CS stage with diode-connected load

Thread Starter

screen1988

Joined Mar 7, 2013
310
Hello everyone,
I am new to electronics. Today, I have a question to ask you.
The circuit attached is CS stage with diode-connected load. My book says that the output voltage equals VDD- VTh2 if Vin < VTh1. I am confused because when Vin < VTh1 M1 is in cut off and it will cause open circuit for the circuit including M2. Therefore, is it possible for M2 to conduct and saturate?
 

Attachments

crutschow

Joined Mar 14, 2008
26,361
M2 will act as a source follower to charge the stray capacitance on its source node. It will stop charging when the voltage reaches the threshold voltage of M2.
 

Thread Starter

screen1988

Joined Mar 7, 2013
310
M2 will act as a source follower to charge the stray capacitance on its source node. It will stop charging when the voltage reaches the threshold voltage of M2.
Thank you, crutschow but I still don't get it. Can you explain a little more?
I don't know how the stray capacitance is created and how to draw it. Do you mean that stray capacitance is caused by oxide layer?
P.S Now I think that you mean stray capacitance is caused between source node of M2 and ground but how can I know the capacitor will stop charging when the voltage reaches the threshold voltage of M2?
 
Last edited:

Ron H

Joined Apr 14, 2005
7,014
Thank you, crutschow but I still don't get it. Can you explain a little more?
I don't know how the stray capacitance is created and how to draw it. Do you mean that stray capacitance is caused by oxide layer?
P.S Now I think that you mean stray capacitance is caused between source node of M2 and ground but how can I know the capacitor will stop charging when the voltage reaches the threshold voltage of M2?
M1 has capacitance from drain to substrate and from drain to gate. M2 has capacitance from source to gate and from source to substrate.
how can I know the capacitor will stop charging when the voltage reaches the threshold voltage of M2?
You just said that M1 will be off if Vin<Vth1. Keeping in mind that Vin=Vgs1, don't you see that the same rule applies to M2?
 

Thread Starter

screen1988

Joined Mar 7, 2013
310
M1 has capacitance from drain to substrate and from drain to gate. M2 has capacitance from source to gate and from source to substrate.
You just said that M1 will be off if Vin<Vth1. Keeping in mind that Vin=Vgs1, don't you see that the same rule applies to M2?
Thanks Ron,
I think I have to learn more about this, it can't get it. I want to ask you more about how to determine capacitors between nodes of transistor.
In NMOS in this case assuming that subtrates B1 and B2 are grounded. Then how can I know which capacitor have to be considered. In my opinion if two nodes that have difference in voltage then I have to consider it capacitance, right?
You just said that M1 will be off if Vin<Vth1. Keeping in mind that Vin=Vgs1, don't you see that the same rule applies to M2?
I only know that in NMOS if Vgs < Vth it will be cut off but I can see the similar in M2. Will you help me again?
 

Thread Starter

screen1988

Joined Mar 7, 2013
310
I don't know how to help you. You tell us how we can help you.
OK, thanks!
I think I have just had my answer. Because M1 is cut off => there is no current through M2 => Vgs (M2) is equal to VTh2 and therefore Vout = VDD - VTh2.
Is it right?
 
Top