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Odd Transistor Choice

Gentlemen,

In the PSU section of that old Tek oscilloscope I've been troubleshooting, I discovered several transistors had died. One of these is puzzling me somewhat. It's a 2N5859 which they're using to drive one of those big ol' TO-3 power transistors for the -15V output section. I can't understand why they chose a fast switching device with a GBP of 250Mhz.

This being a linear PSU, in 1970 when these devices would have been pretty expensive, it seems a curious choice. Fine, it can handle 80V and pass 2 amps which is respectable in TO-39 in those days, but a

250Mhz Ft? What's the point? Some dumb old medium power audio transistor could do that job, surely?

The schematic:

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Datasheet:
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Reply to
Cursitor Doom
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Unless someone remember how much it costs in 1970s, we will never know why. However, when compares to the popular (and easy to remember) 2N2222, the 2A (vs. 0.8A) collector current seems attractive. Both are 250MHz, so freq response is probably not an issue.

Reply to
Ed Lee

Ft 250Mhz was not particularly special even then, the Ft is not relevant to the choice there; they chose because the V/Ic/Hfe etc fitted the need. Even the 60 year old BC107-109 family you probably know well had Ft of circa 200Mhz.

I guess 2N2218A/2219A or BFY50 would do as equivalents to 2N5859. There are zillions likely to work well.

piglet

Reply to
piglet

They may have had commercial reasons to make that choice. Perhaps they were already buying millions to use in other products. Maybe an alternative that might have been a "better" choice would have added another part to the design. In many large companies there is a lot of pressure not to do that. John

Reply to
John Walliker

Good point. Maybe also the low capacitances? Dunno.

Reply to
John S

I wish I knew WTF you were talking about.

Reply to
Cursitor Doom

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