OPAMP buffer or voltage follower is an opamp configuration whose gain is equal unity. Voltage follower is also known as unity gain amplifier, voltage buffer, opamp buffer, or an isolation amplifier. It is used to separate two circuits having the same voltage level but different impedance.
In a voltage follower circuit output voltage and input voltage, both are equal. Due to this, the gain of the opamp is one (unity), and the amplifier does not amplify the input signal. This voltage follower circuit does not require any external components.
What is the need of a voltage follower opamp?
when R1 = ♾️ and Rf = 0 the non-inverting amplifier gets converted into a voltage follower for unity gain (gain of 1) amplifier for unity gain buffer an isolation amplifier.
Circuit diagram of a voltage follower is as shown in the figure:
If we short-circuit the Rf resistor and open circuit the Ri input resistor in a usual non-inverting amplifier, we get a voltage follower circuit.
Since all the output voltage is feedback to the inverting input of the opamp, the feedback factor of the circuit b = 1.
Closed-loop gain Avf:
consider the expression for the closed-loop gain of a non-inverting amplifier,
Avf = 1 + Rf/R1
In this equation substitute, the value of Rf = 0 and R1 = ♾️ to get the closed-loop gain of voltage follower as:
Avf = 1
Therefore the output voltage will be equal to and in phase with the input voltage.
Therefore the voltage follower is a non-inverting amplifier with a voltage gain of unity.
The unity gain amplifier does not behave like a conventional voltage amplifier but it acts as a resistance transformer.
Use of voltage followers:
- For matching impedance between two circuits
- Isolating two circuits
- To provide a constant voltage source
- In A to D conversion
Features of a voltage follower circuit:
- closed-loop voltage gain is equal to one.
- the output voltage is equal to input voltage with no phase shift.
- Input impedance is very high.
- Output impedance is very low.
- Bandwidth is large.