LME48910 biasing...

[Boscoe]

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

 

[qusp]

by using lme49830

 

[h_a]

Hi Boscoe,

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

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

 

[qusp]

yeah its only laterals that are not as sensitive to thermal runaway

 

[astankov]

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...

 

[carlmart]

On the LME48910 datasheet there's also an application board using those very same mosfets you want... with VBE.

 

[Boscoe]

So with negative temp coefficient fets I can just use a resistor to determine volts drop from the constant current source?