Tutorial 7G - Voltage Follower

Learning Objectives

• draw and recognise the voltage follower op-amp amplifier circuit and describe its applications;

• recall that the voltage gain of a voltage follower is 1, but that the current and power gain can be very large;

• recall that the input resistance is equal to the resistance of the op-amp.

This operational amplifier voltage follower circuit is simplicity itself.  There is no input resistor to the non-inverting input, and no feedback resistor to the inverting input.  The circuit is shown below: You will recognise that this is like the inverting amplifier with its negative feedback.  Since the inverting and non inverting inputs are always at the same voltage, and the inverting input is at the same voltage as the output terminal, we can say:

Vin = Vout

We can see easily that the voltage gain is 1.  So this circuit is called the unity gain amplifier.

The voltage gain of the op-amp in this configuration is about 1.  This because of  the feedback factor (the fraction fed back), given the code b (beta, a Greek letter ‘b’) is 1.

We can show this by considering the open loop gain Ao.  The actual gain A is given by

A = ___Ao___

1 + bAo

If b is 1, and Ao is very large, we can say that A » 1.

Although a voltage gain of 1 does not appear to be particularly useful, the op-amp can give out a relatively large current, so that there is a high power gain.

The main use of the voltage follower is as a buffer amplifier, which matches a high input impedance (1 × 1012 ohms) with a low output load (75 ohms).  You would come across such a circuit in the input stage of a digital multimeter, which has a very high input impedance, allowing the voltage read to be the same as the voltage that should be there.

It is possible to make a voltage follower from an inverting amplifier.  However there is the problem of a change of phase, which is avoided in this arrangement.  The output of a voltage follower should follow the input signal as closely as possible.  A real amplifier does not quite do this absolutely perfectly, as there is a very small offset voltage.  However this is much lower than when transistors are used as voltage followers.  And the gain is very close to 1, whereas in a transistor voltage follower, it is about 0.9.

It is possible to add a resistor in the negative feedback loop: The source that supplies the non-inverting input has a resistance and possibly a bias current.  This can result in Vout having an offset.  This can be minimised by having a resistor, Rf, in the feedback loop.  Rf must have the same value as the source resistance.

The diagram shows a voltage follower used in a power amplifier. Links Simple simulation Voltage followers for beginners Video tutorial