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Negative Gain and Resistance

  1. Mar 21, 2015 #1

    Zondrina

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    1. The problem statement, all variables and given/known data

    Calculate ##R_S## given the expression:

    $$A_v = - \frac{g_m (R_L || R_D)}{1 + g_m R_S}$$

    The parameters ##g_m = 1.292 \frac{mA}{V}##, ##R_L = 180 k \Omega##, ##R_D = 18 k \Omega##, ##A_v = 5 \frac{V}{V}##.

    2. Relevant equations


    3. The attempt at a solution

    My question is about the negative sign. When I go to compute ##R_S##, do I pretend the negative sign isn't there or something? If I leave the negative sign in the calculation I get:

    ##R_S = - 4.05 k \Omega##

    Resistances must be positive, so the above does not make physical sense.

    If I pretend the negative sign isn't there then I get:

    ##R_S = 2.5 k \Omega##

    What is the proper way to compute the resistance?

    EDIT: I believe the gain should be ##A_v = -5 V/V## even though it was given as 5, so the resistance ##R_S = 2.5 k \Omega## should be correct.
     
    Last edited: Mar 21, 2015
  2. jcsd
  3. Mar 21, 2015 #2
    This is for a common-source amplifier with source resistance ##R_S##? If so, then you're right about ##A_v##.
     
  4. Mar 23, 2015 #3

    rude man

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    Generalizing post 2 a bit, I'd venture that the circuit is an inverting stage of some kind, be it MOSFET, JFET, BJT, vacuum tube, etc. - based.
     
  5. Mar 23, 2015 #4
    I just guessed it was FET-based from the subscripts, i.e. D, S, L for drain, source, load.
     
  6. Mar 23, 2015 #5

    rude man

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    Sure.
     
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