op amp question

Discussion in 'General Electronics Chat' started by fran1942, Sep 15, 2011.

  1. fran1942

    Thread Starter Member

    Jul 26, 2010
    Hello, I am learning the basics of op-amps and have designed the attached configuration to give 25 gain. I was just wondering, if I wanted to increase the gain to 340, could I simply replace R1 with a 330k and R2 with a 10 Ohm resistor ?
    Is there any reason why I could not do that ?
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    Use a calculator. 8200 divided by 330,000 is .0248, not 25.8. Notice that the signal out is nowhere near 25 times the input - 2.5 volts in, and about that out.

    A gain of 340 is close with R1 = 68,000 ohms, and R2 = 2000. Your resistor values will give a gain of 33,000. DC gain is not 1 - try dividing any value by infinity and see what you get.

    You might be interested in this part of our Ebook - http://www.allaboutcircuits.com/vol_3/chpt_8/5.html
  3. eblc1388

    AAC Fanatic!

    Nov 28, 2008
    One reason being the opamp is not fast enough to amplify signal as high as 20KHz with such gain.

    Looks at the gain/frequency plot inside common opamp data sheets and you will know why. A gain of 340(50dBs) is possible at 200Hz with some opamps but when the frequency is 20KHz, few will do.
  4. colinb

    Active Member

    Jun 15, 2011
    I think you made a mistake there.

    The circuit shown in fran1942's schematic has an 8200 ohm feedback resistor with a 330 ohm dividing resistor, so Av = 1 + (8200 ohm)/(330 ohm) ≈ 25.8, just as he stated.

    (I avoid using the terms R1 and R2 since fran1942's schematic has those references swapped from the AAC ebook op-amp usage.)
  5. atferrari

    AAC Fanatic!

    Jan 6, 2004
    I got used to Rf and Rin which are self explanatory. Hard to make a mistake with them.