more VCO cores

[Martin Czech]

:::I'm sorry to say the answer is no, I just hacked at the circuit until I got
:::it to track right. (So much for "celebrity"!) I don't even remember how I
:::got the idea of varying the OTA drive level. I just wrote the effect off as
:::some non-ideal behavior that I would never understand (especially since I
:::could barely understand the ideal behavior).

Sorry for popping in!
My understanding is that three things can possibly happen:

1. 
If the diff stage is hard overdriven, the overdriven npn can get
into saturation (I hate this saturation confusion with field effect
and bipolar junction transistors, I mean Ube~>Uce here).
This would mean charge storage in the base and considerable slow
down of input response. Could happen and would influence
the high frequency response (flatten). 

Many bipolar diff stages use Darlingtons and I think this saturation/
charge storage slowness get's worse then.

2.
The diff stage sits on top of the Iabc current mirror, it is stearing
the Iabc current (or some mirrored fraction) into the inverting and noninverting
output stage mirrors. In "linear" operation the base current is quite small,
but it will already add to Iabc at the diff stage tail. I think if the transistor
saturates (or comes closer to that), beta goes down, i.e. the base current
s becomes considerable, therefore the error is considerable. 
This would mean problems @ low Iabc currents.

3.
And of course
both diff amp trannys are involved, I don't know if saturation will imbalance
the input stage, so symmetry of the triangle get's lost.
This can happen in a weird way, depending of the actuall chip layout,
I don't know which frequency range will be affected by that.

:::On a related subject, at one point in time I discovered that all of the expo
:::converters I had built had small high frequency oscillations. (See EN #98,
:::p. 15.) Terry recently found a similar effect when he revisited his VCO
:::design. This effect causes the tracking to go sharp at the high end also. I
:::strongly suggesting checking carefully for this problem anytime tracking is
:::a problem. My present solution is to use a BB OPA132 for the op-amp in the
:::converter. This amp is optimized for feedback-loop applications, and greatly
:::reduces the amout of external compensation needed to kill the HF
:::oscillations.

So THAT'S it!! I could never figure out, how sharpness at the high end could
happen. Must be some kind of rectifying of the oscillation (current rectify),
so average currents get higher if the part oscillates.

I allways though that oscillation is critical with the feedback transistor,
so it is compensated with a cap across emitter/collector. This
should then happen when the transimpedance of the tranny is high.

Now it seems to me that oscillation comes when the transimpedance is low,
it is the opamp output stage that is loaded...

???

I guess I have EN #98 reading this evening ;->

m.c.