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
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
Attachments
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.
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.
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.
An externally hosted image should be here but it was not working when we last tested it.
The picture shows how I have glued the BS170 on the IRFD120.
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