The transistor, common base circuit are current follower circuit, with unity gain, and zero degree phase shift , while the common emitter circuit the higher gain offering, voltage follower, with high input impedance, and 180 degree phase shift, good for weak signal to be amplified, though could be changed to unity gain if needed....

Dear Ali Biabanifard ,

The main feature is that:

unity gain,

high input impedance ideally infinite

Out impedance low ideally zero.

Very wide bandwidth

There are many circuits that can be work a a buffer such as:

-The common collector or the emitter flower transistor amplifier.

- Also the common drain or the source follower amplifier.

- The unity gain op amp circuit.

As a practical requirement:

The input impedance must be much greater than the source impedance

The output impedance must be much smaller than the load or the termination impedance.

For the CC OR the emitter follower please see the materials in the link:Book The bipolar transistors, theory and basic applications

Such circuits are used buffer high impedance sources from relatively low impedance loads. Or to drive stronger loads from weak sources.

Best wishes

The voltage followers are considered as voltage buffer, changing nodal impedance to provide better matching. Ali Biabanifard

Dear Ali Biabanifard ,

As has already been mentioned by some other respectable researchers, a voltage follower (or a voltage buffer) is a voltage amplifier with unity-gain (gain=1), along with sufficiently large input impedance and sufficiently small output impedance (I will not repeat the other details).

It may seem like a voltage gain equal to 1 (which may even be slightly smaller than 1, practically) is useless for voltage amplification and it may seem irrelevant to call it an "amplifier". However, since a voltage follower can contribute to amplification by preventing excess attenuation (e.g. caused by excess voltage division), it can well be regarded as an amplifier.

The same conclusion can be reached from power gain point of view: The magnitudes of output and input voltages of a voltage follower is naturally equal, however input current (hence input power) is small (because of large input impedance) whereas output (load) current (hence output power) can be significantly larger (thanks to small output impedance which helps driving small loads without much losses). Thus, a voltage follower is likely to provide significant power gain, which again reveals that it is an amplifier.

Best regards...

It is a non-inverting and unity gain buffer, that uses a single operational amplifier. These have two characteristics like input impedance is high and output impedance is low. They strengthen the signal by allowing high impedance sources & drive a less impedance load.

Voltage follower is used as an impedance matching circuit. It can also be used as an unity gain buffer. It plays a significant role in low signal applications.

A nice and easy to follow article about voltage follower is here:

http://www.learningaboutelectronics.com/Articles/Voltage-follower

A voltage follower (also called a unity-gain amplifier, a buffer amplifier, and an isolation amplifier) is a op-amp circuit which has a voltage gain of 1.

This means that the op amp does not provide any amplification to the signal. The reason it is called a voltage follower is because the output voltage directly follows the input voltage, meaning the output voltage is the same as the input voltage. More about gain, impedance matching, purpose and features can be found there. Best regards.

All the answers here are correct... but Ali Zeki's answer impresses with its originality. Because it is easy to repeat banal explanations of the dominant paradigm... but it is difficult to express a new, own point of view, on such a famous circuit solution. I mean his assertion that "since a voltage follower can contribute to amplification by preventing excess attenuation (e.g. caused by excess voltage division), it can well be regarded as an amplifier"... and then his considerations about the ratio between the input and output currents at the same voltage.

I will accept this "challenge" and try to explain even more simply the philosophy behind this circuit, without using vague terms such as "high input resistance" and "low output resistance". Here is my idea:

1. Additional voltage source. The transistor connected in such a weird way does a simple job - it creates a copy of the "weak" input voltage across a “powerful” load. What is most important here is that, for this purpose, it uses the energy of an additional powerful voltage source (power supply) and not the energy of the low-power input voltage source.

We can see the difference if we disconnect for a while the collector from Vcc. Thus the load will be connected directly (through the base-emitter junction) to the input source… and the full load current will be consumed from the input source… if the latter can provide it... and the base-emitter junction can endure it...

2. Active copying. We can see this powerful idea all around us where weak beings make strong ones fulfill their desires:-) The conventional (meaningless) name of this idea is "negative feedback principle" and a more meaningful name can be "active copying principle".

I will illustrate various implementations of this powerful idea in the comments below (sorry that some labels are written in Bulgarian):

1. Conceptual voltage follower (a block diagram of a negative feedback circuit).

2. Man voltage follower (19th century).

Dear Cyril Mechkov ,

Thank you for your kind words.

However, the ideas I explained are not my original ideas, but ideas shared by many others.

Nevertheless, maybe the way I explained them has been regarded by you as original. :)

In any case, I thank you again for your nice words. :)

Best regards...

3. Transistor voltage follower (in the middle of 20th century).

4. Op-amp voltage follower (after the middle of 20th century).

Hmmm ... thinking in such a way, a very interesting conclusion can be drawn:

The voltage follower was invented in the 19th century?

And really, if we refer to Wikipedia (https://en.wikipedia.org/wiki/Potentiometer_(measuring_instrument)#Constant_current_potentiometer), we will see that the so-called "constant current potentiometer" is actually a voltage follower.

Dear Ali Zeki, I see... but can you give me a link to the source where this idea was expressed:-)? Because I have not met it on the web yet... but it sounds very intriguing...

Dear Cyril Mechkov ,

Many electronics books which explain amplifiers and basic transistor amplifier stages,

1. consider "emitter follower" (or "source follower") as an amplifier.

2. define an amplifier as a block which not only avails an amplified copy of the input signal at the output, but also provides power gain (e.g. A transformer's output voltage signal can be a larger copy of its input voltage signal, however it is not an amplifier because it does not achieve power gain).

Best regards...

Dear Ali Zeki,

I see... Everything is already invented and explained... and we have just to reproduce it:-) But here I will add two more unconventional explanations for those who do not know this truth and continue to think about circuit ideas...

3. Bootstrapping. The voltage follower makes a copy of the input voltage and subtracts this copy from the input voltage. So, it can be considered as another "input voltage source" contrary connected to the original input source. As a result of this "intervention", the input current becomes (almost) zero at the same input voltage... Vin/Iin -> infinity... and, as they say, "there is extremely high input resistance". But this is not a "physical" resistance like the base-emitter resistance; this is a kind of a "virtually increased resistance". This technique is known as "bootstrapping".

So, seen from the side of the input voltage source, the circuit input seems like a "virtually increased base-emitter resistance".

4. Helping current source. It is interesting that, in the case of the emitter follower, the input voltage source still passes its (base) current through the emitter load. But the transistor adds to it its beta times higher collector current thus helping the input voltage source in its desire to change the voltage across the load. Thus it serves as a kind of a "helping source" connected in parallel to the input source; as a result the emitter current is a sum of two currents (Ie = Ib + Ic). In contrast, in an op-amp voltage follower, there is no input current flowing through the load; only the op-amp output current flows through it.

This arrangement reminds the so-called "current-inversion negative impedance converters" (INIC) where a "helping current source" is connected in parallel to the input current source. Typical examples are Howland current source and Deboo integrator.

I did not go in detail through all the comments and answers - nevertheless, I think there is one important application which was not yet mentioned:

Any active filter needs positive feedback - and because the feedback path must not load the forward circuitry, we can use a voltage follower (unity gain amplifier). Such a solution has the advantage that both - forward path and feedback path - can be designed independently. In this case, the classical feedback scheme (feedback model) introduced by H. Black can be used.