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Old 25th July 2002, 08:30 PM   #1
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Question SOS for BJT's in output ???

Hi all

If an output amp of say 200 W into 8 ohms is wanted and I would like to use BJT's. How schould I go about designing the output stage to get within the limits of the SOA ?

Scould I go :

1 : lots of transistors in parallel
2 : 2 or more in series

What is the better solution ?

Please help

\Jens
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Old 25th July 2002, 09:42 PM   #2
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For starters: bridged or unbridged?

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Old 26th July 2002, 07:43 AM   #3
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Default SOA for BJT

The amp is only to be used in non briged mode, with rails of about 65 Volts (+-).

Transistors are MJL3281A/MJL1302A from ON (Former Motorola I think)

Thanks

\Jens
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Old 26th July 2002, 07:52 AM   #4
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Default Re: SOA for BJT

Quote:
Originally posted by JensRasmussen
The amp is only to be used in non briged mode, with rails of about 65 Volts (+-).

Transistors are MJL3281A/MJL1302A from ON (Former Motorola I think)

Thanks

\Jens
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Old 26th July 2002, 08:01 AM   #5
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Groman *please* try to keep to the topic at hand.
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Old 26th July 2002, 08:06 AM   #6
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Thanks

I didn't want to start a fight since I'm new here.

Does anyone have any idears for the SOA ?

\Jens
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Old 26th July 2002, 08:51 AM   #7
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warning -- I don't really know enough about this to give more than my best attempt, so take this with a grain of salt. Hopefully someone who knows more about SOA will jump in.

I would use parallel devices becasue teh real impedance of 8 Ohm speakes is likely to be somewhat less (perhaps a lot less) than 8 ohms at some frequencies, so you might get wuite a bit more current than 65V/8 Ohms.

Then to check SOA, look at page 4 of the MJL1302A data sheet, figure 13.

Unfortunately the SOA plot is on log-log scales, which makes it more trouble ... but you can plot a bunch of points on the line

I=0.5*(1/8)*(65-V)

(the 0.5 from using parallel devices)

Although that is really a line it will be a curve on the log-log plot.

If the curve crosses SOA limit you have to use series devices as well.

Why not just play it safe and use 4 of each, 2 series * 2 parallel per channel?
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Old 26th July 2002, 09:05 AM   #8
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Default To mirlo

Thanks...

I was planning to use 5 transistors in parallel, since the BC voltage if the MJL3281/1302 is rated at 200 V there is no problem there. And the transistors will work at currents where Betadrop has not started yet.

Also i find it harder to design the output with transistors in series.

I would like to use a standard darlington confuration since it's problably the only one that's really suited for parallel cofigurations.

How does that sound ?

\Jens
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Old 26th July 2002, 09:46 AM   #9
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Default How does it sound?

You have to tell us, when it's done!

I suspect that 5 parallel is more than enough without series, but I'd go through the process of drawing that load line with scale factor 1/5 instead of 1/2 anyway.

What I meant to suggest from the Bryston schematics isn't the tricky NPN+PNP per side output stage but the voltage divider thing they use in their higher output amps, would be something like this if translated to Darlington:
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Old 26th July 2002, 10:18 AM   #10
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Default Power dissapation in output

Hi

This is the simulated power dissapation in one transistor with

RL = 4 ohm
Rails = 65 V

I suppose thats ok ?

\Jens
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