Quick Question, What are Amplification Factors in FET?

[freydawg56]

Homework Statement

A BJT has VA of 50 V and MOSFET has lambda(modulation length) = .02 V^-1, and has VGS-VTN=.5

What are the "amplification factors" of the 2 transistors.

Homework Equations

well, not sure, all i really want to know is what do they mean by amplification factors

The Attempt at a Solution

I'm not asking anyone to solve the problem, but I don't know what amplification factors are, or what they mean by them. I assume* they mean the things that increase the gain Vo/Vin. If anyone knows, would they like to push me in the right direction?

 

[vk6kro]

Amplification factor is a measure of the gain of amplifying devices.
It really only applies to triode valves and was always given in the data sheets for triodes.

If you have a triode valve with 10 mA of plate current flowing, it may only have 50 volts of plate voltage on it to produce this current with zero grid voltage.

Now, you make the grid negative and this current will decrease, but you can bring it up to 10 mA again by increasing the plate voltage.

The change in plate voltage needed to do this, divided by the change in grid voltage that made it necessary is the amplification factor.

For example:
Suppose you had a change from zero to -5 volts on the grid and you had to increase the plate voltage to 200 volts from 50 volts to have a current of 10 mA again.
Amplification factor = (200 volts - 50 volts) / ( 0 - (-5) ) = 150 / 5 = 30

It is not the same as voltage gain, although it is convenient as a tool for calculating voltage gain in triodes. It is a property of the valve and does not depend on the circuit components.

It is not applicable for BJTs and Mosfets as these tend to draw much the same collector or drain current regardless of the supply voltage.