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A question about an opamp or comparator application

  1. Jan 7, 2017 #1
    hello dears
    i have a question at op amp
    could i use the comparator to compare between to differential signals .. more specific if i have tow signals out of phase (180) and unequal amplitude could i use op amp comparator or differential amplifier to distinguish which of them is higher amplitude ??
     
  2. jcsd
  3. Jan 7, 2017 #2

    Baluncore

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    An op-amp is designed to have a very small voltage difference between the inputs. It has an output voltage that can be stable at any value.

    A voltage comparator has inputs that are happy to have a big voltage difference. The output is designed to be either high or low, with no stable point between high and low.

    A difference amplifier would be made from one or more op-amps. It would produce an output voltage, (relative to a reference), that was an accurate multiple of the differential input voltage.

    So you should use a voltage comparator.

    Some op-amps might work in your application, but it would not be good design practice. Power consumption and recovery time of the op-amp might be high. Differential input protection clamps might connect the inputs in some situations which could corrupt high impedance input signals.
     
  4. Jan 7, 2017 #3
    thank you .. i thought to use inverting op amp to invert one of these signals (with unity gain) and then use a comparator is this true ?? it just idea not a practical excercise
     
  5. Jan 7, 2017 #4

    Baluncore

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    To compare the amplitude of two sinewaves can be tricky. There are many pitfalls.

    You could put the waves through “rectifiers” then low-pass filter and compare the DC outputs.

    The difference between two in-phase sinewaves will have a phase of 0° or 180° depending on their relative amplitude. Multiplication by a reference sinewave will synchronously detect the phase of that difference. The average, Σ( s1 * ( s1 – s2 ) ) can be fed to a comparator.

    For waves that are 180° out of phase they can be added or averaged with a couple of resistors before being synchronously detected and low-pass filtered. Σ ( s1 * ( s1 + s2 )/2 )

    A specification of the range of frequency, amplitude, and the DC offset of the waves is needed before a reliable circuit can be designed.
     
  6. Jan 9, 2017 #5

    Baluncore

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    Having thought about this problem a bit more, it turns out that there is a neat and reliable way to make the decision as to which is biggest. It works quite independently of whether the waves are in 0° phase, or are 180° out of phase.

    We call the two waves x and y. First we use a couple of op-amps to compute the sum and the difference of x and y. That gives us the two voltages, sum = x+y and dif = x–y. Then we multiply those two together, (x+y) * (x–y) = x2 – y2. The average of that product over time will be a positive or negative voltage depending on the absolute amplitude of the input waves.

    Because the sum and difference were computed, we have derived a big “reference” and a smaller “signal” that flips in phase. The symmetrical multiplication has effectively removed the need to know which is the smaller signal and which is the bigger reference phase, which has also eliminated the need to know the initial phase of the input signals.

    The multiplication is detecting the phase of the smaller signal. In this situation, phase can be detected without an analogue multiplication. Feeding the sum and dif voltages to a couple of comparators, followed by a digital XOR gate will do the job. Low pass the digital output of the gate, then voltage compare that with half the gate supply voltage to decide if x or y had the greater amplitude.
     
  7. Jan 10, 2017 #6

    NascentOxygen

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    What voltage are these sinewaves? Does their amplitude remain fixed, roughly?
     
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