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Old 2nd November 2009, 11:58 PM   #1
IG81 is offline IG81  Canada
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Default Mounting a small signal diode on heatsink - need suggestion

Hello,

I am currently designing a class-A power amplifier.

I was looking to mount the diodes, that I plan to use in the current source that will set the output stage bias, on the heatsink, in order to thermally compensate the BJT's.

I am looking for suggestions as to how to physically mount the diodes on the heatsink.

Thanks!

ig
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Old 3rd November 2009, 12:03 AM   #2
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The old solution was to use diodes in a metal can and a small metal strap clamp. The old SWTPC Tiger amps did this. You could pot the diodes in a small piece of brass tubing and do the same thing using thermally conductive epoxy. You could wrap the diode in a piece of Sil-Pad type material and use a flat spring clamp. You could also find a suitable diode in a TO-220 case and mount it in the conventional way. You could use one junction of a small metal body transistor.
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Old 3rd November 2009, 12:43 AM   #3
GK is offline GK  Australia
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Or just connect a TO-126 transistor such as a cheap BD139 as a diode (c-B) and use that.

Last edited by GK; 3rd November 2009 at 12:47 AM.
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Old 3rd November 2009, 12:59 AM   #4
syn08 is offline syn08  Canada
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Quote:
Originally Posted by GK View Post
Or just connect a TO-126 transistor such as a cheap BD139 as a diode (c-B) and use that.
B-E would be better. Vbe tempco has little variation from device (e.g. BD139) to device (e.g. 2SC5200).
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Old 3rd November 2009, 01:12 AM   #5
GK is offline GK  Australia
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When using a transistor as a diode you generally connect the collector to the base.
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Old 3rd November 2009, 04:07 AM   #6
Enzo is offline Enzo  United States
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Well now, GK, I am unclear on your suggestion now.

Are you saying to connect using the collector AND base as a diode? (as implied in post #3) Or are you suggesting they connect the base TO the collector, leaving the B-E junction as the diode? (as implied in post #5, which was the suggestion syn08 made)

In other words, do you want to say connect the collector to the base, or do you want to say connect to collector and base?
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Old 3rd November 2009, 06:29 AM   #7
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Yes GK is saying to connect the base to the collector and use the BE junction as the diode.
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Old 3rd November 2009, 06:43 AM   #8
GK is offline GK  Australia
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Yes, sorry, by “(C-B)” I meant collector connected to base. I just figured everyone would know that already
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Old 3rd November 2009, 12:17 PM   #9
IG81 is offline IG81  Canada
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Thanks alot guys, it didn't even occur to me to use a transistor junction, duh!

That will greatly facilitate heatsink mounting.

ig
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Old 3rd November 2009, 12:53 PM   #10
IG81 is offline IG81  Canada
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In addition to the above, I assume the best transistor to use as a diode would be the same model as the output transistor to bias, as they will exhibit the same temperature coefficient. Would that just be splitting hair, or would any cheaper transistor behave close enough to be almost just as good?

And when it comes to setting the current through the "diode transistors", I guess the Ic v.s. Vbe plot in the spec sheet is what I should look at to make the initial prediction to determine Vbe voltage drop right? And I would assume the former is the reason why the collector should be tied to the base in this situation.

Thanks,

ig
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