decreasing tempco in this Vbe multipl.

[djdamix]

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

[Mr Evil]

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.

[MikeB]

You could try adding a small emitter-resistor to the mje340, a few
ohms should be enough.

[BrianL]

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.

[Eva]

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

[peranders]

Why don't you use a mosfet as temp sensing element? I tried with BS170 in my monster headphone amp with good results.


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