Op Amp circuit analysis


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...Jim Thompson

| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
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Jim Thompson
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Hello all,

when I started this discussion with Bob Pease, all I wanted was to oppose "the fact" that the input impedance of the circuit in question is not constant, nor that given in most Application Notes. I think that you all can agree with this.

I calculated the impedances a little bit hastily, so there are small errors, as you find out. However, the numbers are not important, but the principle is.

Kindly, EePee (OH2NFI).

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Hello all (after many years), Frank Miles' equation is exact and it becomes even simpler if A approaches infinity (ideal OpAmp), so:

((R3 + R4) * R1) Rin = ------------------ (R3 + R4 - m * R4)

which gives correct values, too. If m = 2, Rin is (almost) infinity and i f m > 2, Rin appears to be negative meaning that output phase reverses. Tak e positive value only. However, if inputs are in quadrature, other equation could be found (but not necessary). Anyway, the original purpose of discus sion seems to be solved. Kindly, EePee (OH2NFI).

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