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transfer characteristics

How to Move a Trip Point of Op-Amp Comparator Circuit?

You all will be familiar of basic comparator using a Op-Amp circuit. So today we will show you how to move a trip point of a Op-Amp comparator circuit. Maybe many of people already know this, but still this is the post for beginners. So if you already know quit reading now only. 🙂

Moving a trip point

As shown in the above circuit, by application of a reference voltage to the inverting input rather than grounding it the trip can be moved. The reference voltage is given to the inverting input of a basic comparator using a potential divider consisting of resistors R1 and R2. The reference voltage Vref is derived using +Vcc and potential divider R1 and R2. Mathematically Vref is expressed as,

Vref = [+Vcc/(R1+R2)] R2

Now as long as the input voltage is less than Vref, the output is low i.e. -Vsat. When Vin becomes slightly greater than Vref, the Op-Amp output becomes high i.e. +Vsat. Thus the trip point is moved from Vin = 0 to Vin = Vref due to reference voltage applied to the inverting input terminal.

A bypass capacitor is used on the inverting input to reduce the amount of power supply ripple and noise appearing at the inverting input of Op-Amp. For effective bypassing of ripple and noise, the critical frequency of bypass circuit must be much lower than the ripple frequency of power supply.

transfer characteristics

The transfer characteristics of such a comparator is shown in above diagram which indicates positive trip point. Such a comparator is also called a limit detector as it detects the particular positive level of the input beyond which input goes high. The resistance R1 and R2 can be used to set the trip point anywhere between 0 and +Vcc.

The cut-off frequency of bypass circuit is,

fc = 1/[ 2π(R1||R2) Cby.

This must be much lower than the ripple frequency of the supply.

15 Comments

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  • hienclover

    hi all, i am vietnamese,i’m not good at english. i have a question for u? can i replace -Vee with GND? please answer me as soon as possible. thanhks!

    • hienclover

      thank you!

    • Jim Keith

      Vee may be GND for the LM324 or LM358 single supply type devices that have a common mode input voltage that extends to GND.

      Note that the LM741 normally requires ± power supply voltages, but the negative rail may be GND, provided that the input voltages are at least 3V above GND.

  • electron_revolution

    R1 and R2 act as a voltage divider and reference voltage… u can apply this as automatic battery charger or low or high temperature indicator and many more.

  • gth85

    hello every one.
    i would like to know ,how to generate threshold value in a CMOS dynamic latch comparator?

    please post ans ASAP.

  • Maureen Mendohlson

    This is a very helpful post. Although, I need time to digest all of this information.

  • sireesha

    actually, in comparator if the difference between voltages a inverting and non inverting is positive,then out put voltage is +Vsat and it becomes -Vsat if the difference between voltages at inverting and non inverting terminals of an op amp.and those are equal to +Vsat and -Vsat respectively…Why those voltages are equal to supply voltages?

  • Srihari Rao

    @Jim Keith thanks for giving such a long information.. But it is not clear that what you want to say about the post..?

    • Jim Keith

      Just adding some additional info to help round out your post. Using an op amp as a comparator does indeed function, but has notable limitations. Perhaps I should have worked up my own submission, but it is easier to ride on your coat-tails–did not intend for it to get so long.

      Have a good day, Jim

    • Srihari Rao

      Oh, thanks for the info Jim.. 🙂

  • Jim Keith

    What is the difference between a comparator and an operational amplifier applied as a comparator? The answer is twofold: surprisingly little at the IC hardware level, but surprisingly great at the application level.

    Here is the run down of the differences:

    Operational amplifiers:
    1. Relatively slow output voltage slew rate as virtually all op amps have internal compensation to add stability to closed-loop amplifier circuits. This adds substantial propagation delay so that it is unable to respond to rapid changes in the input signal. Due to this limitation, an op amp could never be used to generate a logic clock signal.
    2. Active pull-up in the output circuit eliminates the requirement for a pull-up resistor. While it is nice to be able to eliminate this resistor, it precludes the possibility of interfacing the output to logic ICs that often use a lower Vcc.

    Comparators:
    1. No internal compensation–very fast output transition and minimal propagation delay.
    2. Ability to interface directly to logic circuitry that generally operates at a lower Vcc. In this case, the pull-up resistor connects to the logic Vcc. In some high level comparator ICs, the output may also be grounded to the logic common–in this case, there may be up to 4 power supply rails: Vcc (15V), Vee (-15V), logic Vcc (5V) and ground. The venerable LM311 comparator does all this as well as a host of newer designs. Note that the common LM339 (quad) and LM2903 (dual) as well as all op amp devices cannot perform this interface function because their outputs work against the negative rail.

    For hobby applications, op-amps are often used as comparators to reduce parts count because there are often multiple op-amps in a single IC package and some may be unused. (e.g. the ubiquitous LM324 quad and LM358 dual op amps)

    • sireesha

      actually, in comparator if the difference between voltages at inverting and non inverting is positive,then out put voltage is +Vsat and it becomes -Vsat if the difference between voltages at inverting and non inverting terminals of an op amp.and those are equal to +Vsat and -Vsat respectively…Why those voltages are equal to supply voltages?

  • Jim Keith

    R1 and R2 may be replaced by a potentiometer in order to make the threshold (Vref or reference voltage) variable.

    Why the concern over cut-off frequency? Here is a scenario where it could be an issue:

    In some applications, it is necessary to adjust Vref on the fly from another source such as a phase control comparator where the voltage from an AC line synchronized ramp generator is fed into Vin while the error voltage (as in a closed loop voltage regulator) is fed into the other input (Vcc in this schem). In this case, the cut-off frequency comes into play because it adds a delay in the feedback system.

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