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If I have a circuit like this, how can I obtain the loop gain, since the capacitors and resistors are not either in series or parallel? If the feedback resistance is \$R_f\$, then I have something similar to the inverting configuration:

$$ V_o = \cfrac{R_f}{Z_{eq}}V_I $$

Where \$Z_{eq}\$ is the equivalent impedance in the input. But how can I get the expression for that input impedance for this configuration?

I know that the result should be:

$$G(s) = \cfrac{s^3R^2C^3R_f}{3s^2R^2C^2+4sRC+1}$$

enter image description here

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The key here is that the inverting input of the OpAmp is a "virtual ground". When the circuit is in regulation (closed-loop), the OpAmp regulates the voltage at the inverting input to be equal to that on the non-inverting input, which is connected to ground.

With that, your input impedance turns into this circuit:

schematic

simulate this circuit – Schematic created using CircuitLab

From there on, it's just a simple series / parallel configuration.

Note that you'll have to compute the current through C3, not the overall input impedance, because some of the current drawn from point X drains to ground before it reaches the OpAmp (and therefore contributes to lowering the input impedance while not contributing to the gain). The equivalent circuit remains the same, though.

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