Doubt in comparators

Discussion in 'General Electronics Chat' started by Shikhar, Feb 4, 2010.

  1. Shikhar

    Thread Starter New Member

    Dec 21, 2009
    I have a very basic doubt in schmmit triggers...suppose if I connect the required power supplies to the op-amp (ie. V+ and V-) and make the desired connections(see attached circuit). Now, what would be the output when Vin=0, ie. before the signal is applied.
  2. thyristor

    Active Member

    Dec 27, 2009
    That's a very perceptive question.

    If you switch on with Vin at 0v then the Schmitt will not yet be acting as a Schmitt as you haven't yet passed a trigger point.

    Vout will adjust itself until the the 2 inputs (+ and -) are the same. So if Vin = 0 at switch on, then Vout will also equal zero.

    In fact, until a trigger point is passed, Vout = Vin (1 + R2/R1) as for a non-inverting amplifier. Once a trigger point is passed, it stops becoming a non-inverting amplifier and becomes a Schmitt trigger.
  3. steveb

    Senior Member

    Jul 3, 2008
    This isn't right because the OPAMP has positive feedback. The solution Vout=0 is like trying to balance a ball on a pencil. It is theoretically possible, but does not happen in practice.

    Any disturbance will cause the output to go to one of the rails (V- or V+). I don't think you can say which way it will go unless details of the exact opamp used, as well as the startup conditons are given. Basically, you don't know which way the output will go.

    You either need to install additional startup circuitry to be sure of which state occurs at power-up, or you need to bring the input signal outside the indeterminate hysteresis range of voltage.

    The same type of problem occurs with flip-flops in digital circuits. That's why reset and preset pins are usually provided.