Vbe Multiplier for LM4702 Output Stage - diyAudio
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Old 27th May 2006, 09:43 AM   #1
rspamr is offline rspamr  United States
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Default Vbe Multiplier for LM4702 Output Stage

Short question:
How do I go about choosing a transistor to serve as the Vbe multiplier to avoid thermal runaway issues? What specs do I want, and how do I go about matching them to the darlingtons I have chosen?

Long Question:
This is my first post here, but I've been reading for a while, trying to absorb as much information as I can.

The LM4702 will be my first project beyond a chipamp, so I'm hoping I can learn a bit about choosing transistors. I would like to build a (relatively) simple amp using the typical circuit schematic in the LM4702 datasheet. It's quite possible that this project is too much for me to handle, but I'm hoping that by narrowing my scope to just the output stage, I'll be able to do it. If this question indicates that I'm in over my head, please let me know.

I recognize that most people have chosen to ignore the recommendation of the darlington output stage, but I'm looking for a learning experience, and not necessarily the best amplifier. Anyway, I've run into this issue and I don't really know where to go to find a solution.

I understand the thermal runaway issue, and I follow National's recommendations of using a heatsink and emitter degeneration resistors (page 11), and I also understand how a Vbe multiplier, or Qmult as they refer to it, can help solve the problem. How do I choose a BJT to act as my Qmult?

Also, if anybody knows of any websites or books where I could learn more about this, I'd love to check them out too.

Thanks in advance,
Robert
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Old 27th May 2006, 01:59 PM   #2
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Robert -- I was able to purchase the Sanken power darlingtons from Profusion in the UK -- I received them in less than a week -- here is link:
http://www.profusionplc.com/pro/gex/...%20darlington#

beefy little guys with tempco built in.

this is a schematic -- showing one channel -- using the LM4702a and SAP16's:

Click the image to open in full size.

fwiw -- I have 2 versions of the PCB on the way -- one sites the transistors along one vertical plane, the other on two parallel vertical planes.
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Old 27th May 2006, 04:23 PM   #3
rspamr is offline rspamr  United States
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That is really neat. I had no idea there were darlingtons with built in temperature compensation. Thanks a lot! That should make the project simpler as well.
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Old 30th May 2006, 08:33 AM   #4
glennb is offline glennb  Australia
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The Vbe multiplier transistor is not critical. Almost any general purpose NPN small signal transistor can be used! It must be mounted on the heatsink near the power output transistors to be effective. Its Vbe needs to be able to follow the thermal variation of the Vbe in the output transistors.

BTW, National Semiconductor have just published a new Application Note #1490 "LM4702 Power Amplifier" in May 2006. The 10 page PDF is available at
http://www.national.com/an/AN/AN-1490.pdf
Good reading!

Glenn
Melbourne, Australia
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Old 30th May 2006, 01:09 PM   #5
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Quote:
Originally posted by glennb

BTW, National Semiconductor have just published a new Application Note #1490 "LM4702 Power Amplifier" in May 2006. The 10 page PDF is available at
http://www.national.com/an/AN/AN-1490.pdf
Good reading!

Glenn
Melbourne, Australia

Glenn -- thanks for pointing this out -- although the link may be broken.
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Old 30th May 2006, 01:41 PM   #6
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Quote:
Originally posted by jackinnj

Glenn -- thanks for pointing this out -- although the link may be broken.
Why do you think so? It works splendidly for me, and doesn't look strange either.

Rune
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Old 30th May 2006, 01:50 PM   #7
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now it's working -- perhaps the server was taxed.

jack
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Old 30th May 2006, 02:24 PM   #8
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"sounds" like we need a servo -- and have to take apart a 'scope for some mu-metal.
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Old 1st June 2006, 06:21 AM   #9
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Default AN1490

Hi,
I notice the following about the paper:
1. Cin is a Panasonic FC, not a non-polar cap as drawn on the schematic.
2. MN2488 and MP1620 are long obsolete. So are 2SD2488 and 2SB1620 (There must be a dusty warehouse somewhere in NS).
3. No V/A rating for the transformers.
I would also say that there should be overload protection and relay, but that is personal I guess.
I would trade my tin boxes for the case any day though.
Cheers,
KK
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Old 1st June 2006, 02:17 PM   #10
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most important take-away from the paper is the strict adherence to a layout regimen, ground point (not plane), and routing of Sink1 and Sink2 apart.

They may have experienced some EMI/RFI problems with the amp input as there are now 30pF caps on the inputs.
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