[sdiy] multiplexed synthetic transistor pair

René Schmitz synth at schmitzbits.de
Sat Sep 16 10:24:06 CEST 2023

On 16.09.2023 01:27, Paul Perry via Synth-diy wrote:
> Might it be possible to overcome the matched pair problem by multiplexing
> a single transistor at a sufficiently high frequency?
> Extra points if you also use the same transistor for heating and 
> sensing its own temperature.

Some thoughts:

How would you get a continuous current into the output to drive say an 
oscillator? If not handled you get a steppy discontinuous waveform.

Offsets or drift in this, e.g. sample and hold or the like, on the Vbe 
offset, can easily outweigh any advantage of doing this.

Any temperature variation in duty cycle of your chopping could result in 
offsets and inaccuracies.

I'd say its not worth the effort given that a good integrated pair can 
be used easily. There is no problem to begin with.

And while were at it matching is not the same as thermal coupling:

Thermal coupling is what is crucial for stability.

Two transistors can be matched to 1uV, but if they are on opposing sides 
of the pcb (different temperature) this won't do.

We want them to be on the same die or at least in the same package to 
have them at the same temperature.



synth at schmitzbits.de

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