[sdiy] TB303 / Polysix VCO current mirror thingy.

[rburnett at richieburnett.co.uk]

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?