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Old 14th December 2011, 04:50 AM   #1
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Join Date: Dec 2010
Location: Malaysia, Selangor
Default Transistor role in amplifier ? Properties

For input transistor, their role is to 'process' small signal, thus their important properties (when choosing transistor) is :
(I) quiet (low noise)
(II) high Hfe (amplification)
(III) Fast (Ft)

As for output transistor, their important properties is :
(I) Good/suitable SOA
(II) Moderate fast (compare to input transistor)
(III) Low output capacitance (linear over frequency)

But for Predriver/Driver/VAS transistor/etc, I don't have any idea what is their properties which need to be emphasis when choosing transistor for those roles.
So hope experienced forum member could point out to me what are the parameter for those missing/incorrect information as above stated.

Actually I wasn't planning to ask, but after all the materials arrived now and I found out my 'complementary' driver transistor (BD139/140) aren't complementary that much.... (different manufacturer, thus different batch, then sure significant different in parameter of course)
So I'm planning to use other transistor I have in hand to replace that spot without ruining my amplifier design.
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Old 14th December 2011, 08:32 AM   #2
Enzo is offline Enzo  United States
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Join Date: Sep 2004
Location: Lansing, Michigan
Even from the same manufacturer, the PNP and NPN transistors will, by definition, be from different batches.

If you are that concerned over matching, get or make a curve tracer and plot out the characteristics of your parts. That matters more than brand, in my view. There is no guarantee a PNP and an NPN part will exactly mirror one another just because they come from the same maker.
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Old 14th December 2011, 08:42 AM   #3
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Join Date: Dec 2010
Location: Malaysia, Selangor
well.... in my opinion, it is just like tolerance. +-5% in the same batch, will have about +-10% on different batch, and even more/wider different between different manufacturer.
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