Source impedance

Discussion in 'General Electronics Chat' started by atferrari, Apr 4, 2011.

  1. atferrari

    Thread Starter AAC Fanatic!

    Jan 6, 2004
    I am trying to grasp properly the concepts of output impedance and input impedance.

    Regarding the first, I think I have a minimal idea but trying to calculate it in a real case, I fail miserably.

    For the attached circuit I need to know the source impedance.

    My questions:

    a) Given the filter, what is the source impedance? Can you tell briefly how do you calculate (or just estimate) it?.

    b) If I apply Vout (as Vsig) to the resistive divider, what is the source impedance at the output where I get Vdiv? Can you tell again, how do you calculate (or just estimate) it?

    Gracias for any help on this.
  2. steveb

    Senior Member

    Jul 3, 2008
    My opinion is that calculating output impedance gives people much more trouble than calculating transfer functions, or even input impedance. I think this is because the idea of putting an input into an output is counter intuitive, but this is the way to calculate it.

    I'm heading out the door now, so I'll just post a PDF of something I wrote up for someone else before. I'll check back later and maybe update the document to something more relevant to your examples. Just treat this PDF as an example problem. Most likely t_n_k and The Electrician will join in on this to help you. They always have a good perspective on this subject.
  3. Jony130

    AAC Fanatic!

    Feb 17, 2009
    Without any calculate I can estimate input impedance to be equal
    Zin = R2 + ???
    To find Zin you need connect Voltage source Vin and find

    Zin = Vin/I1

    For simplify the calculations we can replace all component be Z inpedance.


    So we can write the nodal equation:
    For Va

    (Vin - Va)/Z1 = (Va - Vout)/Z2 + (Va - Vout)/Z4

    Next for Vout (non-inverting input)

    (Va - Vout)/Z2 = Vout/Z3 (2) Form this we can solve for Va

    Va = Vout* (Z2 +Z3)/Z3 (3)

    Combining equations (1) and (3) and find Vout

    Vout = Vin * (Z3*Z4) / (Z1*Z2 + Z1*Z4 + Z2*Z4 + Z3*Z4)

    And now

    I1 = (Vin - Va) / Z1 = (Z2 + Z4)/(Z1*Z2 + (Z1 + Z2 + Z3)*Z4)


    Z1 = 1/I1
    = (Z1*Z2 + (Z1 + Z2 + Z3)*Z4) / (Z2 + Z4)

    Now we can replace
    Z1 = R2 ; Z2 = R1; Z3 = 1/sC2 ; Z4 = 1/sC1
    And we get full expression for Zin

    I don't understand what you mean by "source impedance".
    Vsig/Isig = R3+R4
    Vdiv/Idiv = R3||R4 for Vsig=0V