Author Topic: Why high Vceo transistors for Vbe multipliers in class AB PAs?  (Read 423 times)

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Offline SacodepatatasTopic starter

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I thought that they would use the same type of transistors than any of the the input BJT in a Darlington/Sziklai pair at the output stage, because if they are thermally linked by sharing the heatsink, then the thermal curves would match (because they are of the same type or either complementary) and there would be some Iq bias balance.

But i've seen some designs that use a 2SD669A as Vbe multiplier but the driver BJTs in the darlington pairs were other references such as the pair 2SB1369/2SD2061. In such a case, the 669 is not thermally linked to any other. Because the Vce can't go over a few volts due the negative feedback, why this transistor can't be replaced with, for example, a 2SD882? Is there another factor in play that i'm missing?
 

Online David Hess

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Re: Why high Vceo transistors for Vbe multipliers in class AB PAs?
« Reply #1 on: April 01, 2023, 10:55:55 am »
Since the 2SD669A is a high voltage transistor, wouldn't it match the output transistors better than the low voltage 2SD882?

I do not know why they would not use the same transistor as one of the driver transistors though.
 


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