On 2/17/2012 12:30 AM, Bil Herd wrote: > Jim Brain asked: > > Questions: > > * Why was the darlington pair needed with the 2n3904/2n2222 setups but > not with the 2sc1815/7406/7416 setups? Is the TIP29 that hard to > toggle? > > The TIP29 has the gain(hFE) of a small brick typically, as low as 15 at > current so I assume if I run the numbers that the 4401 is used to source > more current when the TIP29 was on and to keep it in saturation at full > current. If a small value resistor (higher current) was used to bias the > TIP when on then the 2N3904 would have to sink too much current when the > TIP29 was off to be practical. So it is hard to drive because it doesn't > help that much by adding DC gain. I can understand that if the 2n3904 was used to drive the TIP29, but the TIP29 was biased on with a 1.5K resistor. The 2n3904 merely drove the base to ground (.6V or so), thus removing current from the base. Obviously, it would need to sink the 7.5mA through the resistor (11.4/1500), but where was the resit of the current required? > > * The 2n3904/2n2222 and TIP29 are commodity. Why did the VIC go with > more specific transistors? > > They were overseas sourced, if they used US transistors they would have > had to ship them from US to Japan for stuffing, plus more expensive in > general. This was back when the Japanese were kings of the .5 cent > transistor. Ah, that makes sense. > > > I didn't like the interaction of two transistors by the time you run > min/max gains on both it felt a little sloppy, I designed it out on > TED/Plus4 to use my favorite transistor... the output stage of an '06/'07 Yep, I noticed that, and when I saw it first on the C128, I went to look at the 264 series, as I assumed you brought that over from the +4 design. Can you expand on your "sloppy" note, though? Message was sent through the cbm-hackers mailing listReceived on 2012-02-17 07:00:22
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