How does FET amplifier amplify?

[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

 

[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.

 

[Outrageous]

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.

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,

 

[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.

 

[alphy]

for your question. FET amplifiers, or Field Effect Transistor amplifiers, work differently than BJT amplifiers. In a FET amplifier, the input signal is applied to the gate terminal, which controls the flow of current between the source and drain terminals. The voltage supplied to the gate determines the amount of current that flows through the FET, which in turn affects the output voltage.

Unlike BJT amplifiers, FETs do not require a separate DC voltage supply to keep them active. The gate voltage itself is enough to control the flow of current through the FET. This means that FET amplifiers can operate using only a single power supply, making them more efficient and simpler to design.

In summary, FET amplifiers amplify the input signal by controlling the flow of current through the FET using the gate voltage. The output voltage is then amplified based on the amount of current that flows through the FET. I hope this helps clarify how FET amplifiers work. Let me know if you have any further questions.