mosfet biasing for micro port drive

[bouvett]

hi,

i was studying mosfets from a textbook in order to learn how i should exactly bias the mosfets correctly.

an equation was derived to calculate the current both for the saturation and triode region. It included parameters such as the width of the channel, length and certain capacitances... whcih are not included in the datasheet...

so please can anyone tell me how to do such a basic thing as biassing a mosfet without actually having the parameters at hand..



any help is appreciated

thanks a lot

 

[blueroomelectronics]

Probably get more replies if this question wasn't in the microcontroller forum.

 

[Electronworks]

Most designs that use MOSFETS just use them as switches. Turn them on with a gate source voltage (greater than Vgs) and the channel (drain to source) goes low resistance. I have very rarely seen them used in their linear region. Engineers use op amps for such applications.

MOSFET amps are the exception, but they tend to use feedback to generate the bias, so the need to know the operating parameters of the FET are largely removed.

What is your application? Why do you need to know?

 

[bouvett]

i just need to know so that i can have an idea of the that will pass exactly...
i guess that is up to experimenting it's true... i just wanted to have a guidline before going to the experimenting procedure... what's is the average resistance of a mosfet biassed at some 4 volts greater than the threshold... for eg between 200 to 400 ohms? (please consider high and low current mosfets seperatly)

thanks a lot regards

 

[Noggin]

You mean when the gate is 4v above the threshold? At that point, most FETs would be fully on (assuming 4v won't kill it) and the drain to source resistance would be under a couple of ohms. High current fets could be as low as a handfull of milli-ohms.

As electronworks stated, FETs are most often used as switches.