# Should a linear amplifier be considered as a current source or a voltage source?

Linear amplifiers such as class A, B, and AB can deliver current to the load. Then should it be regarded as a current source?

However, a current source should have a high output impedance while a voltage source should have a low output impedance. Now consider the following class B amplifier. It is in the emitter follower configuration. The impedance looking into the emitter is low and hence the output impedance should be low. So in this case, should this linear amplifier be regarded as a voltage source instead of a current source?

Consider another class B amplifier configuration in the following diagram. Now it is in a common source configuration. The impedance looking into the course is high and the output impedance is also high. So in this case, should the power amplifier be considered as a current source?

So as a whole, can we say that a linear amplifier can either be a current voltage or a voltage source depending on its configuration?

From the Thevenin and Norton theorems we know that any source (aside from an ideal voltage or current source) can be considered as either a voltage source (Thevenin equivalent) or current source (Norton equivalent).

Practically, if the source has a low output impedance, it will behave more like an ideal voltage source (its output voltage will be insensitive to the load impedance) and it's usually more useful to describe it as a voltage source. And if it has a high output impedance, it will behave more like an ideal current source (its output current will be insensitive to the load impedance) and it's usually more useful to describe it as a current source.

A class B amplifier is designed to have a low output impedance, therefore we will usually describe its output as a voltage source.

Even a class A amplifier, which is a common emitter or common source stage, and has a higher output impedance than a class B amplifier, usually has a lower output impedance than the load it's designed to drive, so it will usually be described as a voltage source.

If you want an example of a current source, look for things like current mirror circuits, or op-amp circuits designed to deliver (near) constant current to their loads.

All of these are linear circuits. But that is a very wide category --- basically meaning the output is proportional to the input --- and isn't restricted to only voltage sources or only current sources.

In the great majority of cases, there will be voltage negative feedback such that the output impedance of the entire circuit can be considered very low - being much lower than the load that will be connected.

As a result, the circuit can be considered a voltage source.

In very rare cases there will be current negative feedback to give high impedance, in such cases, it can be considered a current source. The applications for a current source are very limited though are unusual.