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Calculating ballast resistor values to to control current hogging in paralleled BJTs

Posted 3rd January 2017 at 08:32 PM by bwaslo
Updated 3rd January 2017 at 08:41 PM by bwaslo

I started a thread at http://www.diyaudio.com/forums/solid...t-hogging.html on this subject, and after some time and many helpful suggestions, here is the calculation I came up with.

The goal is to find out how large the ballast resistors must be to prevent current runaway and to control "current hogging" to a given limit. Current hogging is a phenomenon with bipolar transistors when operated in parallel in which the hotter of the set draws more current than the cooler one(s) and so heats up even further. The fix is to put series resistors ("ballasts") in the emitter circuits (see the drawing), but I've not been able to find anything published that says how to calculate the value of the resistors.

The model is for two BJT transistors, with their collectors and bases commoned and their emitters summed through individual ballast resistors. You need to know what the process-variation-caused...
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File Type: zip Current Hog Calc v2.zip (27.8 KB, 9 views)
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Ground loop noise, why it is and what you might do about it

Posted 19th January 2016 at 12:37 AM by bwaslo

I recently was motivated to figure out numerically what might be happening when you get hum out of your power amplifier only when connecting other cables to it. And why a fix I sometimes use works so well.

Before you go blowing your budget on expensive audio isolation transformers, please take a look!

http://libinst.com/GroundNoise/The%2...se%20issue.pdf

Hope someone finds this useful.

Bill
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