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Homework Help: Voltage at the output/Parallel resistors

  1. Aug 30, 2009 #1
    1. The problem statement, all variables and given/known data

    Reading the voltage taken at the output, Vo.

    2. Relevant equations

    Resistors in parallel carry the same voltage.

    3. The attempt at a solution

    The circuit shown in the diagram, the output is taken directly in parallel with Ro, so shouldn't that mean that:

    VRL = VRD = Vro ?

    In calculating the output, they combine all those 3 resistors in parallel into RL', I don't understand why.

    In other words, why doesn't Vo = VRL ?

    Attached Files:

  2. jcsd
  3. Aug 31, 2009 #2


    User Avatar

    Staff: Mentor

    The output voltage is indeed the voltage across those three parallel resistor components. It doesn't matter if they are drawn as 3 resistors or the equivalent single resistor, unless some individual current is needed for some other part of the model calculation.
  4. Aug 31, 2009 #3
    I might be repeating the above post:


    You don't know the voltage across either of those three. But the total current passing through all three resistors is g_m*V_gs which is known as I am assuming. You can go solve the circuit in either way: it's a simple circuit with three resistors in parallel and one current source. Simplest possible method is to combine all three ll resistors and find the voltage.
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