Best MOSFET for output stages

[nigelwright7557]

IRFP240/9240 are by far not complimentary.
One Rds is well below the other for starters.
This seriously affects the damping factor by being different on different phases.

This doesnt mean you cant use them as complimentary as feedback takes care of most evils in them.

[nigelwright7557]

Originally Posted by ilimzn

Argh always this story about IRF/IRFP not needing thermal compensation..

It depends on how much standing current you want to pass.

If you dont mind settling for just enough to get rid of crossover distortion on the scope then a simple Vbe multiplier works not on the heatsink.

If you want 100mA or more then you need the Vbe multiplier on the heatsink or glued on an output transistor.

I found using 240/9240 I had no problems.
However, it got a bit more difficult when using a quasi output stage.
I found I needed the Vbe multiplier on the heatsink.

I have built about 40 complimentary pcb's and 2 quasi's to get my data.

[tiefbassuebertr]

Most topologies used so called "true complementary" power stages, even by IRF MOSFETs. If such power stages runs in pure class A, no matter regarded the sound.
But if such stages runs in Class AB (mostly 50-150mA idle current/each pair) the sonic results always worse, because the term "true complementary" itself is an error (present only in therory and schematic).


Only inexperienced developers choose "true complementary" power stages topologies. But experienced developers like L. Ollson or diyaudio member "X-PRO" choose only N-Channel MOSFETs in the output with the related front end and get better results especially by not "pure ClassA" resp. hard running bias adjust. My favorite topology is the CSPP/PPP circuit (inside also by commercial brand SUMO).

Check out this threads:

Creek 5350MK2 (5350-II 5350 Mark 2 5350-MK2) - circuit descr. arround Q12a wanted
http://www.ant-audio.co.uk/Theory/N-...0linearity.pdf

Only N-Channel MOSFETs (NMOS); better Audio from non complements by Audio Power?
URL collection of Totem Pole and CSPP (Single Ended related) Solid State Output Stage

It's cheap, it's N, it's dirty, it's.... The CIRCLOMOS!!!

How to identify a quasi-complementary amplifier?

SUMO Power Amp

[ontoaba]

Originally Posted by nigelwright7557

I found using 240/9240 I had no problems.
However, it got a bit more difficult when using a quasi output stage.
I found I needed the Vbe multiplier on the heatsink.

Thats is right. It has thermal problem (thermal runaway) when junction temperature start to reach 70C or above. Better to keep the temperature below 65C. DMOS working faster and stronger than LateralMOS, but lateral mosfet has exelent linearity and thermal stability.

[wahab]

Originally Posted by ilimzn

There is no speciffic sweet spot except that it's usually around 100mA - as there are tolerances. I've seen variance from about 70 to 200mA across many dozens of devices (had to make several sets of matched pairs)...

Wahab, Toshiba and IR can both give very good results if you know how they work. It does sometimes require some specialcircuits - like error correction or feedforward, or similar techniques (eg. CFP-like drive, but this tends to be difficult to tame re oscilaltions). Toshibas are more forgiving. Also, adding degeneration can help but it increases loss quite a bit, as rather high value source resistors are required to make any difference. When using simple circuits, I've found that Toshibas definitely have a sweet spot regarding bias current, usually between 150 and 200mA per pair. IR and the like seem to work the better the higher the bias, but with Toshibas, if you go too high, they tend to start sounding dull. Of course, it's not a simple matter of comparison as this depends largely on the rest of the amp, what I'm talking about are simple linn-derived topologies, with refinements such as cascoding and current mirrors - kind of what you would expect if someone took a proven classic BJT output design and replaced the output pair(s) with MOSFETs.

Thank you for all these infos, Ilimzn , you experience is much
apreciated..

[Mooly]

Originally Posted by okkyn

Hi.. I'm resurrecting zombie thread

Is is possible to directly replace (with minor modification) those Hitachi mosfets with TO3 IRF230/9230??

Thanks
okky

The simple answer to that is no

Whether it's possible depends on the circuit... the bias generator would need to provide the required voltage for the IRF's and as others have eluded, also be able to provide thermal tracking.

The laterals are far superior for a class b (ab) design at low bias current.
Why are you looking to replace with IRF's ? have the originals failed ?

[ilimzn]

I have built several versions o a design that is extremely stable thermally, using a single pair of IRFP240/9140 without any source resistors at all. The bias generator is a simple Vgs multiplier - a MOSFET version of the classic Vbe multiplier - using an IRF610. This particular part was chosen simply because it was at hand and because it uses the exact same geometry on the silicon except fewer cells, so thermal tracking is almost perfect. The metal tab makes it very easy to track the temperature of the heatsink.
The exact factor of Vgs multiplication is usually a bit over 2 but depends on the ratio fo desired bias current in the outputs and current through the Vgs multiplier.
One particulairly interesting version of this output stage was biassed to 100mA, and it's driver also uses 100mA as the standing current, so this same current passes through the Vgs multiplier (the current sounds high but it's perfectly easily attainable since the driver is actually a MOSFET 'VAS' - higher current in it just makes it more linear and capable of driving the output stage and the only extra cost is the heat). As an experiment, an IRF640 was sued in the Vgs multiplier, since it is very close in specs as the outputs and it passes the same idle current. Predictably, the multiplication factor was near exactly 2, and the bias was actually very slightly overcompensated (falls from 100mA at 20C to about 97mA at 50C). This means the compensating MOSFET needs to work with a current closer to it's zero tempco than the outputs, in other words, it's better if it's a lower Idmax device compared to the outputs. Needless to say, in a situation where there are no source resistors, you want the most complementary devices you can choose.

[CBS240]

Hi

The amp I'm currently working on uses comp hexfets with HEC. I took a chance with the PCB and tried an experiment using SOT-23 RF transistors for the error amplifiers. With HEC, the error devices are part of the thermal transfer function so they must be within the thermal compensation loop. I mounted them on the PCB so they are directly under and in contact with the drain pin right next to the case of the respective output transistor. I was worried about how it would track the bias, but it turns out that it seems to be working. Right now, it has a slight negative coefficient, which is better than a positive. I left room so that the position could be adjusted, a few mm closer or further from the transistor, sort of a "thermal pot" so to speak. The idea is that since the drain is where the heat is generated, the pin temp would better represent the die temperature. The tracking appears to have little delay, which can sometimes happen with the thermal compensating device mounted on the heatsink. I find bias for these type of outputs needs to be at least 150mA, which by observing the error signal seems to be enough to keep Gm more constant wrt Id. Gfs = 22S for these devices.

[okkyn]

Originally Posted by Mooly

The simple answer to that is no

Whether it's possible depends on the circuit... the bias generator would need to provide the required voltage for the IRF's and as others have eluded, also be able to provide thermal tracking.

The laterals are far superior for a class b (ab) design at low bias current.
Why are you looking to replace with IRF's ? have the originals failed ?

Hi Mooly,

Thanks for the answer. Actually I am looking for good high power class AB MOSFET amplifier to drive my new magneplanar. I feel that my DIY amplifier (using a pair of Hitachi) doesn't deliver enough power. I prefer MOSFET's sound than BJT. Lateral MOSFETs are very expensive nowdays, but I don't want high power class A IRF's either, too much heat (and more expensive)..

[wahab]

Originally Posted by okkyn

. Lateral MOSFETs are very expensive nowdays, but I don't want high power class A IRF's either, too much heat (and more expensive)..

They doesn t cost too much if they live long enough...
I bought 6 pairs of Hitachi s back in 1988 , and they
still work flawlessly on the same amp..