op amp question stuked... help needed

Discussion in 'Homework Help' started by shuchi12singh, Jun 14, 2013.

  1. shuchi12singh

    Thread Starter New Member

    Jun 14, 2013
    An amplifier using an opamp with slew rate SR=1v/μsec has a gain of 40db.if this amplifier
    has to faithfully amplify sinusoidal signals from dc to 20 KHz without introducing any slew-rate
    induced distortion, then wht is the max input signal value??????? :confused:
  2. tshuck

    Well-Known Member

    Oct 18, 2012
    You need to start by telling us what you have done to solve the problem, or, if you feel you can't, where you think you should start.

    We do not do your homework for you.
  3. WBahn


    Mar 31, 2012
    Q1) What signal (input or output) is going to hit the slew-rate limit first?

    Q2) What part of that signal requires the highest slew rate?

    Q3) How does that required slew rate relate to the amplitude and frequency of the input signal?
  4. gootee

    Senior Member

    Apr 24, 2007
    Define your input signal to be Vin = (A)sin (2π f t)

    Then Vout = (gain)(A)( sin (2π f t))

    The output slew rate at any time is the rate of change with respect to time. i.e. the derivative!

    You also know that dVout/dt can have a maximum of 1V/usec.

    Using the Chain Rule to differentiate, Slew Rate is

    dVout/dt = -2π f(gain)Acos(2π f t)

    Generally, you would find the values of t where max/min occur by differentiating again and setting the derivative to zero and solving for t, but in this simple case we can see that when cos = -1, it will give the maximum dVout/dt, which gives

    Max dVout/dt = 2π f(gain)A

    We need the worst-case frequency. Then we could solve for A, with the max dVout/dt value given.

    By inspection, larger f will give larger Max dVout/dt.

    So worst case would be

    1 V/usec = 2π (20000)(gain)A

    Solve for A.