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Old 28th February 2012, 08:12 PM   #1
Boscoe is offline Boscoe  United Kingdom
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Default LME48910 biasing...

The schematic is in the datasheet here.

If someone good help me with some of these it would be great!

1. I wonder about the Vbe multiplier they have displayed and talk about in the text. It says that 2.8mA is a good value to flow from BIASm to BIASp but out of interest how would I work this out not know the voltage presented by these two pins? I know it's knot a simple as putting a resistor there but I would like to know more.

2. Also they talk about a bias formula which I haven't got a clue about.

3. I prefer to use FETs so could I just use diodes and forget the Vbe because I don't have to worry about thermal runaway? I want to use IRFP240 and it's partner. How would I implement this?

Thanks
Boscoe
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Old 29th February 2012, 04:57 AM   #2
qusp is offline qusp  Australia
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by using lme49830
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Old 29th February 2012, 06:20 AM   #3
h_a is offline h_a  Europe
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Hi Boscoe,

Quote:
It says that 2.8mA is a good value to flow from BIASm to BIASp but out of interest how would I work this out not know the voltage presented by these two pins?
Datasheet says

The LME49810 has two dedicated pins (BIASM and BIASP) for Bias setup and provide a constant current source of about 2.8mA.

so there's a current source and whatever voltage difference you want you can obtain by choosing suitable total resistance in your vbe-multiplier

Quote:
because I don't have to worry about thermal runaway?
Fets are sensitive to thermal runaway! Their zero tempco point you refer to is at several amps for vertical types as the IRFP240!

Hannes
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Old 29th February 2012, 06:32 AM   #4
qusp is offline qusp  Australia
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yeah its only laterals that are not as sensitive to thermal runaway
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Old 29th February 2012, 06:44 AM   #5
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Quote:
Originally Posted by qusp View Post
yeah its only laterals that are not as sensitive to thermal runaway
Absolutely true!

IRFP240/9240 are very sensitive to thermal changes and if you don't fit some thermal feedback be ready for nasty surprise...
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Old 29th February 2012, 11:42 AM   #6
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On the LME48910 datasheet there's also an application board using those very same mosfets you want... with VBE.
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Old 1st March 2012, 06:59 PM   #7
Boscoe is offline Boscoe  United Kingdom
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So with negative temp coefficient fets I can just use a resistor to determine volts drop from the constant current source?
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