What causes slew rate to be low in op-amps?

Discussion in 'General Electronics Chat' started by Thecomedian, Sep 16, 2015.

  1. Thecomedian

    Thread Starter New Member

    Oct 12, 2013
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    What properties of op-amps cause low slew rate? Is it best described as resistance to change? Does it have to do with the type of transistors inside the op amp, BJT vs FET? Is there some involvement of resistors or capacitors inside an op-amp? A combination of all of the above?
     
  2. #12

    Expert

    Nov 30, 2010
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    My best take on this is capacitance. Sometimes capacitance is intentionally designed into an op-amp. Only when you get to really high speeds do the type of transistors come into play. Can't remember what it's called right now but, clearing the charge out of a saturated base-emitter junction slows things down.

    The amount of complication matters, too. You can do a basic op-amp with a dozen parts, but some of them have hundreds of parts and way better accuracy. There are people a lot smarter here than I am. One or two will soon describe some other causes of low slew rate.;)
     
  3. dl324

    Distinguished Member

    Mar 30, 2015
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    It's primarily caused by compensation in the second gain stage to prevent phase instability. In addition to affecting slew rate, it also limits bandwidth.

    But there are high slew rate opamps.
     
  4. #12

    Expert

    Nov 30, 2010
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    Slew Rate = 2 Pi F Vp (for a sine wave). This can often help you sort choices at a vendor site.;)
     
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  5. crutschow

    Expert

    Mar 14, 2008
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    As dl324 noted, it's largely determined by the built-in compensation of the op amp. It takes a lot of current to charge and discharge the compensation cap and how rapidly this can be done determines the slew rate.
    It has little to do with the type of transistor or their design.
    Op amps with higher gain-bandwidth generally have a higher slew rate.

    For comparison, comparators, which are internally very similar to op amps but have no internal compensation, have a much higher slew rate.

    Also, note that current-feedback op amps don't require significant internal compensation and thus generally have much higher bandwidths and slew rates as compared to standard voltage feedback op amps.
     
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  6. Brownout

    Well-Known Member

    Jan 10, 2012
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    I disagree inasmuch as you might be referring to the transistor design of the op-amp. See equation 18 here: http://users.ece.gatech.edu/mleach/ece4435/tutorial.pdf

    SR = IQ/Cc. IQ is a design parameter arising from the transistor bias design.
     
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  7. crutschow

    Expert

    Mar 14, 2008
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    No, I was referring to the design or type of the transistor (such as whether it's a BJT or FET), not the design of the op amp (which does obviously affect the slew rate).
     
  8. dannyf

    Well-Known Member

    Sep 13, 2015
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    Their designs.

    Output's inability to follow the input.

    No.
     
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