Cascading op amps

Discussion in 'The Projects Forum' started by goodbyegti, Mar 22, 2010.

  1. goodbyegti

    Thread Starter Well-Known Member

    Apr 28, 2004
    I have a quick question about a circuit I am using to amplify a very small voltage. Any help would be much appreciated. Here is the diagram:


    IC2B is configured as a transimpedance amplifier with a high gain. I've built this part and it works fine.

    I now want to amplify the output from IC2B using IC1A and IC1B, configured with gains of 1 and 10 respectively. I wish to measure these channels simultaneously with an ADC.

    From my naive point of view I have assumed that R5 and R6 are connected to virtual earths. I will choose R5 and R6 to be around 10k to avoid overloading the output of IC2B.

    My question is can i connect the inverting input of IC1A and IC1B directly to the output of IC2B without any adverse effects? Are there any obvious improvements i could make?

    The circuit is operating at low frequency (maximum ~2kHz).

    Thank you for your time.
  2. Audioguru


    Dec 20, 2007
    R5 and R6 are important to set the gain of IC1A and IC1B together with R1 and R2.
    The inverting input of each opamp will be almost like a dead short to ground and IC2B will not be able to drive them if R5 and R6 are missing.
  3. goodbyegti

    Thread Starter Well-Known Member

    Apr 28, 2004
    Yeah, i guessed that because of the virtual earth. I'm planning on 10k for R5 and R6 with R1 and R2 at 10k and 100k which should give me a gain of -1 and -10. I just simulated it in multisim and it seems to work. Any suggested improvements are much appreciated!