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!
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!