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Little question

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Franzisko

New Member
Hello people! I'm Francesco from Rome, Italy. I'm trying to learn electronics. I'm stuck on exercise... I don't understand a stupid thing.

I've posted the exercise...
My italian edition of sedra smith, says that because of the Vg=+5V
the mosfet is certainly ON. We only have to understand if it is in triode or saturation region.

My question is: How we can say it is ON!?!?

We know that Vg=+5V but we don't if Vgs>Vt ...! Please, help me :)
 

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tresca

Member
Hm..i believe its actually just an educated guess.

Most of the times when dealing with diodes and mosfets, you have to guess and then work out the math to see if you are right.

Vds < vgs - vt for triode
vds > vgs - vt for saturation { both of these requice vgs > vt}
and vgs < vt for cutoff

Since the gate voltage is 5V and Rs goes to ground, you can 'guess' that Vgs > vt, so it it should be triode or saturation. If the math does not work out, then that leaves cutoff.
 

Franzisko

New Member
Thanks a lot... in fact, in this one and in the other exercises I always proceed step by step :) The strange thing is that for Sedra the nmos is obviously on :D
Excuse me for my bad english... This summer I'll be in Vancouver LOL
 

tresca

Member
I have an Sedra book and sometimes he doesnt explain things that well. He just assumes that you already know (which I hate).

Your english is ok.

Good luck.
 

audioguru

Well-Known Member
Most Helpful Member
The source resistor is the same as the drain resistor. So the gate-to source voltage is divided twice to only 2.5V. Even though the Mosfet current is very low at 833uA many Mosfets need a gate to source voltage of 4V (the threshold voltage) to conduct only 250uA.
Look at the spec's for the Mosfet in its datasheet.
 
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