JFET voltage gain.

Discussion in 'Homework Help' started by Petrucciowns, Jul 21, 2009.

  1. Petrucciowns

    Thread Starter Active Member

    Jun 14, 2009
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    Hi, does anyone know the equation to calculate voltage gain on a simple JFET circuit with 1 drain resistor ,1 gate resistor, and 1 source resistor?

    Thanks.
     
  2. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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  3. scythe

    Active Member

    Mar 23, 2009
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    do you have any specs of the JFET? that would be needed for any type of analysis, as well as information on how the JFET is hooked up to anything else in the circuit
     
  4. Petrucciowns

    Thread Starter Active Member

    Jun 14, 2009
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  5. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    In the absence of any other information a reasonable approximation for the gain of the amplifier is

    Av=-Rd/Rs = -5000/500 = -10

    [More accurately Av=-gm*Rd/(1+gm*Rs)]

    So if gm*Rs >> 1 then the approximation is reasonable.

    Note : For gain Av above the output is taken at the drain and the input at the gate.

    The 1MΩ gate resistor to ground establishes the self bias condition - which you are told gives Id of 1 mA. It won't contribute to the gain other than when it might load the signal source, but that's unlikely at 1MΩ and we don't have any information regarding the nature of the signal source in this example.
     
  6. Petrucciowns

    Thread Starter Active Member

    Jun 14, 2009
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    Awesome, thanks TNK you are always so helpful, and detailed.
     
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