Schmitt Trigger

Discussion in 'Homework Help' started by Cesc_1984, Jan 6, 2010.

  1. Cesc_1984

    Thread Starter New Member

    Dec 28, 2009
    If an op amp is used to form a basic inverting schmitt trigger, what resistor values would produce a hysteresis value of 5V.

    How is this worked out.

    Also what would be the switching points if a sinusodial wave having a peak of 5V and frequency of 10 kHz is applied to the above circuit and what would be the max and min output voltage.
  2. JDT

    Well-Known Member

    Feb 12, 2009
    A schmitt trigger is basically a comparator, where the input is connected to the -ve input and the reference (the voltage at which it switches) is connected to the +ve input. The +ve input is connected to the op-amp output via a voltage divider network.

    In this mode, the maximum and minimum output of the op-amp will depend on the supply voltage rails less some value that can be determined from the data sheet and the load current. Many "single supply" op amps will swing between zero and the positive supply less about 1.5V. This assumes no load current. Most datasheets will show graphs of output voltage against load current.

    Once you know the output voltage levels, you can design a resistive voltage divider network between the output and the +ve input that gives you the required positive and negative switching points.

    There are many textbooks and online resources that will help you.
  3. thyristor

    Active Member

    Dec 27, 2009
    You asked specifically about an inverting Schmitt trigger. For the avoidance of doubt, this is where a (say) positive input of sufficient magnitude causes the output of the Schmitt to go negative and vice versa.

    There exists also a non-inverting Schmitt trigger whereby a positive input, of sufficient magnitude, causes the output to swing positive and vice versa. This latter circuit does not utilise the potential divider type circuit of the inverting Schmitt shown below.


    The hysteresis (the gap between the trigger points) will be 2xVxR2/(R1 + R2)

    If you applied a sinewave of sufficient magnitude to trigger the Schmitt, the output will be a square wave of the same frequency as the sinewave.

    (Remember also that, in reality, op-amps don't necessarily swing to full output rail voltages)