more VCO cores

[Ian Fritz]

Hi Martin --

> 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.
>

This  all sounds plausible.  I'm not sure how you could check which of these
are occurring and to what extent without having probes within the IC.  And
I'm not sure I see how the mistracking would change sign as a function of
drive level.

>
> 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.
>

Or maybe because of the exponential response the high side of the excursion
has a larger effect than the low?

> 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...
>

Let's see ... you need more compensation when the gain is low, right?

  Ian