[sdiy] TB303 / Polysix VCO current mirror thingy.

[Tom Wiltshire]

> On 6 Apr 2018, at 09:33, rburnett at richieburnett.co.uk wrote:
> 
>> Is there any downside to this cheap-n-cheerful alternative, Richie?
> 
> Two that I can think of:
> 
> 1. The trigger threshold of the thyristor has a temperature coefficient.  So the trip point will vary significantly with temperature, whereas a decent comparator combined with a precision voltage reference should do much better in this regard.  (However, the AN299 paper that Neil mentioned explains how this temperature coefficient was purposely used to cancel out the approximately equal and opposite temp coef of a particular type of capacitor used for the VCO integrator.  So I guess the temp coef of the SCR is not always a downside!)   I suspect the additional diode-connected transistor Q24 in the TB-303 schematic is an attempt to compensate for the temperature coefficient of the Q25/Q27 thyristor in some way.
> 
> 2. Thyristors (SCRs) have a holding current.  Once the thyristor fires and starts to conduct, this is the value below which the current must then fall before the thyristor un-latches and ceases conducting.  The problem here is that the current from the expo current source must stay below the holding current of the Thyristor.  Otherwise the Thyristor will fire, and then the charging current from the expo current source flowing through it will be sufficient to keep the thyristor latched permanently.  This imposes a limit to the upper operating frequency with a given integrator capacitor.  The TB-303's VCO simply stops oscillating if you try to drive it much above it's designed pitch range using an external CV.
> 
> Despite these potential disadvantages, I still think it's a neat bit of cost engineering.  Clever Japanese :-)
> 
> -Richie,

Very interesting. Thanks Richie.

Tom