I highlighted the importance of the choice of the transistor for bipolar discrete saturation logic before. The relevant parameters were already well understood in the 1960ies and devices like the 2N709 were devised, based on the technology that was available back then.
What about 2020, sixty years later? Clearly, discrete logic is not within the focus of any transistor manufacturer anymore, so we have to pick what is available. What are important criteria (to me)?
- Speed - low propagation delay.
- Availability - I don't want to work with NOS bought from some obscure vendor. Availability at PCB assemblers would be the best case.
- Package size - SMD is a must. Packages smaller than SOT23 would be perfect.
The last two criteria can be easily evaluated from datsheets and vendors listings. It turns out that the first criterion, speed, is not so easy to asses and merits actually testing things with real hardware.
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