Marcel Birthelmer wrote:
> apologies for the n00b-ness of my question but being a first-year EE
> student I haven't learned enough useful things yet to figure out the
> workings of the following circuit:
>
> http://marcel.carrietech.com/phase.png
>
> This is a phase shifting circuit (Rv being variable) that I found
> online.

Not really.

> Could someone please explain how to calculate the output
> voltage given the input voltage and the various components? I've
> checked all my EE books (which are all intro level, unfortunately)
> and have come up dry.

There is a lot that can be learned from this circuit.  It will be well worth
spending some time analysing and understanding it.  However, if I just tell
you the answer you won't learn as much, and I don't want to do your homework
for you anyway.

Look at the circuit as two parts, the part controlling the opamp positive
input, and the part controlling the negative input.  Analyse what happens if
the input is driven by a symmetric sine wave around ground and the negative
input were held at ground.  For example, let's say it's a perfect opamp with
+5V and -5V supply and the sine peaks also go from -5V to +5V.  Try to
figure out what the opamp will do.  If you get stuck, look up "positive
feedback" or "hystersis".

Now forget about what the opamp is doing and look only at the signal
presented to the negative input.  It's a perfect opamp, so the negative
input has infinite impedence.  In other words, it's not there for purposes
of analysing what the voltage will be.  That leaves a capacitor in series
with the input and a resistor to ground.  What will the result look like for
a very low frequency sine wave input?  A high frequency sine wave input?

That's enough for now.  Answer the questions above to show you understand it
to this point and we can continue.


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