Re: why do we prefer to supply voltages at the inverting terminal of an op-amp???


"archie" <haughtynarcissist@xxxxxxxxx> wrote in message
news:1163086722.766134.62130@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
why do we prefer to supply voltages at the inverting terminal,or
conventionally known as the negative terminal of an op-amp???

I'm assuming that when you say "supply voltages," you are
asking why the input signal to an op-amp circuit is very often
connected to the inverting input - is that the case?

If my assumption is correct, then the answer is as follows:
Many circuits which are constructed from op-amps make
use of a technique known as "negative feedback" to
stabilize the performance of the circuit and set such
parameters as the overall gain, etc.. In this method, a
portion of the output signal is basically being subtracted
from the input signal (or a portion of it). Since in most
cases it is not important to preserve the absolute "polarity"
or phase of the output relative to the input, it is simpler to
just use the inverting input of the op-amp to achieve this,
as opposed to having completely separate "gain" and
"invert and add" blocks.

Most such circuits rely on the fact that the gain of the
op-amp is extremely high (what's called the "open-loop
gain," i.e., the gain in the absence of such external
feedback), such that when all is said and done the
behavior of the complete circuit is actually determined by
the components external to the op-amp. If you Google
for "inverting amplifier op-amp" you will no doubt find
any number of analyses with circuit diagrams which will
explain why this is so. The analysis of such circuits will
typically assume that the op-amp in fact has "infinite"
gain, as well as "infinite" input impedance (meaning that
its inputs do not represent a load on the input signal or
on the portion of the output which is being "fed back").

Bob M.




.

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