opamp

Discussion in 'Homework Help' started by aanchal, Jan 10, 2008.

  1. aanchal

    Thread Starter Member

    Jan 10, 2008
    11
    0
    why does output of comparator changes to plus or negativ saturation voltages(+Vcc nd -Vcc respectively) only,when difference input is given
     
  2. eeboy

    Active Member

    Sep 27, 2007
    90
    1
    The output of the ideal op amp is Vo=A(V+ - V-) where A is your open loop gain. A difference, however slight, multiplied by the open loop gain (extemely large value) will drive the output to rail.
     
  3. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    Your title contained the sole word "opamp", while your question was concerning the behaviour of a comparator. That leads me to the conclusion that you perhaps are unaware that the two components while sharing the same schematic symbol have significantly different characteristics.

    The comparator as eeboy has indicated is, as the name indicates, a device used to compare to different input voltages responding based on the sign of the difference by driving its output either high or low. Feedback when alpplied is usually positive in nature and used to provide hysterysis.

    An operational amplifier on the other hand, opamp for short, is designed to respond to the magnitude of the difference between the two applied input voltages. Feedback in an opamp is generally negative and is used to set the gain and response of the opamp

    An opamp can serve in the role of a comparator with some compromise regarding the output signal risetime when compared to a comparator. Due to the internal non-linear response of the comparator together with its propagation delay input to output is not usable as an opamp.

    These are but a few of the major differences.

    Hope this helps clarify things a bit anyway.

    hgmjr
     
  4. techroomt

    Senior Member

    May 19, 2004
    198
    1
    a comparator is usually made of 3 op amps - with a differential amp in front to monitor the two inputs, with an open loop amp as the output device. usually another differential in the middle. as explained the open loop output with large gain will drive the outputs to the maximum value which falls ~ volt short of the bipolar supply voltages.
     
  5. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    You are misusing the term "op amp", which is short for operational amplifier (Google). There are no commercially available IC comparators that contain op amps. The comparator most commonly used by hobbyists, LM339/393, has only one differential amplifier (the input stage), followed by two single-ended gain stages.
     
  6. techroomt

    Senior Member

    May 19, 2004
    198
    1
    oh wow! my bad. i shouldn't have put the "op" in there. i guess i was starting at the title too long. sorry guys!
     
  7. aanchal

    Thread Starter Member

    Jan 10, 2008
    11
    0
    sorry for if i have used the title "opamp",for my question on behavior of comparators,but what i know bout opamp is that it is made up of a balanced differential amplifier stage followed by an unbalanced differencial amp. stage,following it a level translator and an output pushpull amplifier stage,nd this when used in an open loop configuration work as a comparator.
    eeboy said that its output is=A(V+-V-),where V+ nd V- are the inputs given,when A>>large,how come the o/p=Vcc+ nd Vcc-,the answer is nt very clear.for if i see this way that since Vcc+ and Vcc- are the maximum supply voltages given nd when A>>large,o/p becomes Vcc+ nd Vcc-,but i would prefer the answer interms of stages of comparator,illustrating the whole operation for if how this supply voltages are actually transferred at the output,considering it ,say,made of BJT,as the basic device,for eg.if Vi of BJT is below cut in,transistor remains in cutoff mode and whole of the voltage (supply voltage) appears across at the collector junctn.(considering CE config.)
     
  8. aanchal

    Thread Starter Member

    Jan 10, 2008
    11
    0
    wher my msg gone?
     
  9. aanchal

    Thread Starter Member

    Jan 10, 2008
    11
    0
    so stupid of me!i m not able to search my own confusion still left after getting answers from so many whenever i login.it isn't displayed
     
  10. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
    10
    A comparator is nothing more than a particular application of the op-amp. In fact, it is the simplest application possible with an op-amp.
     
  11. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    Except that an op amp is designed to have feedback around it, and so is either internally compensated or has pins for external compensation. That compensation compromises the speed of the amplifier when it is used open-loop as a comparator. Also, comparators are generally designed to have logic-level outputs, or they have open-collector outputs so the positive level can be externally selected or they can drive a load such as a small relay.
     
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