feedback techniques

Discussion in 'General Electronics Chat' started by lean, Oct 15, 2004.

  1. lean

    Thread Starter New Member

    Oct 15, 2004
    3
    0
    Hello everybody. I would like ask you for a curious phenomemnon by
    measuring
    the output impedance in a circuit. Take a look to this
    (www.eurobotics.org/vco/circuito.doc) simple circuit. When I meassure
    the
    output impedance using the convencional way, that is, using a voltage
    source
    at the output while shortcutting the dependences ones I get 6 MOhmns. If
    instead of this I use feedback techniques to calculate the output
    impedance I
    get 30 GOhmns... What is going on?.

    If transistor is a Mosfet.
    When I meassure the output impedance using the convencional way, that is, using a voltage source
    at the output while shortcutting the dependences ones I get 60 GOhmns. If
    I use feedback techniques to calculate the output
    impedance I
    get 60 GOhmns.
    when I work with BJT the values are different.
    What happen with rpi and ru?

    Thank you all.
     
  2. pebe

    AAC Fanatic!

    Oct 11, 2004
    628
    3
    The circuit shown on your link is an op-amp with its output fed back to its inverting input via an emitter follower. So Voutput = Vinput - approx 0.7V
    How is this used to measure output impedance? and of what?
    Can you explain how you measure output impdance 'using conventional way' and 'using feedback techniques'?
     
  3. lean

    Thread Starter New Member

    Oct 15, 2004
    3
    0
    In Mosfet:

    When I meassure the output impedance using the convencional way,

    Vx/ix= RE +ro(1 + gmRE(1+Ad))= 60Gohmios

    If I use feedback techniques to calculate the output impedance,
    I have to calculate zoA,

    and Zof= zoA(1 + A*f)

    A=Io/Vi=gmAd/1+gm*RE+RE/ro =2.47
    because Io= gm(AdVi - IoRE) -IoRE/ro

    zoA= V2/I2= RE + gmREro + ro = 6.064Mohmios

    f=RE

    Zof= 60Gohmios
     
  4. pebe

    AAC Fanatic!

    Oct 11, 2004
    628
    3
    I'm afraid that if you don't quantify the measured terms in your equations, they have no meaning to me.
     
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