Thu May 29 23:32:40 EDT 2014

Coupling signal/feedback into the filter section

3 points:
1) RC to end of ladder
2) RC to beginning of ladder (at bias collector)
3) as small signal part of bias input

I tried 1) which has the disadvantage of coupling into a low
resistance at high current/frequency.  Signal looks ok but a little
distorted.  Let's try coupling into 2)

Coupling into 3) is a high-impedance / current coupling as opposed to
low impedance voltage coupling, so have to figure out what the result
of that is.

Coupling into 2) is problematic because that point is at one diode
drop below V_cc so opamp can't deal with it.

So now I have it coupled into the base of the bias transistor, which
+- works.  Trying to make a non-inverting amplifier with DC gain of 1
but that doesn't seem to work (bypassing the resistor from - to GND).
Maybe best to AC-couple it into one that's biased around 1.5V

Go over these scenarios again with a fresh mind..

Some conclusions for now:
- collector is high impedance, so might be best for input

- terminating diode is low impedance: best for output?  AC couple into
  1M/1M bias network?