Is there any difference between comparator and op-amp?

Discussion in 'Electronic Basics' started by fred, Feb 2, 2004.

  1. fred

    fred Guest

    Does it make any difference if I buy the lm324
    which has 4 op-amps, or the lm339 which has
    4 comparators, these are the same thing in essense
    aren't they, i.e. I can use op-amps as comparators,
    and comparators as op-amp, it all depends on how
    I wire them up, can anyone confirm this?
     
    fred, Feb 2, 2004
    #1
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  2. "fred" <> wrote in message
    news:...
    > Does it make any difference if I buy the lm324
    > which has 4 op-amps, or the lm339 which has
    > 4 comparators, these are the same thing in essense
    > aren't they, i.e. I can use op-amps as comparators,
    > and comparators as op-amp, it all depends on how
    > I wire them up, can anyone confirm this?


    Op amps and comparators are only "mostly" the same. Comparators are
    usually op amps that are optimized for high slew rate (that means real fast
    voltage swing) and also have a different output stage- one that is usually
    an open collector transistor that requires a pullup or pulldown resistor to
    function as you would expect.
    In a pinch, you can use either as the other, but remember that you get
    what you pay for.

    Cheers!

    Chip Shults
     
    Sir Charles W. Shults III, Feb 2, 2004
    #2
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  3. fred wrote:
    >
    > Does it make any difference if I buy the lm324
    > which has 4 op-amps, or the lm339 which has
    > 4 comparators, these are the same thing in essense
    > aren't they, i.e. I can use op-amps as comparators,
    > and comparators as op-amp, it all depends on how
    > I wire them up, can anyone confirm this?


    There are several important differences between opamps and
    comparators.
    Opamps can pull up or down (source or sink current). Many
    comparators, including the 339 sink current only. They rely on some
    external means to pull the output positive when the output is off.

    Comparators are usually designed to switch their outputs as fast and
    hard as possible for the least input differential voltage. Opamps are
    purposely slowed a bit, and their high frequency response rolled off,
    to let them stabilize the output voltage at any point in its swing
    whenever the inputs are brought to a match by negative feedback. If
    you connect negative feedback around a comparator, it can be expected
    to oscillate.

    Opamps usually limit output current to some maximum value with an
    active current measuring circuit, while comparators carry as much
    current as the the output switch can carry. So the maximum current
    the output will carry generally varies more with comparators than it
    does with opamps, unit to unit.

    Opamps are designed to produce an output voltage that is an arbitrary
    function of input signals, based on the input and feedback networks.
    Comparators are designed to tell you which input is more positive, as
    quickly as possible.
    --
    John Popelish
     
    John Popelish, Feb 2, 2004
    #3
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