Gerhard Fiedler wrote:
> Rather than working with the mental image of an infinite gain, I found
> it quite useful in my early days to think of the opamp as a device
> that sets its output so that both inputs are equal.

I think this is a bad way to teach opamps and deliberately avoid it.  The
problem is that this rule of thumb sounds deceptively simple and
authorotative, and people don't think about the limitations when they apply
it.  To know when to apply it, you need to understand opamps to the point
where this crutch is no longer necessary.

One of the questions I ask when interviewing EE candidates is to draw a
simple opamp circuit with hysteresis.  It's just the opamp and two resistors
(output to +, and + to ground).  I tell them this is a ideal opamp powered
from -5 and +5 volts, and both resistors are 10Kohms.  The problem is to
explain what happens as the input ramps from -5 to +5 volts.  You'd be
surprised how many people mess this up, and how many of those mumble about
the opamp keeping the two inputs at the same voltage.

Some opamp *circuits* will try to keep the - input at the same voltage as
the + input.  The opamp itself merely performs:

  Out = Gain(PosIn - NegIn)

If you can't understand how that causes the circuit to have the - input
follow the + input in some cases, you won't be able to know which cases that
applies to and which cases it doesn't.  In other words, the real rule isn't
any harder to understand than when it's OK to apply the simple rule.


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