Circuit Analysis of a Positive Offset Clamping Diode Circuit

  1. 1. The problem statement, all variables and given/known data

    Plot the waveforms for capacitor voltage VC, output voltage Vo, and diode voltage Vd given that Vs is a 20 Vpp triangle wave with period T. Use CVD model with diode VON = 0.7 V.

    [​IMG]

    [​IMG]

    2. Relevant equations

    KVLs?

    3. The attempt at a solution
    From my basic understanding of a clamper, I can see that the output is offset by +2 V. Thus Vo begins at +2v, Vc begins at -2V, and the diode voltage begins at 0V and heads towards -10V (off, reverse bias).

    However, I can't manage to show this analytically.

    KVL around the left side gives:
    Vs - Vc + Vd - 2V = 0

    At the same time, I know that Vo + Vd - 2 = 0.

    I can't really solve anything with just these two equations though.

    I can say that Vs - Vc = Vo, but these equations just take me in circles.

    Again, I understand that Vo starts at 2V and rises in step with Vs. With that said, Vo = 12 V at when Vs reaches its first 10V peak. At T/2 when the input becomes negative, the diode turns on and the capacitor can start charging. With the diode on, output Vo is clamped to 2V - 0.7V = 1.3V. The 0.7V is the diode drop from the CVD model.

    It stays on until 3T/4. By that point, the capacitor has charged to -11.3V. From 3T/4 onwards, diode remains off. Capacitor has no discharge path and remains at -11.3V. At the second 10V peak, Vo is 21.3V.

    I just don't know how to show ANY of that with work, which doesn't earn me any points when I have to analyze this on a test.
     

    Attached Files:

    Last edited: Oct 21, 2012
  2. jcsd
  3. NascentOxygen

    Staff: Mentor

    Hi stn0091. [​IMG]

    Do you have simulation software that you can construct this and see precisely what it does? It helps a lot when you know what you are aiming to explain. :smile: There can be a big difference between how a circuit behaves for the first so many cycles, and how it operates well down the track once it has settled down to a steady state.
     
  4. Yes, I simulated the circuit using PSPICE. Just from observation of the circuit, I can tell how it behaves for the first few cycles, even without the simulation. I just can't manage to write it down with some work.

    At 0 seconds, is the circuit assumed to be in DC steady state? Is it valid if I say the entire circuit is assumed to have been off for a long period of time such that the capacitor is discharged and open and there is no current anywhere in the circuit?

    If that's the case, that might help a bit.

    Under DCSS at 0-:

    Vs = 0V

    Vc = Vs - Vo

    Vo = -Vd + 2

    There is no current, so there's no drop across the diode so Vd = 0V.

    Working back up that list up equations,

    Vo = 2V
    Vc = -2V

    Those numbers agree with my intuitive understanding of the circuit and what PSPICE says.

    Question: I said that the capacitor was fully discharged at DCSS for t = 0-. How can Vc = -2V in that case? Shouldn't it be zero? Or does "discharged" just mean any value < 0? This stuff from basic circuits is really throwing me off.

    That was all at DCSS at 0- seconds. Now turn Vs on.

    Diode will be in reverse bias so the circuit is still "off."

    With KVL, I again have:

    Vs - Vc - Vo = 0

    Capacitor has no path so Vc remains constant at -2V. Let's say I'm at the first 10V peak at T/4 seconds.

    10 - (-2) - Vo = 0
    Vo = 12V

    Is that all logically sound so far? Are my assumptions valid?
     
    Last edited: Oct 21, 2012
  5. NascentOxygen

    Staff: Mentor

    Umm, I was hinting that you should post a graph of vā‚€(t) over multiple cycles. [​IMG] This might allow us to cut to the chase with the explanation.
    For this circuit, the only current path includes the capacitor, and since we can't have a steady current into a capacitor without its voltage soaring towards infinity, then, yes, the current must have zeroed.
    The 2V source is not part of the input and is not switched in at t=0; that 2V is always present so it takes part in determining the equilibrium conditions before t=0.
    It's right so far.

    We need your graph of vā‚€(t).
     
  6. Not exactly a triangle wave, but it still follows the same general shape. Top/pink is Vo, middle/purple is Vs, bottom/red is Vc.

    [​IMG]
     

    Attached Files:

  7. NascentOxygen

    Staff: Mentor

    Are those waveforms close to what you expected to find? Can you explain what they show, and some of their features?
     
  8. This is a clamper circuit.When the input is at its negative peak,the diode is on and the capacitor is charged opposite to what is shown in the circuit.The capacitor is charged to V=20+2-0.7=21.3V.
    Now when the input is at its positive peak,the diode is off and the capacitor stays at the same charged voltage.
    The output in first case is (2-0.7)V=1.3V.Voltage across diode is 0.7V
    The output in second case is 21.3+10=31.3V.Across diode is also 31.3V since open circuit.
    The output is never negative.
     
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