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Homework Help: Z(in) of circuit help?

  1. Jun 19, 2013 #1
    Z(in) of circuit help??

    1. The problem statement, all variables and given/known data
    A. 10+j2.5 (Ω)
    B. 10-j0.4 (Ω)
    C. 10-j2.5 (Ω)
    D. 10+j0.4 (Ω)

    2. Relevant equations

    3. The attempt at a solution
    the problem asks for z(in) without any frequency w given? i think i was just confused by the wording of this problem. s=sigma+jw so does that mean, j2 given means w=2?? then answer would be 10+j2.5 (A). but i'm unsure how to do this without any w.
  2. jcsd
  3. Jun 19, 2013 #2


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    Staff: Mentor

    Yeah, it looks like they are asking for Z(w=j2), so j2 must be the frequency w. 2Hz shifted by 90 degrees...?
  4. Jun 20, 2013 #3
    Remember your transfer function can be expressed in the form H(jω) = Vo(jω)/Vi(jω), where we can denote jω by s.

    Here you are trying to find the input impedance Z(i), or in other words Z(jω), where Z(jω) = Z(2j).

    So you can conclude that ω = 2 rad/s or ω = 2*pi*f = 2 and find the frequency.

    Find the total impedance of the system and substitute your value of ω.
  5. Jun 21, 2013 #4


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    Staff: Mentor

    The general expression is V(s). To express in the frequency domain, we simply substitute s with , giving V(jω)

    So V(j2) indicates V(jω) with ω=2 radians/sec.
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