# Need Help With Op Amps

#### chet47

Joined Mar 4, 2012
3
I have 2 inverting op amps connected with a resistor. Eventually I'll be finding the transfer function, but right now I'm just stuck on KCL.

Here's my attempt so far:

0 = i1 + i2 + i_ *since i_ = 0, we have i1 + i2 = 0

i1 = (Vi - VA)/R1
i2 = (V1 - VA)/(R2 in parallel with C1)

What is R2 in parallel with C1?

Then, after I find Vi, it will be the input voltage to the second op amp, correct?

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#### mlog

Joined Feb 11, 2012
276
You're on the right track. For low frequency signals, you can ignore C1, which is an open circuit. C1 is added to create a 1st-order low pass filter (LPF). The LPF corner frequency is 1/(2*pi*R2*C1). The corner frequency is where the power drops to half or the voltage drops to a ratio of 0.707. In other words, C2 will roll off the higher frequencies, and above the corner frequency it should not be ignored. The same is true of the capacitor in the next stage.

One more thing. The R2-C1 combination also adds adds a phase lag at the higher frequencies. Sometimes that is important.

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