size of feedback resistor in op amp

Discussion in 'General Electronics Chat' started by tpny, Dec 10, 2012.

  1. tpny

    Thread Starter Member

    May 6, 2012
    For example, non-inverting op amp configuration where gain = 1+(R1/R2), how does the size of R1 and R2 matter? For example, for gain = 2: whats the difference between 1+(1k/1k) and 1+(10k/10k) and 1+(100k/100k)... Thank you!
  2. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    Much of the time it will be of no matter as both input and output impeadance are functions of the op amp itself.

    I had a case once when I was making a high gain amp, something like an Av of 33 at 3 MHz. For THAT amp I had to use very small resistors, R2 was something like 39 ohms (not K, just ohms) to get any operation at that frequency. Seems there is a small capacitance on each pin, and that forms a break with the feedback resistors, so since the cap is fixed you need to decrease the resistor to increase the break and the overall frequency response.
  3. Audioguru

    New Member

    Dec 20, 2007
    Most opamps have difficulty driving a feedback resistor less than 2k ohms.
    With a higher value for the feedback resistor then the load current can be a little more, or the output voltage swing can be higher.
  4. ramancini8


    Jul 18, 2012
    High resistors decrease bandwidth because stray feedback resistor capacitance forms a pole that acts like a low pass filter. High resistance causes output offset because of the input current. High resistance causes more noise. Low resistance dissipates more power and robs output drive current capability.
  5. Ron H

    AAC Fanatic!

    Apr 14, 2005
    High value resistors interact with stray and device capacitance on the inverting input, adding another pole in the feedback loop, which causes high frequency peaking on the output.