My question is, how can I calculate the input impedance of this circuit? I want to find the expression, that describes the input impedance.
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\$\begingroup\$ Use maths and yes, it will be a tad complex but if you pay attention to what you are doing you'll get there. \$\endgroup\$– Andy akaMay 25, 2017 at 13:40
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\$\begingroup\$ Since small positive feedback is used it reduces the current load on input by the (1+ |-gain|) Thus get voltage ratio of 2 stage filter and subtract from node as a current reduction on intermediate node. using KCL to find input current divided into Vin \$\endgroup\$– Tony Stewart EE75May 25, 2017 at 13:51
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\$\begingroup\$ So you are asking for the input impedance of the whole circuit - and not only for the opamp (as indicated in the title), correct? \$\endgroup\$– LvWMay 25, 2017 at 15:18
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\$\begingroup\$ That is correct \$\endgroup\$– Willy WonkaMay 27, 2017 at 3:22
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1 Answer
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You've posted a Sallen-Key Band-Pass Filter. Personally I'd avoid doing the math -- The way I would approach it is to build the circuit in LTSpice then plot V(in)/I(in) vs. Frequency to get a plot of the input impedance vs. frequency.
That said, you can gain some insight by considering extreme cases when
frequency->infinity
and frequency->DC.
As f->inf, you're going to see R1 looking in as C shorts to ground. As f->DC your C's are like open-circuits. The V+ terminal of the op-amp will be pulled to ground through R2 so the output of the op-amp will be 0V, so it will be like R3 is connected to ground. Thus, looking in at DC you'll see R1 + R3.
