Theory for beginners (opamps)

Daniel Gendreau gendreau at rochester.rr.com
Mon Nov 8 09:59:41 CET 1999


>     It seems to me that the op amp is a major player in the whole synth
> thing. I see these in every schematic. maybe one of you could give a brief
> bit of the theory or point me to a location on the web with more
> in formation on op amps.

Probably the first thing to learn in opamp analysis is when they are in
negative feedback mode (the most frequent use for them).

You can tell an opamp is in negative feedback mode if you can trace a
connection (ie wire, resistor etc.) that allows DC current to flow from the
opamp output back into its negative input. Capacitors do not count here
since they block DC current.

In this "negative feedback mode", the opamp always seeks to keep its
negative input voltage the same as its positive input voltage. In other
words, the opamp output will produce whatever output voltage it can (limited
by the +/- rails) to make the negative input voltage the same as the
positive input voltage. My electronics professor called this a "virtual
short", meaning that - must be the same voltage as +, but no current flows
between the +/- inputs AND no real current(just nano amps) flows into/outof
the +/- inputs.

Given this theory, go back to your radioshack opamp cookbook and look at the
inverting/summing amplifier circuit. If its the one I remember, you will see
several 1K input resistors linked to the negative input, the positive input
is linked to ground and there is a 1K feedback resistor linking the opamp
output back to the negative input.

Looking at this circuit, we see negative feedback. Therefore, a "virtual
short between the +/- inputs. This means the voltage at the -input wants to
always be 0V (since the +input is grounded). Lets say we apply +1V to one of
the input resistors. Since the -input is 0V(virtual short to the
+input/ground), we know 1ma current is flowing toward the -input, but we
also know the current cannot go into the -input, so it must be going through
the feedback resistor and toward the opamp output. 1ma Going from
0V(the -input) through the feedback resistor causes a 1V drop. So the output
yeilds -1V. Result: output voltage = -(input1+input2+input3...) A simple
inverting mixer!

Another very useful opamp circuit is a non-inverting buffer. Here you will
see a wire running from the output to the -input and an input signal
connected to the +input. Since there is a virtual short, the output=the
+input. The advantage here is that there is almost no current drawn from the
signal going into the +input, but the output can source a very large amount
of current. That sort of thing is useful in signal input jacks.

These are just some simple uses for an opamp. There are MANY others and they
get much more complicated, but its a good starting point.

-Dan G.




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