Re: TIS98 and TPS98 transistor questions

Phil Morris wrote:
>
> <snip extremely useful info>
>
> MANY thanks for that, marvellous stuff - I'll have to re-read it a few
> times to *fully* understand it (I'm still learning, remember :) but it's
> great. Hopefully it will also be of use to others on this list.
>
> >What does it all mean? Use a fairly fast (over 1 MHz) NPN transistor
> >with good gain in the 1-5 mA range and a voltage rating of at least 40
> >volts (the more the merrier). Whatever you use USE THE SAME THING FOR
> >BOTH Q701 AND Q702. Why? You want these two as close to matched as you
> >can get. Most important matching is emitter to base voltage. If these
> >aren't the same you get something called "offset voltage" which will
> >cause centering to be off. With no offset voltage, zero voltage from
> >the game board will give you zero current through the yoke (and that's a
> >good thing!).
>
> So now all I have to do is check the data sheets again for the 2SC2240 and
> see how it matches up to your recommendations. Sounds easy? Well, it would
> be if only for the fact that the sheets don't indicate the speed of the
> transistor (or it's more than likely I'm reading them wrong!).

I wouldn't worry about it, I doubt anyone sells a small signal
transistor that would be too slow. The only way you would have speed
problems (or gain problems for that matter) is if you tried to use some
old slow power transistor like a 2N3055. Those are the same case style
as the 2N3716/2N3792 output transistors --- not much danger of anyone
using those for preamps.
 
> Re the gain - as I understand it, this is represented by the hFE
> classification, is that correct? If so, my 2SC2240's are GR classification,
> which according to the data sheet is 200 ~ 400. How does that equate to mA
> then?

Yes, hFE is the gain, and is usually given at a given collector
current. Don't worry about it. If hFE is 200, for example, a .1 mA
current into the base will cause the collector to draw 20 mA --- 200
times as much. Any modern small signal transistor will have plenty of
gain. You CAN have too much gain, if you put a darlington transistor
with a gain of 10,000 in there the circuit might get squirrelly. A
darlington in the Q703 position wouldn't work at all in this circuit.
But for differential amplifier use a gain of 100 to 500 should be fine.
Just be sure to use the same part as the transistor pair (Q701/Q702 in
my example).
 
> All the small transistors are the same type (they were previously TIS98's,
> now they're all 2SC2240's) so all will match up.
>
> Many thanks for your excellent help,
> Phil
>
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Received on Tue Jan 22 01:06:08 2002