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schematic

Ideal Operational amplifier

Zi=(R1+R2)||Zc2

Zo=0

741 Operational amplifier

Zi=(R1+R2)||Zc2

Zo=0 (at DC)

For the input impedance when using a 741 I am not sure if I should consider the Fairchild version of the 741 graph showing input resistance vs frequency impedance (at DC is about 6Mohm) so It would be Zi=((Zc2||R_OA)+R2))||Zc1+R1

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The input impedance of an opamp is that between the inputs. In the Sallen Key circuit, the amplifier is used as a follower (the other input is connected to the output), so the input impedance is 'bootstrapped out' and becomes very high within the bandwidth of the amplifier.

Even a single transistor used as an emitter follower gives very little error (except for DC offset of course) in this circuit configuration.

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  • \$\begingroup\$ This is a typical configuration of a second order low pass filter, there is no a closed form expressions for Zin y Zout? \$\endgroup\$ – Fran Aug 3 '17 at 6:50

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