Bias stabilty tests.

nigelwright7557 · 11:34 PM

I have done some research on a bias circuit in a quasi complimentary circuit with two output vertical MOSFET's.

The bias circuit works well so long as there is not too much bais current in which case thermal runaway occurrs.

With 20mA bias current it is very hard to get thermal runaway.
I tested the circuit using a household iron to heat up the heatsink to above 70 degrees C and had no problems.

However, with 90mA starting bias the bias current just kept on rising out of control.

I used a simple Vbe multiplier with no emitter degeneration glued to the heatsink.

Bigun · 18th November 2009, 02:56 AM

sounds interesting and I probably won't have a clue as to the issue but a diagram would help me follow along and contribute if possible.

Hopefully you'll have this ironed out soon

nigelwright7557 · 18th November 2009, 09:35 AM

Quote:

Originally Posted by Bigun

sounds interesting and I probably won't have a clue as to the issue but a diagram would help me follow along and contribute if possible.

Hopefully you'll have this ironed out soon

richie00boy · 18th November 2009, 11:01 AM

The source resistor is in the wrong place on your lower mosfet, it should be between the output rail and point where your cfp transistor joins the mosfet. This will help thermal stability of the lower half.

What have you got mounted on your heatsink? Outputs and drivers and bias generator? Or just outputs and bias generator?

nigelwright7557 · 18th November 2009, 12:03 PM

Quote:

Originally Posted by richie00boy

The source resistor is in the wrong place on your lower mosfet, it should be between the output rail and point where your cfp transistor joins the mosfet. This will help thermal stability of the lower half.

What have you got mounted on your heatsink? Outputs and drivers and bias generator? Or just outputs and bias generator?

Just the bias and output transistors.

The driver transistors just have small bolt on heatsinks on them.

richie00boy · 18th November 2009, 12:41 PM

OK good, that is the preferred setup as generally it stops the drivers getting as hot as the outputs.

I can only suggest trying the modification I described to see if it helps. The rest of your circuit seems well thought out.

17th November 2009, 11:23 PM	#1
nigelwright7557 diyAudio Member Join Date: Apr 2008 Location: Carlisle, England	Bias stabilty tests. I have done some research on a bias circuit in a quasi complimentary circuit with two output vertical MOSFET's. The bias circuit works well so long as there is not too much bais current in which case thermal runaway occurrs. With 20mA bias current it is very hard to get thermal runaway. I tested the circuit using a household iron to heat up the heatsink to above 70 degrees C and had no problems. However, with 90mA starting bias the bias current just kept on rising out of control. I used a simple Vbe multiplier with no emitter degeneration glued to the heatsink. __________________ http://www.murtonpikesystems.co.uk PCBCAD40 pcb design software. Last edited by nigelwright7557; 17th November 2009 at 11:34 PM.

18th November 2009, 02:56 AM	#2
Bigun diyAudio Member Join Date: Jan 2009 Location: K-W Ontario Blog Entries: 2	sounds interesting and I probably won't have a clue as to the issue but a diagram would help me follow along and contribute if possible. Hopefully you'll have this ironed out soon __________________ "The test of the machine is the satisfaction it gives you. There isn't any other test. If the machine produces tranquility it's right. If it disturbs you it's wrong until either the machine or your mind is changed." Robert M Pirsig.

18th November 2009, 11:01 AM	#4
richie00boy Did it Himself diyAudio Member Join Date: Nov 2003 Location: Gloucestershire, England, UK	The source resistor is in the wrong place on your lower mosfet, it should be between the output rail and point where your cfp transistor joins the mosfet. This will help thermal stability of the lower half. What have you got mounted on your heatsink? Outputs and drivers and bias generator? Or just outputs and bias generator? __________________ www.readresearch.co.uk my website for UK diy audio people - designs, PCBs, kits and more

18th November 2009, 12:41 PM	#6
richie00boy Did it Himself diyAudio Member Join Date: Nov 2003 Location: Gloucestershire, England, UK	OK good, that is the preferred setup as generally it stops the drivers getting as hot as the outputs. I can only suggest trying the modification I described to see if it helps. The rest of your circuit seems well thought out. __________________ www.readresearch.co.uk my website for UK diy audio people - designs, PCBs, kits and more

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