[sdiy] Transistor questions

[René Schmitz]

On 06.03.2025 07:56, Spiros Makris via Synth-diy wrote:
> An interesting observation in regards to the sawtooth core:
> Its operation has an inherent non ideality that can be optimised but 
> never eliminated (you will never switch "instantaneously" no matter 
> what); the apparent simplicity comes at the cost of balancing 
> transistor capacitance, conduction resistance, trace inductance, 
> comparator hysterisis and delay etc. The reset portion of the waveform 
> will last a constant amount of time, which eventually becomes 
> comparable to the adjustable ramp portion, and that messes your 
> tracking, as well as the waveform itself (although I'd argue you won't 
> be able to hear that difference in texture).
> In contrast, the triangle core oscillator does not have this 
> inherently unachievable feature, since the discharge/charge currents 
> are not required to ever become infinite. Of course, the comparator 
> propagation delay, frequency dependence of hysteresis, capacitor 
> leakage will create a set of constraints in operation. However, it is 
> admittedly easier to make a very fast accurate comparator than making 
> an adequate reset circuit for a "wideband" tracking sawtooth core. The 
> cost you have to pay is a significantly more complex bidirectional 
> current source, which cannot be realised easily with discrete components.

In practice these problems are all manageable. In the end its all about 
trade-offs. You don't ever need infinite speed, e.g. your processing 
will always wash it down to some finite speed.

Reset time can be compensated for. And you have non-ideal behaviour in 
the current source either way. (Rbe) for that alone you need some form 
of compensation.

Triangle to sawtooth waveshaping tends to introduce its own artefacts, 
though they are less audible (yet for an LFO can be noticeable).

There are triangle oscillators that don't need a bipolar source, that 
give you inherently good symmetry .


Cheers,

  René


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