Class A with LME49811...

[Dxvideo]

Hi all,
Do you think is it possible to design a really good 20-25W Class A amplifier with LME49810?
Why I am asking that? Because this opamp has really good THD figure and I guess it has a class A biased driver stage (consider that it has just 9mA output capability while 49810 and 49830 has ~50mA).
So with gate drivers (AN1645 pg:14) two pairs of HEXFETs like IRFP240/9240 can handle 20W or more class A output with 2.5-3A bias..
It would be an easy and good Class A amplifier...
What do you say?

[EWorkshop1708]

Sounds like a plan.

Are you planning a CCS or Push-Pull Class A output stage?

I'm pretty sure the chip will run on the lower voltage rails, most chips do.

Keep us posted.

Eworkshop1708

[Dxvideo]

Yes.. A plan for now!
I will use P-P output stage.. And will feed the LME with higher voltage than the outputs.
Will keep you informed.

[Dxvideo]

What about this one;

[AndrewT]

Hi,
what is the Vgs of your output FETs when they pass your intended bias current?

Now apply that Vgs across the 100r emitter resistors of the driver pair.
What current are you passing through the drivers?

[Dxvideo]

In fact I expect to adjust the Vgs (and of course the bias current) via the trimpot..
This output design is from National AN1645 pg:14.. I did not make a simulation yet. But with this way I expect to have 2.5A at least... What do you think?

[bobodioulasso]

one pair of mosfets at 1.3A bias would do the job? Look at Mr Pass F5.

[AndrewT]

Originally posted by Dxvideo
In fact I expect to adjust the Vgs (and of course the bias current) via the trimpot..
This output design is from National AN1645 pg:14.. I did not make a simulation yet. But with this way I expect to have 2.5A at least... What do you think?

you have not thought yet.
What is the Vgs of your output FETs when they pass the intended bias current.
Go and measure them, then do the rest.

[Dxvideo]

Originally posted by bobodioulasso
one pair of mosfets at 1.3A bias would do the job? Look at Mr Pass F5.

But my amplifier will give 25W RMS into 8R.. So;
25/8 = 3,125, SQRT(3,125) = 1,767 A RMS into load
That makes 1,767 * SQRT(2) = 2,489 A peak into load
And if you take 20% more for bias that makes ~3A...
Do you still think that 1,3A will enough?

Originally posted by AndrewT
you have not thought yet.
What is the Vgs of your output FETs when they pass the intended bias current.
Go and measure them, then do the rest.

Ohh.. Sorry for the misunderstanding..
As I read from the datasheet of IRFP240 then the Vgs for 3A is just ~5v @ >10v Vds...
Ok I'll make the circuit, but I'd like to make a simulation first.. And have no model for LME49811 !! Do you know any solution for this?
Thanks..

[AndrewT]

Originally posted by Dxvideo


But my amplifier will give 25W RMS into 8R.. So;
25/8 = 3,125, SQRT(3,125) = 1,767 A RMS into load
That makes 1,767 * SQRT(2) = 2,489 A peak into load
And if you take 20% more for bias that makes ~3A...
Do you still think that 1,3A will enough?



Ohh.. Sorry for the misunderstanding..
As I read from the datasheet of IRFP240 then the Vgs for 3A is just ~5v @ >10v Vds...
Ok I'll make the circuit, but I'd like to make a simulation first.. And have no model for LME49811 !! Do you know any solution for this?
Thanks..

do the calculation in your head, or if you must use a calculator/slide rule or whatever.


A single ended amplifier current limits at the bias current level.
A push pull amplifier reaches the ClassA limit at twice the bias current. Most will then go further operating in ClassAB. Read the F5 to see how it's done.