CS stage with diode-connected load

Discussion in 'General Electronics Chat' started by screen1988, Mar 8, 2013.

  1. screen1988

    Thread Starter Member

    Mar 7, 2013
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    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?
     
  2. crutschow

    Expert

    Mar 14, 2008
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    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.
     
  3. screen1988

    Thread Starter Member

    Mar 7, 2013
    310
    3
    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: Mar 8, 2013
  4. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    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?
     
  5. screen1988

    Thread Starter Member

    Mar 7, 2013
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    3
    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?
    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?
     
  6. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    I don't know how to help you. You tell us how we can help you.
     
  7. screen1988

    Thread Starter Member

    Mar 7, 2013
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    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?
     
  8. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    That's correct.
     
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