3 Stage FET-BJT Amplifier Analysis

Discussion in 'Homework Help' started by eceturtle, Nov 24, 2015.

  1. eceturtle

    Thread Starter New Member

    Jul 18, 2015
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    upload_2015-11-24_20-3-48.png
    We're supposed to analyze this circuit and this particular question asks us to find the drain current of the mosfet. I keep getting an answer of Id = 3.54mA and Vds = 6.3675V. Since the gate voltage is zero Vgs = 0-Vs= -Vs. Using the current flowing through the source resistance, I came up with Vs = IsRs - 7.5V. Since the source current is equal to the drain current, Id = 0.5*kn*(Vgs-Vtn)^2 = 0.5*kn*(-Vs-Vtn)^2. I plugged in the eqn for the source current in the expression for Vs which leaves Vs as the only unknown and then obtain Vs = -1.8411V, then I plug in Vs into the eqn for the drain current and solve for Vd, for which I get Vd = 4.5264V. Thus Vds = 6.3675V.
    Can someone point out where I'm making an error?
     
  2. WBahn

    Moderator

    Mar 31, 2012
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    Are you taking into account lambda? Though it is small enough I would not expect it to have a huge effect.

    Also, of the answers offered, how many are physically possible?
     
  3. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    But can you explain me one thing. How can this N-MOS be ON when his gate is at 0V and Vt has positive value Vt = 1V?
     
  4. WBahn

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    Mar 31, 2012
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    The gate is at 0V, but the source is pulled down toward -7.5 V through a 1600 Ω resistor.
     
  5. Jony130

    AAC Fanatic!

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    Are you trying to say is that M1 is a depletion MOSFET ?
     
  6. WBahn

    Moderator

    Mar 31, 2012
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    Nope.

    The gate voltage itself is irrelevant except in relation so the source voltage. It is the gate-source voltage that matters. If the gate is at 0 V and the source is at -2 V, then Vgs = 2 V and you have plenty of overdrive available.
     
  7. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    OK I see it now. I simply didn't notice that the source is not at ground as usual.
     
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