From: Bil Herd <>
Date: Fri, 17 Feb 2012 01:30:32 -0500
Message-ID: <>
Jim Brain asked:


  * 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

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.

  * 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

  * The 2n3904/2n2222 circuits have a 10K to ground on the first stage
    transistor base.  I assume this weakly holds base at GND until a
    signal appears.  This does not show on the VIC,64.  Why not?

My preference is always to tie something down to create a known voltage
drop .  With that said, the C64 was cost reduced by various entities, they
would have known if not stuffing the part decreased yield if it existed in
the first place.

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

Bil Herd

Jim Brain

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Received on 2012-02-17 07:00:18

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