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Old 16th December 2014, 03:02 PM   #1
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Default Bias question for class B output

Hi All,

This is my first post on this forum so please be gentle
I am designing a small class B amplifier for my computer speakers. The output power is about 30 W max with 25 V rails, which should be plenty enough.

I have a question concerning the bias of the output stage. As you can see it is a CFP stage with about 30 mA of quiescent current, which gives about 10 mV between collectors of the power BJTs (by the way they are BD911/912, scm is incorrect)

My questions is, do you think the solution I use will be good enough ? In simulations, the quiescent current decreases a bit as temperature rises, which is the effect I was aiming for. To me this solution is simpler than the Vbe multiplier and for some reason seems to yield lower (simulated) distortion figures. By acting on R10 you can divert some of the cureent source's current away from the diode connected transistors and this way act on the bias voltage. The transistors are the same as in the CFP stage, and on the pcb they are mounted together for good thermal coupling.

Thank you for your help
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Old 16th December 2014, 03:19 PM   #2
kevinkr is offline kevinkr  United States
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Just wanted to say welcome to the forum.

I'll let the guys more versed in discrete semi-conductor amplifier design comment on your thermal compensation scheme, the rest of it looks fine on a quick look. I am wondering about the purpose of Q8 - over current protection or ? Assuming Q11 and Q12 are mounted on the heat sink.
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Old 16th December 2014, 03:50 PM   #3
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Hi kevinkr,

Thanks for the kind words. Yes Q8 is to protect the VAS emitter follower Q6 in case of negative clipping. And yes the idea is to have Q11, Q12, Q13 and Q14 grouped together on the PCB, and Q15 and Q16 on the heatsink. But not sure if it will work or not..

Cheers !
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Old 16th December 2014, 04:00 PM   #4
kevinkr is offline kevinkr  United States
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Not sure either, but it seems worth trying - it's the best way to learn.

Others should be able to weigh in and advise on any particular design issue.

I'd recommend prototyping it with provisions for reasonable heat sinking so you can debug the design.
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Old 16th December 2014, 05:39 PM   #5
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Welcome in diyaudio.com

Quote:
My questions is, do you think the solution I use will be good enough ?
Yes. Actually I have a little amp (about 100W) with 13mA quiescent for the whole amp and it sounds very well.

Look at the low-pass in the input. Only C7 is may be to hard for the preamp. Better take a RC-pass. E.g. 680 ohm and 1n.

The feedback resistors R15/16 are very high. Oscillations could occur.
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Old 16th December 2014, 06:52 PM   #6
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Hi moschfet !
Thanks for your comments. So you think this very simple scheme will work well and provide effective thermal compensation ? I am especially worried about how things go when the amp becomes hot.

For your other comment, do you mean that I should use 680Ohm + 1n in series instead of C7 alone ? I take note of your comment about R15/16, will try different values when I build it.

Thank you !
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Old 16th December 2014, 11:43 PM   #7
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If the amplifier has a standing current through output transistors it is class AB not class B.

If the amplifier is stable across the entire power range then I would be happy with that.
You really need to thrash the amplifier to see if the thermal feedback is working OK.
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Old 17th December 2014, 02:31 PM   #8
wg_ski is offline wg_ski  United States
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Epoxy all 4 (Q11-14) to a common strip of metal. This is how you need to do it anyway - no problem with TO-92's Heat it up with a soldering iron and see where your bias goes. Overcompensated is bad (unless it's a 5kW PA amp), "a little undercompensated" is good, but of course a lot is bad.
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Old 17th December 2014, 09:18 PM   #9
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Hi hadrianus,

You're absolutely right - having CFP output stage topology, you need to track the temperature of the drivers (not the big output devices). So, having Q11, Q12, Q13 and Q14 on a metal strip or local heatsink, will work fine.

Cheers,
Valery
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Old 19th December 2014, 02:54 PM   #10
AndrewT is offline AndrewT  Scotland
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You could use sot23 (or smaller) for Q11 & Q12.
Glue them to the wide part of the collector lead of the Drivers, just at the edge of the plastic package, Q12 & Q13.
Since you have omitted the resistors from Vbe and using them as diodes, you can solder tiny wires from the collector and base to the Driver base leads.
Then you have just one wire from each emitter to run parallel to the driver base leads to the Vbe multiplier resistor, VR10.
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Last edited by AndrewT; 19th December 2014 at 02:56 PM.
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