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Homework Help: Question about bistable multivibrator op amps

  1. Feb 22, 2014 #1
    1. The problem statement, all variables and given/known data
    Here is the problem:

    So this is a bistable multivibrator.

    2. Relevant equations
    KVL and KCL laws

    3. The attempt at a solution
    I don't entirely understand bistable multivibrators. I get what the solution is, the input voltage caps at 0.1V and the output caps at 0.7V and hysteresis occurs. Apparently, the output voltage will feed back into the + terminal of the op amp and this loops repeats itself until saturation... Why doesn't this happen with a regular non-inverting amplifier. The configuration is almost the same except the terminals are flipped... What does this mean?

    However, when I do some circuit analysis I notice that if the input is greater than 0.1V weird things happen. For example take a look at what I did here:


    So when you have 5V as the input, what happens to the current flow, does some of it just magically get capped?

    Another question I have is what is that 10k resistor right after the op amp used for?
  2. jcsd
  3. Feb 22, 2014 #2


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    Staff: Mentor

    The equality v(+) = v(-) for an op-amp holds only while the op-amp is operating linearly and ouput is not saturated.

    If those conditions are not met, then it's the case that
    v(+) v(-)
    Last edited: Feb 22, 2014
  4. Feb 23, 2014 #3

    rude man

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    Homework Helper
    Gold Member

    Your feedback here is positive. In a linear noninverting amplifier the feedback is negative. BIG difference!

    Use the fact that the op amp output is +12V if input V+ is higher than input V- , and -12V if V- > V+.

    So you're always comparing input voltages V+ vs. V- as you vary the V- voltage back & forth. Hint: limit your V- input swing to +/- 0.2V.

    The 10K output resistor limits the op amp output current to a safe value.
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