation amplifier) is an op-amp circuit whose output voltage equals the input voltage: it “follows” the input. A voltage follower op-amp therefore does not amplify the signal and has a voltage gain of 1.

The voltage follower neither amplifies nor attenuates; it buffers the signal.

A voltage follower circuit has very high input impedance. That characteristic makes it a common choice wherever a circuit needs isolation between the input and output sides of a signal.

The circuit of a voltage follower is shown below.

An important law that underpins the voltage follower is Ohm’s law.

Ohm’s law states that a circuit’s current equals its voltage divided by its resistance.
As noted above, voltage followers have very high input impedance, which here means high input resistance.

Before discussing circuits with high impedance, it helps to see what happens in a circuit with low impedance.

A low input impedance, which in this case means low resistance, makes the R term of Ohm’s law small.

With a fixed supply voltage V, a low-impedance load therefore draws a large current.

The circuit then takes a large amount of power from the source, producing strong disturbance there.

Now consider giving the same power to a voltage follower circuit.

A voltage follower circuit is shown below.

Notice that the output connects directly to the inverting input.

This feedback connection forces the op-amp to adjust its output voltage until it equals the input voltage.

The output voltage thereby “follows” the input voltage, which gives the circuit its name.

As mentioned, the voltage follower is an op-amp circuit whose impedance is very high on the input side.

More precisely, the op-amp input presents 1 MΩ to 10 TΩ of impedance, while its output side does not.

Ohm’s law must still hold true.

If the voltage stays the same from the input side to the output side while the resistance drops sharply, what happens to the current?

It rises enormously: with R small, I = V/R becomes large.

A voltage follower keeps the voltage the same; nothing guarantees that the current stays the same too.

Although its voltage gain is unity (1), a voltage follower provides very high current gain.

So on the input side: very high impedance and very little current drawn.

And on the output side: very low impedance driving the load with ample current.

The voltage level passes through unchanged while the available current increases, because the input and output impedances differ so greatly.

To recap: the input impedance of the op-amp is very high (1 MΩ to 10 TΩ).

That high input impedance means the follower barely loads the source, drawing only a small current from it.

Because its output impedance is very low, it drives the load almost like a perfect voltage source.

Both the connection into and out of the buffer are therefore bridging connections.

The result is reduced power draw at the source and less distortion from overloading and other causes of electromagnetic interference.

Voltage Follower Gain

A voltage follower has a voltage gain of 1 (unity) because its output voltage follows its input voltage. Although the voltage gain of a buffer amplifier is approximately unity, the circuit provides considerable current and power gain. It remains common to quote its gain as 1, referring to voltage gain, equivalent to 0 dB.

Voltage Follower in Voltage Divider Circuits

In every circuit, voltage distributes itself across the impedances or resistance values of the connected components. When an op-amp is connected, its high impedance takes a correspondingly large share of any voltage placed across it.

In voltage divider circuits, a voltage follower ensures that adequate voltage reaches the load.

Consider a voltage divider circuit with a voltage follower, as shown in the figure below.

The divider sits between two 10 KΩ resistors and the Op-amp. The Op-amp input offers some hundreds of megaohm; assume 100 MΩ. The equivalent parallel resistance of the lower leg is then 10 KΩ || 100 MΩ.

That works out to approximately 10 KΩ, so the voltage divider consists of two practically equal resistances and delivers half of the power source voltage.

We can prove this using the voltage divider formula:

Thus 5 V drops across the upper 10KΩ resistance and 5 V drops across the lower-leg resistance, leaving the follower to place that same 5 V across the load resistance 100Ω (components in parallel share the same voltage).

The Op-amp thus works as a buffer that delivers the desired voltage to the connected load. Without the voltage follower, the same circuit would fail for lack of sufficient voltage across the load.

Circuits implement the voltage follower mainly for two reasons: isolation, and buffering the output voltage of an electrical or electronic circuit so the connected load receives the desired voltage.

Advantages of Voltage Followers

The advantages of voltage followers include:

• Voltage followers provide large power gain and current gain.
• Low output impedance toward the circuit that uses the output of the voltage follower.
• The Op-amp draws virtually zero current from the input.
• Loading effects are avoided.

Applications of Voltage Followers

Some of the applications of voltage followers include:

• Buffers for logic circuits.
• Sample-and-hold circuits.
• Active filters.
• Bridge circuits via a transducer.