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Homework Help: Small-signal input and output resistances of the circuit

  1. Jun 3, 2013 #1
    1. With the circuit provided determine the small-signal input and output resistances of the circuit.

    2. I was either given or found all the following info for the circuit:

    VA = 110V

    β = 115

    T = 300k

    quiescent base current is 12.0 microA

    R1 = 12 kOhms, R2 = 6 kOhms and Rc = 2 kOhms

    Ic = 1.38 mA, IE = 1.392 mA and IB = .012 mA

    3. Now I am stuck on how to find the small signal info. This is what I am thinking though:

    Would I change it to a small signal circuit to find the info? Then find input current and divide it by Vi?

    Am I going down the right path?

    Attached Files:

    Last edited by a moderator: Jun 3, 2013
  2. jcsd
  3. Jun 4, 2013 #2

    rude man

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    Homework Helper
    Gold Member

    That would be one way to do it.

    You realize this is a terrible design since it's overwhelmingly dependent on beta. For example, Rf is not a feedbak resistor. It should have gone from the collector to the base. Also, the computed Vcc is only about 0.84V to get ib = 12uA. I have no idea what you meant by VA = 110V.

    I have computed the approximate input & output impedances but you probably don't want those. I'll keep tabs on the thread to see what others come up with & possibly offer comments.
  4. Jun 4, 2013 #3


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    Staff: Mentor

    After determining Zin of the transistor model, you have R2 in series with it. Then R1 in parallel.

    (Ditto for query on VA?)

    I would have expected R1 to be some MΩ, and rude man is right to be suspicious about the figures you provide.
    Last edited: Jun 4, 2013
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