I have been designing few current mode circuits in ORCAD through BJTs... now i have begin to use MOSFETs and encountering problems...
i) Which MOSFET should be used preferably?
ii) One of the MOSFETs is inoperative irrespective of the Vcc and current values...(it was inserted from BREAKOUT)... what can the problem be?
iiii) How can we change the operation modes of the MOSFETs in ORCAD?

 

I have been designing few current mode circuits in ORCAD through BJTs... now i have begin to use MOSFETs and encountering problems...
i) Which MOSFET should be used preferably?

Well, it would help to have an idea of what you need to do (voltage and current wise), however very generally, N-ch enhancement mode MOSFETs are generally preferred over their P-ch counterparts, as the gate charge is about 2.5 times higher for an equivalent P-ch MOSFET - as such, P-ch MOSFETs are more scarce.

ii) One of the MOSFETs is inoperative irrespective of the Vcc and current values...(it was inserted from BREAKOUT)... what can the problem be?

It's broken.
Seriously, you have not provided any information about it other than you imported it from somewhere, and it doesn't work.
Where is the SPICE model? Can you post it?
What are the connections to your symbol that you are using? Perhaps you have the symbols' pin assignments wrong?

iiii) How can we change the operation modes of the MOSFETs in ORCAD?

What do you mean by "operation modes"? Linear vs saturated? If that's what you mean, varying Vgs will give you an idea of where the threshold of the model is, and what it needs to be in order to be saturated, or considered off.

 

Thanks for the reply.... the "MOSFET not working" problem is solved.
The model that I used was MBREAKN3 from BREAKOUT library...

Yes, by mode of operation, i meant liner and saturation...the thing is can we know threshold voltage direclty instead of varying Vgs...

 

You need to read the datasheet of a MOSFET to determine the "real world" threshold range.

If you have a Spice model for "MBREAKN3", look in the file and find "VTO=" - the number immediately after the "=" is the zero-bias threshold voltage for the model.