Text 263, 81 rader
Skriven 2005-02-24 20:06:46 av Roy J. Tellason (1:270/615.0)
    Kommentar till text 262 av MIKE ROSS (1:167/133.0)
Ärende: Fan Of True Multitasking
================================
MIKE ROSS wrote in a message to Roy J. Tellason:

 MR> "Roy J. Tellason" bravely wrote to "James Bradley" (23 Feb 05 
 MR> 20:06:19)  --- on the heady topic of "Fan Of True Multitasking"

 RJT> A horizontal output transistor is usually one that's got a *very* high
 RJT> breakdown voltage rating,  depending on the application.  Ratings of
 RJT> up to 1500V are not all that uncommon.  They don't typically have as
 RJT> much dc current gain as other parts of similar power ratings.

 MR> I think the designers have to give up current gain for increased
 MR> voltage.

Yes,  it would seem so.  I've been collecting a lot of datasheets lately,  and
very high current,  voltage, or power ratings seem to give up on such
characteristics as gain.  It also seems that the very high frequency stuff
doesn't typically have much in terms of breakdown voltage,  etc.  So there are
definitely tradeoffs.

 MR> One would suppose the collector to base junction would have to be 
 MR> a lot thicker, perhaps this is what makes it less sensitive to 
 MR> influence from the base?

The junction being a boundary layer I don't see how it could be "thicker".  I
did read one book (online,  let me know if you want a URL) that discussed
linear chip design and a number of the tradeoffs were talked about in there.

 MR> Horizontal Output transistors are also relatively slow and the 
 MR> best rated are good at switching at perhaps a maximum just shy of 
 MR> 100KHz.

I've seen f(sub)T ratings going as high as maybe 15 MHz,  but a lot of
small-signal stuff goes way higher.

 MR> They tend to be able to carry quite a bit of current as some have 
 MR> ratings of 15 amperes or more.

Yep.

 MR> Most these days have a built-in low value resistor (~10 ohms)
 MR> across the base to emitter terminals.

I'm thinking 47 ohms is more typical.  Makes it a bit tough to do resistance
checking to see if it's okay...  :-)

 MR> This is for damping the input waveform which is typically injected 
 MR> using a small pulse transformer. The low resistance also helps 
 MR> swamp the Icbo (collector to base leakage) to ensure cutoff in 
 MR> case there is a runaway condition.

I didn't know about that part.

Some of them also have built-in damper diodes.

 MR> There are many part numbers but they are all pretty much the same.
 MR> Some have higher power ratings for the sloppier designs. In fact
 MR> they should be running cold since all they do is switch but many
 MR> run warm in fact. If you have a hot HOT then something is wrong in
 MR> the circuit!

I have one monitor that died on me here recently,  and that's one of the parts
I need to get around to checking.

 MR> BTW:
 MR>  How to multitask MS-DOS: Run PCTask twice on an Amiga!
 MR>  Multi-tasking:  Crashing both hard drives at once.
 MR>  Multitask - Make twice the mistakes in 1/2 the time.
 MR>  Multitasking - screwing several things up at once!
 MR>  Multitasking environment: bathroom with magazines!
 MR>  True multitasking - 3 PC's and a chair with wheels.

 MR>  ;-)

Heh.

Got three monitors here,  four machines,  and although the chair has wheels I
don't need 'em to switch back and forth (my linux workstation just pulled in a
pile of email)...

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 * Origin: TANSTAAFL BBS 717-838-8539 (1:270/615)