"compensated" RC network

Discussion in 'General Electronics Chat' started by fez, Nov 18, 2012.

  1. fez

    Thread Starter Member

    Dec 6, 2009
    I was reading this article on oscillating op amps on the TI E2E community, and while I got the gist, I have a little trouble understanding how. See Figure 1. I understand the op amp oscillates because of the delayed feedback caused by the opamp's input effective capacitance. Figure 2 solves the problem by adding another capacitor, this one across R2, and apparently the entire feedback network now has a constant impedance and no delay.... How the heck? I can't even understand the network. How does having two series impedances, both being a resistor parallel to a capacitor, cause that? Where did the R1∙Cx = R2∙Cc formula come from? I know the capacitive impedance formula and all, but I'm having trouble resolving the situation both mathematically and intuitively.

    PS: Another query. In figure 1, how can we be sure Cx sees a resistance of R1//R2? There isn't a ground at the Vout node, there's actually a controlled voltage source there (as per the op amp model usually taught/known)! You don't ignore controlled sources when figuring out a 'seen' resistance, you only ignore the independent sources!
  2. crutschow


    Mar 14, 2008
    You must remember that the op amp negative input is a summing junction and the op amp drives the output so that the summing junction voltage is equal to the input voltage at the plus input.

    Capacitor Cc across the feedback resistor R2 provides extra feedback for the step edge of the input that helps rapidly charge Cx and minimizes the delay caused by Cx. The value of Cc has to be properly selected based upon Cx and the circuit gain so that Cx is properly compensated. You want the ratio of Cx over Cc to equal the ratio of R2 over R1 (since capacitive impedance is inversely proportional to its value). Often this is done experimentally since you usually don't know the value of Cx exactly.

    I don't understand you query in PS:. :confused:
  3. ramancini8

    Active Member

    Jul 18, 2012
    Bruce neglected to include the details that are included in "Op Amps for Everyone" which is available for free on the TI web. Look for compensated attenuator under the voltage feedback compensation section ---section 7.