# Mosfet Analysis

Discussion in 'Homework Help' started by dalam, Sep 8, 2014.

1. ### dalam Thread Starter Member

Aug 9, 2014
58
6

How should I analyse this circuit?
I did it assuming PNP MOSFET on and NPN off and then change the assumption. It became too complex. Can I solve it simply be looking at options available?

2. ### crutschow Expert

Mar 14, 2008
13,515
3,386
NPN and PNP refers to the polarity of BJTs. MOSFETs are defined as N-channel or P-channel.

The circuit is in the linear region when both MOSFETs are conducting at the same time. So under what input voltage conditions is that true? Do you know what Vtp and Vtn means?

Feb 17, 2009
3,993
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4. ### dalam Thread Starter Member

Aug 9, 2014
58
6
Yes, I know what Vtp/Vtn means. It specifies the minimum |Vgs| needed to turn the mosfet on. This is required to create the channel.
Which region should I choose ? Linear or sturation?

5. ### dalam Thread Starter Member

Aug 9, 2014
58
6
Last edited: Sep 8, 2014
6. ### #12 Expert

Nov 30, 2010
16,705
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Because A only defines one end of the region in question. Right, Jony?

7. ### Jony130 AAC Fanatic!

Feb 17, 2009
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Good question. But forget about that, I wrongly understood the question. They ask only about M1 transistor. And ask about the linear region (triode region). So in this case I choose A as a right answer.

8. ### crutschow Expert

Mar 14, 2008
13,515
3,386
Answer A might be correct if it was >1.875V but it says <1.875V.

9. ### Jony130 AAC Fanatic!

Feb 17, 2009
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M1 is a P channel so for Vin<1.875V M1 will be in triode/linear region.

10. ### crutschow Expert

Mar 14, 2008
13,515
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Egg on my face. I was thinking of M2.

11. ### dalam Thread Starter Member

Aug 9, 2014
58
6
Jony, I understand that A is correct answer among these. But what does the value of 1.875V signifies here? Why it is a boundary? Or is it to be solved by just looking at options?

12. ### Jony130 AAC Fanatic!

Feb 17, 2009
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Well for Vin>1.875 the M1 transistor leaves the linear region and enters the saturation region.
So, M1 is in linear region and M2 is in saturation.

ID1 = ID2
Id1 = Kp*(Vgs - Vtp)*Vds - Vds^2/2

Id1 = 0.5*Kn*(Vgs - Vtn)^2

Then
(Vin - Vdd - Vtp)*(Vout-Vdd) - (Vout - Vdd)^2/2 = 0.5*(Vin - Vtn)^2

And M1 leaves the linear region when Vds = (Vgs - Vtp) hence Vout - Vdd = Vin - Vdd - Vtp
then
Vout = Vin - Vt

And after we solve this for Vin I get 1.8333V

http://www.wolframalpha.com/input/?i=(X+-+5+-+(-1))+*+(X+-+1+-+5)+-+(X+-+1+-+5)^2/2+=+(X+-+1)^2/2

But I made a error so this solution is not true.

http://oldeee.see.ed.ac.uk/private/ee3/med/rc/week5MOSFET.pdf
http://www.prenhall.com/howe3/microelectronics/pdf_folder/lectures/tth/lecture11.fm5.pdf