1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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

    berkeman

    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:

    Yes

    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.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook