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Old 26th February 2005, 08:31 AM   #1
djdamix is offline djdamix  France
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Default decreasing tempco in this Vbe multipl.

I've build an amp which sounds good too me. The bias current of the output stage is set at about 200mA with the VBE multiplier. It works well, but after a few time outside (winter at -5°C), when I turn it on, I get more than 1A bias current ... It comes from the vbe multiplier, and that's why I would like to decrease its tempco. (The Vbe x is fixed on one of the output transistor).

I've calculated that for 1°C, the current increase in the mosfet of 0,008A.

1 Volt less on ther gate give about 1A less, so I need to decrease 0,008V the gate to get 0,008A less on the output.

Actually this VBE give about 0,029V / °C which is too much. I searching for idea to decrease it ...

Thanks.
David

Attached is the schema of the vbe x
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Old 26th February 2005, 01:05 PM   #2
Mr Evil is offline Mr Evil  United Kingdom
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You could try placing a Zener diode in parallel with R1, which will limit the maximum output for m the Vbe multiplier. A 9.1V Zener should be about right.
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Old 26th February 2005, 01:25 PM   #3
MikeB is offline MikeB  Germany
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You could try adding a small emitter-resistor to the mje340, a few
ohms should be enough.
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Old 26th February 2005, 02:18 PM   #4
BrianL is offline BrianL  United States
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See Doug Self's books or his EW/WW articles or possibly his web
site for discussion. Your main problem may be lack of thermal
feedback from the output transistors to the Vbe multiplier.
I believe that Doug also covers the changing of the tempco
of the multiplier. If not there is a JAES paper by Malcom Hawksford
on the topic.
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Old 26th February 2005, 07:39 PM   #5
Eva is offline Eva  Spain
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Try placing the Vbe multiplier directly on the heatsink just near one of the output devices

The plastic resin of the cases of the TO-247 devices has very high thermal resistance so extreme ambient temperatures make bias setting error unacceptable
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Old 26th February 2005, 07:57 PM   #6
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Why don't you use a mosfet as temp sensing element? I tried with BS170 in my monster headphone amp with good results.
Click the image to open in full size.

The picture shows how I have glued the BS170 on the IRFD120.
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