# Questions and Answers regarding Amplifiers

1. Nov 2, 2009

### Air

Hello PF Members! I'm revising amplifiers so will post question here occasionally on topics which I don't understand. This isn't homework question as I am asking for my benefit but if you wish for it to be moved, it's fine. :)

Here's my first question:
I've seen the circuit model for voltage amplifiers. Is the current amplifier circuit model the same but the only difference is that we consider the current gain and the input current (Aiii instead of AvVv)? I didn't see a different chapter for current amplifier so I assume it is the same?

2. Nov 2, 2009

### vk6kro

They are similar.

Usually you consider something as a current amplifier if there is appreciable current flow at the input (like a bipolar junction transistor for example) and if the current gain tends to be linear, again like a BJT.

Sometimes the real intention is to amplify current. You might be trying to operate a relay or light a LED.
Sometimes you really intend to produce a larger voltage but the device you are using has a non linear resistance vs voltage relationship on its input, but a linear current gain. So, it is easier to work in current.

With devices like FETs, you have no choice. There is essentially no current input so you have to regard it as a voltage amplifier or a voltage to current converter.

So, it is not really a clear-cut decision of whether something is a current amplifier or a voltage amplifier.

When modelling a current amplifier, you have a current generator as an output and the signal current out is a fixed factor (hfe) times the input signal current.

You convert these values to voltages by multiplying by the resistances they pass through.
The input voltage is the input signal current times the resistance of the base emitter junction (if it is a BJT) and the output voltage is the output of the current generator times the load resistance (in parallel with the output resistance of the transistor).
The voltage gain is the ratio of these two voltages.