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How do I go about designing a threshold amplifier?

Discussion in 'Electronic Basics' started by MRW, Mar 23, 2007.

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  1. MRW

    MRW Guest

    Greetings everyone!

    I was using a comparator as a threshold detector for part of my
    circuit. Then, I realized that without any sort of feedback, the
    output will most always be close to the VCC supply levels. I'm trying
    to envision a threshold amplifier circuit, but my head is clouded.

    Pretty much I need something that will output zero if an input signal
    does not meet the threshold level and output a replica of the input
    signal with no gain if it surpasses the threshold level. I am trying
    to come up with this circuit using only the following components:
    opamps, comparator, resistors, capacitors, etc.

    Any tips to get me started?

  2. Bob Myers

    Bob Myers Guest

    So consider this - what you're looking for is basically an
    amplifier whose output is gated by a comparator
    looking at the same input signal, no?

    Bob M.
  3. MRW

    MRW Guest

    What means "output is gated by a comparator"?
  4. Rich Grise

    Rich Grise Guest

    You have an ordinary amplifier, with an electrically controlled switch
    in its output lead, and alongside it, you have a comparator, that
    determines if the input is above your threshold. When the input is
    above the threshold, the comparator's output turns the EC switch to
    "amp output", and when the input is below the threshold, it turns
    the EC switch to "threshold level".

    Hope This Helps!
  5. MRW

    MRW Guest

    Hmm.. Thanks, Rich! I think I see what you are saying.

    I can get a CMOS SPDT and have one of the inputs connected to the
    amplifier output and the other input to one of the comparator input
    connected to the threshold, reference voltage. Then, I can also tie
    the amplifier output to the other comparator input and tie the
    comparator output to the logic input of the SPDT. The output of this
    SPDT would either be amplifier output or threshold output.

    Does this sound about right?

    I'm a bit concerned with tying the amplifier output to the SPDT input
    since I've read some application notes suggesting that the switch
    inputs should be tied normally to high impedance inputs.

  6. jasen

    jasen Guest

    have a look in the op-amps chapters of "art of electronics"

  7. Rich Grise

    Rich Grise Guest

    As long as you don't exceed the input spec (which will be on the datasheet
    somewhere), you should be fine.

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