Operational Amplifier + Diode

Discussion in 'Homework Help' started by gianx80, Feb 22, 2011.

  1. gianx80

    Thread Starter New Member

    Jan 23, 2011
    Hi, I have to do this exercise in preparation for my first and only exam about electronics. This is the scan of the exercise (the first one):


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    and this is the scan of the proposed solution:


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    I have to find the "transfer function" Vo/Vi (is this the correct English name?), Bode plot. For point 2, I can't it into English, sorry.

    I'm a total newbie, so I can't understand the proposed solution. Why, for example, Vx/Vi = 1+sRC1? You you explain me the solution step by step?

    Thanks in advance!
  2. Audioguru


    Dec 20, 2007
    Helo again.
  3. gianx80

    Thread Starter New Member

    Jan 23, 2011
    You can find the scan attached to this post.


    Exercize 1
    In this circuit we have a real diode and a real operation amplifier.
    Io = 1 microAmpere, C1 = C2 = 10 picoFarad, R = 10 kilo Ohm.

    1) Find the transfer function Vo/Vi and draw the related Bode plot.
    2) I can't translate this point, so I'm not searching any kind of advise about this point


    In the small-signal model a the diode acts like a resistor with this value:
    rd = Vt/Id = 26mV/1microAmprere = 26 kilo Ohm.
    In order do find the tansfer function we use this equivalent circuit:


    We find that:

    Vx/Vi = 1 + R/(1/sC1) = 1+ sRC1

    Vo/Vx = (1/sC2)/(rd + (1/sC2)) = 1/(1 + srdC2)

    From this we obtain:

    Vo/Vi = Vo/Vx*Vx/Vi = (1 + sRC1)/(1 + srdC2)

    [The final part of the solution is about point 2].

    Now you have all the exercise in English. Could you help me?