> On 3/12/07, Harold Hallikainen <harold@hallikainen.org> wrote:
>> In teaching this stuff, I use the "theory of the happy op amp." The op
>> amp
>> is "happy" when its two inputs are at the same voltage. So, assume they
>> are at the same voltage, then use Ohm's law to analyze from there. This
>> generally means determining the voltage at the non-inverting input,
>> assume
>> the inverting input is at the same voltage (and both inputs draw no
>> current), then analyze from there. An example (instrumentation
>> amplifier)
>> is here:
>> http://sujan.hallikainen.org/cuesta/et113/InstrumentationAmpAnalysis.pdf
>>
>> Harold
>
> Harold,
> this approach is the way I was taught about op-amps also, but the one
> thing that bothered me is that it did not address the reason that the
> inverting and non-inverting inputs are different.
> But other than that short-coming, this method of analysis works really
> well.
> - Marcel
> --


The op amp output is A*(Vni - Vi) where A is the open loop gain of the
chip, Vni is the voltage on the non-inverting input, and Vi is the voltage
on the inverting input. As A gets very large, to get a finite output
voltage, Vni-Vi has to approach zero. The "difference between the
inverting and non-inverting inputs" is which input voltage is subtracted
from which. To get a positive output, Vni > Ni , though this difference
may be in the millivolts or microvolts. For a negative output, Vni < Vi

Harold

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