What are the practical reasons to use a voltage-follower?

Could you give a particular example why is a voltage follower is used?

Could you give a particular example why is a voltage follower is used?

A voltage follower typically has a very high input impedance and a very low output impedance. Why is this important or necessary?

Well, it is often the case that a voltage amplifier stage will have a moderately high output impedance. This output impedance will form a voltage divider with the load impedance and reduce the voltage gain of the stage. For example: simulate this circuit – Schematic created using CircuitLab

Here, I model a voltage amplifier with an open circuit (unloaded) gain of 100 and an output resistance of $1k \Omega$.

But, with an $8 \Omega$ load connected, the loaded gain drops to

$$A_v = 100 \dfrac{8}{8 + 1000} = 0.794$$

The voltage gain of the stage is reduced to less than 1!

However, by inserting an (ideal) voltage follower between the amplifier and the load: simulate this circuit

The overall loaded voltage gain is now 100, the unloaded voltage gain.

This may seem paradoxical since the voltage follower has a voltage gain of 1 but remember, the voltage follower is still an amplifier. That is, it increases the power of the signal.

Specifically, by presenting an open (or effectively open) circuit to the preceding voltage amplifier stage, no (or effectively no) signal power is required to drive the voltage follower and thus, no signal power is lost in the output resistance of the voltage amplifier

And, by presenting a zero (or effectively zero) output impedance to the load, there is no (or effectively no) power lost in the output resistance of the voltage follower.

We say then that the voltage follower is a buffer between the voltage amplifier and the load.

• nice explanation that was I was asking for. but does the voltage from from 1 kohm happens in a scenario where we have very long cable? thnx Nov 17 '13 at 12:48
• @user16307, a very long cable will have some resistance associated but how much depends on a number of factors and, depending on the frequency of operation, the resistance might be a minor factor compared to others. Please provide more context. Nov 17 '13 at 13:16
• i mean what can that 1 kohm Rout be in your figure in a real life scenario if not the cable? what i understood from your explanation is that the voltage follower is preventing the voltage to drop across that 1kohm. because normally the current is following through it in case of no voltage follower. but when we have a voltage follower there is almost no current drop because the voltage divider's input impedance is too high and current is almost zero so no voltage drop occurs. Nov 17 '13 at 13:42
• @user16307, for a real life example, the output resistance of a common-emitter voltage amplifier is approximately equal to the value of the collector resistor. But, the voltage gain is proportional to this value so, the higher the voltage gain, the higher the output resistance. See: en.wikipedia.org/wiki/Common_emitter#Characteristics Thus, you often see a cascade of a voltage follower (e.g., common-collector amplifier) stage immediately following a common-emitter stage. Nov 17 '13 at 13:50
• so that 1kohm is inside the IC or? Nov 17 '13 at 14:01

Voltage followers are generally used to isolate stages from each other. A voltage follower generally has a high input impedance and a low output impedance. This means that whatever circuit is supplying the input signal does not have to provide much current, while the output of the voltage follower can supply significantly more current to the next stage.

Voltage followers can be used to isolate filter stages from each other when building multistage filters. They can be used to isolate sensors from readout electronics - e.g. separate a thermocouple or thermistor from an ADC. They can be used for driving ADCs as ADCs can draw current in large bursts when they sample their input, and this can be disruptive to whatever circuitry might be sourcing the signal.

2 examples:

1) imagine you have a fixed voltage source of 8V and you need to get a 4 V out of it. A simple voltage divider would not work since the voltage would depend on a load. A voltage divider followed by the voltage follower would work.

2) imagine your microcontroller can supply 1 mA but you need at least 10 or 20 mA to drive something (e.g. LED, relay, etc). Most op-Amps used in voltage followers can supply more current than MC IO pins

It does what it says on the can; the emitter follows the voltage on the base but that in itself is not even half the story because a bit of wire could do the same.

The point is that an emitter follower gives the same voltage but with much magnified current capacity. The emitter can drive a motor or loudspeaker in circumstances where the signal to the base might not be even powerful enough to turn a led on.

Unity voltage gain capability of voltage follower circuit makes it eligible for obtaining desired output voltage that is analog as well as as digital

• How can voltage be digital? Aug 2 '17 at 16:39