# How does FET amplifier amplify?

1. Dec 25, 2013

### Outrageous

BJT ,the output voltage is amplified because there is dc voltage supply to keep BJT active ,but what about the FET? The voltage supplied only control the gate only, anything to do with the output?
Thanks

2. Dec 26, 2013

### vk6kro

FET amplifiers also get a power supply to the output.

In a typical amplifier, the FET is placed in series with a resistor across the supply voltage.

The resistance of the FET is then varied by controlling the gate voltage and this causes the voltage across the FET to change. It behaves as part of a voltage divider where it is varying in resistance.

3. Dec 26, 2013

### Outrageous

Thank you. If I want to calculate the output impedance, I have to off the input signal, yet there are still 2 dc supplies ,one to gate-source, another is the $V_cc$ , and there should be current pass though , but my book said $g_mV_{gs}$ =0?
From the picture you gave,

#### Attached Files:

• ###### image.jpg
File size:
51.5 KB
Views:
56
4. Dec 26, 2013

### vk6kro

There is not a supply to the input (left hand side) in this circuit.

The control voltage (your input signal you want to amplify) is across from Gate to ground.

In other circuits, there may be a bias voltage in the input circuit.

Your book is using small signal parameters to calculate the output impedance of the amplifier. There are figures you can get from the manufacturers data sheets and refer to a particular type of FET. You can't read some of these figures directly from a circuit diagram.