Mosfet Analysis

Thread Starter

dalam

Joined Aug 9, 2014
58

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?
 

crutschow

Joined Mar 14, 2008
34,201
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?
 

Thread Starter

dalam

Joined Aug 9, 2014
58
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?
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?
 

Jony130

Joined Feb 17, 2009
5,487
Thank you for the video. It was helpful.
Option C and D are ruled out. Because for PMOS to turn on Vin<4V.
Why did you prefer B to A?
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.
 

crutschow

Joined Mar 14, 2008
34,201
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.
Answer A might be correct if it was >1.875V but it says <1.875V.
 

Thread Starter

dalam

Joined Aug 9, 2014
58
M1 is a P channel so for Vin<1.875V M1 will be in triode/linear region.
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?
 

Jony130

Joined Feb 17, 2009
5,487
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
http://nptel.ac.in/courses/117101058/downloads/Lec-15.pdf
 

Thread Starter

dalam

Joined Aug 9, 2014
58
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

Id2= 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
http://nptel.ac.in/courses/117101058/downloads/Lec-15.pdf
Thanks a lot Jony!! You have helped me a lot here.
I guess in case of PMOS the Vgs,Vds should be Vsg and Vsd respectively.
 
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