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Old 7th September 2011, 09:33 AM   #1
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Join Date: Jun 2009
Default Transistor case coupling??

Hi everyone, doing some repairs at the moment, and have came across something odd. On the amp pcb there are a few pairs of transistors, that have been placed face to face with the flat part of each case touching, and then held together with a piece of heatshrink.

I thought they may have been optical phototransistor couplings, but after looking at the schematics, they just appear to be standard transistors in darlington pair configurations. It even shows on the schematic what pairs are placed together, and in what configuration.

What is the reason for this? Is it for thermal reasons? Capacitive? Electromagnetic? All else I can think of is to prevent vibration relative to each to help reduce noise.

Any thoughts would be appreciated.
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Old 7th September 2011, 09:59 AM   #2
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that's right, it is so the two transistors can thermaly track each other...
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Old 7th September 2011, 10:40 AM   #3
AndrewT is offline AndrewT  Scotland
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yes, this thermal coupling is commonly used in Long Tail Pair (LTP) and Current Mirror (CM) implementations. It massively reduces thermal drift of the output offset of a Power Amplifier.
regards Andrew T.
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Old 7th September 2011, 10:41 AM   #4
DF96 is offline DF96  England
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They will probably either be in a LTP (to maintain balance) or one will be part of a Vbe multiplier biassing the other. Keeping them at the same temperature means they track Vbe changes.
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Old 7th September 2011, 02:49 PM   #5
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Makes sense! Thanks for your answers
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