[sdiy] QVCO

[JH.]

> > But I found it really makes a difference here wether you use MAT
> > and SSM pairs, or cheaper transistors, when it comes to low offset.
> > And even with these, low offset discrete OTAs are not trivial.
>
> Indeed.
>
> > (It's not the offset per se - it's the offset variation from altering
> > Iabc.)
>
> Indeed.

In VCA's, there is no problem: Iabc < 1uA you're already 60dB down
with your _signal_, so any excessive CV feedthru won't matter anymore.
For VCAs, the MAT pairs are a *dream*.

For VCO's (well: for sine VCOs, where the pairs must be operated
in linear mode), you see the offset voltage at low Iabc currents to
full extend. Only consolation: You're below audio range then.

JH.







>
> > Some details you might find interesting:
> > The collector of the variable gm pair that dosen't run into the
> > virtual GND node of the opamp integrator, is working in
> > a cascode configuration. In order not to draw a base current
> > (whith beta chanhing over Iabc), I'm not using a BJT, but a JFET
> > here.
>
> Good old cascode... never underestimate a BJT/FET combo in cascode!
>
> > The 3rd transistor in the current mirror must be a low beta (A)
>
> The good old BC560A.
>
> Thanks for sharing.
>
> I would love to measure the tracking and stability of this thing. Any
plans to
> come by? ;O)
>
> Cheers,
> Magnus
>