[sdiy] TB303 / Polysix VCO current mirror thingy.

[Neil Johnson]

The two transistor SCR was also used by Jim Williams in the AN299 VCO.

Neil


On Thu, 5 Apr 2018, 20:16 Tom Wiltshire, <tom at electricdruid.net> wrote:

> So this transistor arrangement replaces the comparator+hysteresis that
> you’d see on a typical ramp osc?
>
> > On 5 Apr 2018, at 14:41, rburnett at richieburnett.co.uk wrote:
> >
> > The back-to-back arrangement of NPN and PNP transistors in the TB-303
> oscillator forms a Thyristor (SCR).  Once the voltage is sufficient to turn
> on one transistor, each of the transistors in this pair provides the base
> current for the other.  So the pair of transistors remain latched on, until
> the capacitor is recharged.  Once it's recharged there is no current flow,
> so the SCR drops out of the latched state and the discharging ramp
> commences again.
> >
> > -Richie,
> >
> >
> >
> > On 2018-04-05 14:26, Gordonjcp wrote:
> >> If you look at the VCO in a TB303 or in a Korg Polysix they appear to
> >> use a kind of current mirror arrangement with two NPN trannies, and then
> >> a PNP tranny with its base wired to the collector of one and its
> >> collector to the collector of the mirror through a diode.
> >> There's a capacitor - to the "negative" rail in the 303 (really ground
> >> but that's referenced to the +5.33V suppy) or to the "positive" rail in
> >> the Polysix - which charges up and is discharged through the expo
> >> converter.
> >> At some point for a reason I can't entirely figure out, as the capacitor
> >> discharges something in the current mirror conducts and it charges again
> >> briefly, but I don't get why.
> >> Can anyone shed any light?
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