what about diamond buffer MOSFET?

[padamiecki]

Hello!
I was wondering why nobody did not try diamond buffer built of mosfets? or did not write about it? Is it obvious?
Please look at the circuit.
The first pair is IRF540/9540 (or any low power mosfets pair), output pair IRFP140/9240.
Large heatsinks have to be used.
I only do not know if there is a need to mount all of the mosfets on it, or only 140/9240? In this case small heatsinks should be use for 540/9540.

[padamiecki]

Please look at the simulation results, input voltage = 1V~, output almost the same.
Nice distribution!
Large offset might be a problem, maybe there will be necessary to use a DC-servo?
However 0.1 Ohm resistors stabilises output current during changing of the temperature.
Besides some amplifiers have at the output +-0.6V DC.

[padamiecki]

And this is the graph of +-15V~ at the output.
Nice too!
The circuit has relatively high input impedance (and capacitance?).
It seems to be ideal choice to use tube or good quality opamp preamp!

[Upupa Epops]

All is nonsens, my friend...poor efficiency, poor temperature stability, big distortion, big input capacity...stay on earth

[padamiecki]

Originally posted by Upupa Epops
...poor efficiency

yes, indeed, class A has poor efficiency!

Originally posted by Upupa Epops
poor temperature stability

not so poor!

Originally posted by Upupa Epops
big input capacity

only low output impedance preamp is needed!

Originally posted by Upupa Epops
big distortion

what???
where???
did you see the graphs?
I think this is very good result for non global NFB circuit!!
I could bet that the sound will be better than in conventional circuits (GNFB).

Originally posted by Upupa Epops
All is nonsens, ...stay on earth

sorry I cannot
better join my heaven

[peranders]

I wouldn't say nonsens but as Pavel is in to: What are the good properties and what are the bad?

I can't find very much good, can you help me here?

[padamiecki]

Originally posted by peranders
I can't find very much good, can you help me here?

maybe not very much good,
but good:
1. low distortion
2. good harmonic character
3. simplicity
4. relatively high power as for a class A

not much? maybe.

sending this circuit on the forum I was curious about possible improovments proposed by the members.

I do not think that it is a top

I thought that it will be a nice gift for the tests and improovments, but does not?

anyway, during this weekend I am going to build it!

[zinsula]

High input capacity? Note that these parts are used as followers, Cgs does not come into equation. Driven with a decent frontend, this is not an issue.
Btw, Padamiecki, take the IRF610 and 9610 for input, they have even much lower capacitance.
Also, I think you should use IRFP240/9140 for output, not the other way around. They are a better complement.

It would be maybe better to feed the input FET's with a constant current source (replace R1+R2 with ccs), that way the source resistors will drop a more constant voltage.

Thermal stability should be OK, when you fit the input FET's to the same heatsink as the output FET's.

P-A,
"why a MOS-FET diamond buffer?"
I may ask you, why a BJT diamond buffer?
In both cases, I find it an elegant way to bias the output stage, while at the same time having the current boost of a driver stage.
Btw, regarding efficiency: This is better efficiency than using a source follower driver stage. You can swing closer to the rails.

Go for it, Padamiecki, and report the results.

Tino

[padamiecki]

[peranders]

Maybe I should ask what are you going to use it for?

Which currents do you have in mind? EDIT: 1.7 A Wow.

A few possible drawbacks:

Lower transconductance => more distortion, higher output impedance

Unpredictable Vgs, needs carefull matching

Very unlinear capacitance behavoiur => distortion maybe

Lot's of output offset (may not be a problem though)

Much more temperature coefficient, -10 mV/ deg vs. -2 mV/deg

Much more input capacitance, possibility to reduced bandwidth.

If you really want to know, just do some P2P experimenting.

I maybe be totally wrong you know.