Voltage Follower

A voltage follower (also called a unity gain amplifier) has the output connected directly to the inverting input.

Remember that the basic rule of the op-amp is that it will change the output voltage to whatever it takes to make the inputs equal. If the voltage at the non-inverting input is +2.5 volts, the op-amp will change the output voltage until it sees +2.5 volts at the inverting input. Since the inverting input is connected directly to the output, the output voltage must also be +2.5 volts. The output voltage will simply be whatever voltage is applied to the non-inverting input.

The high input impedance of the op-amp requires very little current from the preceding circuit so it is a good match for a circuit with a high output impedance. The low output impedance of the op-amp means that it can deliver the substantial current that may be required by a circuit with a low input impedance. This makes the voltage follower a good go-between (a buffer circuit) to couple a circuit with high output impedance to a circuit with low input impedance.

Occasionally, you may find a resistor in the connection from the output to the inverting input. This may not make sense because there should be virtually no current flowing in this connection. Some op-amps are sensitive to the tiny current that flows into the input. This resistor reduces such current.