>A simple way to analyze op amp circuits is to take advantage of the fact
>that in an op amp negative feedback circuit, the differential input
>voltage is ideally zero. I call this the "Theory of the Happy Op Amp."
>Generally, you can determine the voltage at the non-inverting input and
>assume the voltage at the inverting input is the same, then analyze the
>rest of the circuit using Ohm's Law based on that assumption. A sample of
>using this technique (analysis of an instrumentation amplifier) is at
>http://kauko.hallikainen.org/cuesta/et113/InstrumentationAmpAnalysis.pdf

This is usually summed up as follows:
In an op-amp circuit with negative feedback,
a) the difference in voltage between the positive and negative inputs
  (or inverting and non-inverting, if you prefer) is ZERO.  This is called
  VIRTUAL SHORT.
b) the current flowing into/out of either input is ZERO.  This is usually
  referred to as VIRTUAL OPEN.

Caveats:  Real op-amps don't work quite that well.  Input bias currents
mean that the current into/out of the inputs is very small, but non-zero.
Many factors affect the voltage between the inputs.

In general, the op-amps output will slew in whatever direction is
needed to maintain the virtual open and virtual short, UP TO THE
LIMITS.  And bear in mind that "rail-to-rail" op-amps usually aren't;
they are just "almost rail-to-rail" (within a few tens of millivolts).

This e-mail got rather out of hand; I'm not exactly sure of the
original intent; at any rate, these are the things I wish someone
had explicitly told me when I was a student rather than leaving
them for me to discover in order to move beyond the "cookbook"
they gave me.  :-p

Mike H.

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