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Old 28th March 2005, 05:27 PM   #1
djdamix is offline djdamix  France
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Default thermal compensation

Still having problem with my thermal compensation : I use IRFP240 & 9240 and the VBE multiplier is on the heatsink (MJE340) however the compensation is too high I got distortion when the temperature rise because the quiescent current decreases too much (=0mA!)

I've also try the compensation use by A.Holton in the AV800 : IRF610 and 100Ohm but it seems to be too low the quiescent current rise until 600mA or more

I've read S.Douglas's book has it described some way to change the tempco by adding voltage source and else but it seems quite a hard solution ...

At this point I plan to use a diode in serie with the vbe multiplier (MJE340) and to put only the diode on the heatsink to have small tempco.

Any help or comment is welcome !

David
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Old 28th March 2005, 05:59 PM   #2
K-amps is offline K-amps  United States
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Here's a suboptimal solution but works... add multiple layers on insulation on the bias device and heatsink to be tracked...
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Old 28th March 2005, 07:55 PM   #3
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Hi David !
You can reduce the steepness of your temperature compensation
quite easy.
If you put a series resistor to the emitter of the MJE340 in your
Vbe multiplier.
The value of this resistor depends on the current which is running through MJE340 in your design.
I am now assuming about 10mA steady state current in the VAS:
A good starting point may be a 4.7 Ohm resistor in series to the emitter and then slowly increasing values if the compensation is still
to massive. Please note that you may have to readjust the idle current depending on the additional resistor.

If there is already resistor at the emitter of your
MJE-340- Vbe-multiplier, then you simply could increase this value.

BTW:
Which resistor values do have in series with the
source of your output MOSFETS?

Good luck
Markus
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Old 28th March 2005, 08:15 PM   #4
djdamix is offline djdamix  France
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K-amps => It could be a solution, I'll try but I think i'll increase the delay before the current adjusts ...


Markus, Thank you I'll try ! Last time your advices about DC offset were helpfull !! Now my amp is working I just have this quiescent current problem to solve and it will be ok.

The resistor in serie with the mosfet are 0.22 ohm

Thanks

David
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Old 28th March 2005, 09:59 PM   #5
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Thanks for the flowers....
Let's hope that I understood your current fight correctly
and that my hint is helpful this time, too.

I think 0.22Ohms at the source of your output Fets
should be OK.

Bye
Markus
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Old 29th March 2005, 12:37 AM   #6
anatech is offline anatech  Canada
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Hi djdamix,
Markus was referring to a series resistance in the emitter of your MJE340. This is exactly what they did in the Counterpoint amps.

When I changed to a bipolar type output the sound became much cleaner, less distorted. Never mind the bias was now easy to control.

-Chris
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Old 29th March 2005, 05:03 PM   #7
djdamix is offline djdamix  France
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I'm working on the thermal compensation & I was wandering what was the recommended quiescent current for AB mosfet amp. It seems that I need about 400mA to make the distortion disapear at 20Khz at about full power (according to my scope)

Does it seems ok ? Can somebody confirm ?

Thanks.
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Old 29th March 2005, 05:10 PM   #8
djdamix is offline djdamix  France
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=>Chris, what is the Counterpoint amps you mentionned ? Is there any schematic somewhere ? I'm interested !

David
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Old 29th March 2005, 07:50 PM   #9
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Hi David !
I am also curious about the counterpoint amps.
My idea was simply to reduce the impact of the
temperature on the collector current of the Vbe multiplier BJT.
Emitter resistor are the most traditional way to reduce the temperature dependency of a transistor current....

I think your observation of distorsions at high frequencies is
reasonable. I guess your amplifier is the one which you have posted here:
improving my VAS
The open loop gain of this design is not very high and may additionally decrease already below 20 kHz. Your closed loop gain is high.
About 100. So there is not much feedback. You will have to work with
quite some idle current. 400mA do not make me wonder.
Probably you will find output powers which show even higher distorsion than at full load. Did you also try about 10V-20V peak output at 4 Ohms load .... ?

Good luck
Markus
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Old 29th March 2005, 07:57 PM   #10
Allexx is offline Allexx  Ukraine
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In my amplifier I use CFP output with BJT driving MOSFETs with drain connected to load. Vbe multiplier transistor of the same type as driver is installed on the same small heatsink of a driver transistor (does not matter which one). This way compensation of quiescent current is amazingly good. Maybe small common heatsink for drivers with Vbe mounted there is the best solution. Do not forget that output is CFP.
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