Voltage Follower

Voltage follower is an Op-amp circuit whose output voltage straight away follows the input voltage. That is output voltage is equivalent to the input voltage. No amplification is provided by the Op-amp circuit. Thus, voltage gain is equal to 1. They are similar to discrete emitter follower. The other names of voltage follower are Isolation Amplifier, Buffer Amplifier and Unity-Gain Amplifier. The voltage follower provides no attenuation or no amplification but only buffering. This circuit have an extremely useful characteristic of very high input impedance.

This high input impedance of voltage follower is the reason of it being used in several circuits. The voltage follower gives an efficient isolation of output from the input signal. The circuit of voltage follower is shown below.

Now, let us go through the most fundamental law; that is Ohm’s law

So, we can say that when resistance increases, the current drawn from the power source decreases. Thus, we conclude that the power is unaffected if the current is feeding a load of high impedance. For understanding this concept and the use of voltage follower, we can go through the following examples. First, we can consider a circuit of low impedance load and a power source is feeding it shown below. Here, a large amount of current is drawn by the load due to the low resistance load as explained by Ohm’s law. Thus, a large amount of power is taken from the power source. This results in high disturbances in power source.

Next, we can consider the same power is given to the voltage follower. Because of its very high input impedance, a very small amount of current is taken by this circuit. The output of the circuit will be same as that of the input due to the lack of feedback resistors.

Voltage Follower in Voltage Divider Circuits

In every circuit, voltage is shared or distributed to the impedance or resistance of the connected components. When Op-amp is connected, the major part of voltage will drop across it due to high impedance. So, if we use voltage follower in voltage divider circuits, it will let adequate voltage to be supplied across the load. Let us go through a voltage divider circuit with voltage follower as shown in figure below.

Here, the voltage divider is in the middle of two 10 KΩ resistors and the Op-amp. This Op-amp will offer input resistance of some hundreds of mega ohm. Now, we can assume it to be 100 MΩ. So the equivalent parallel resistance will be 10 KΩ || 100 KΩ.

So, we get 10KΩ || 10KΩ. We know that the voltage divider which is comprises of two similar resistances will offer exactly the half of the voltage in the power source. We can prove it using voltage divider formula as follows:

Thus, this 5V will drop across the 10KΩ resistance in the top and 5V drop across the resistance 10KΩ in the bottom and the load resistance 100Ω (since 10 KΩ||100 Ω, same voltage will drop in resistors which are in parallel). From this, we have seen how the Op-amp works as a buffer for getting the desired voltage to the connected load. In the same circuit with the absence of voltage follower, the circuit will not work. This is due to the lack of supply of sufficient voltage across the load. Mainly, voltage follower is implemented in circuits for two reasons. One is isolating purpose and the other is for buffering the output voltage from an electrical or electronic circuit to get the desired voltage to the connected load.

 Advantages of Voltage Follower

  • Provides power gain and current gain.
  • Low output impedance to the circuit which uses the output of the voltage follower.
  • The Op-amp takes zero current from the input.
  • Loading effects can be avoided.

Applications of Voltage Follower

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

1.Bunty B. Bommera
2.Dakshata U. Kamble

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