Paralleling Exicon LMOSFETS

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#23
I would say that you have already adjusted for the different gate capacitance of the N and P Fets by using a 680R for the N channel gate and a 470R for the P channel gate.
The Exicon data sheet shows input capacitance at 500pF for N channel and 700pF for P channel, The resistors match that pro-rata by about 2%. so this should ensure the bandwith of the fets will be balanced.
 
#25
Well there it is; that ti lm4702 paper supports what have I heard.
Cool, thanks for sharing that.
The Exicons can benefit from a different gate resistance than the Toshiba also.
The Borbely servo 50/100 shows the use of a 33pf between gate and rail, to help with oscillations too.
 
#26
Yeah thanks for the article...

The zener protection circuit is unnecessary in my design because the exicon devices already incorporate them into the devices. I thought I read that in the exicon site.


Maybe one day just build one using the ti device

Thanks
 
#27
AndrewT said:
Have you read about cross over distortion?
Have you read that the crossover artifacts are at the higher harmonics and sound particularly nasty?
Have you read that mosFETs do not have an optimal bias for minimising crossover distortion?
Have you seen N.Pass recommending increased bias for mosFETs to minimise crossover distortion?
He and others state the higher the mosFET bias the better they perform.
Click to expand...

No, no I haven’t ever read anything. See I actually can’t read. I’m in awe of people like you who can read, especially those who have an ability, as you do, of regurgitating other’s words completely uncritically, without independent thought ever intruding. Just amazed.

Of course you can waste power to reduce crossover distortion in an amp with inadequate slew rate and loop gain. It’s the (lack of) thinking that leads rapidly to class-A “warm” room heaters.

Or you can design a high performance amp that’s fast enough to correct output stage distortion without wasting power.

Dunno about you, but I live in a pretty hot climate, and the air conditioner makes way more noise than any of my amps distortion products. Adding to the need for air conditioning is never helpful.
 
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#29
Here's a quote from Rod Elliot on his LMOSFET design - High Power, High Fidelity Lateral MOSFET Power Amplifier

"In tests thus far (both measurement and listening), I have been unable to detect even a hint of what is commonly referred to as the 'MOSFET sound'. The relatively high levels of low order distortion and susceptibility to crossover (or 'notch' distortion that plague many MOSFET designs are completely missing - indeed, even with zero bias on the MOSFETs, crossover distortion below 10kHz is barely measurable, let alone audible!"

For the one amplifier that I do have elsewhere in similar working order, it's certainly the best sounding i've ever heard. Biased @40mA. Jus hoping to take it up a notch.. TI seems to have found a sweet spot under 150mA for their design.

Does anyone here measure then solder permanently, a resistor for the bias?

Thanks everyone
 
#30
I use a soldered in fixed resistor that limits bias to a safe levels (a couple of hundred milliamperes, but not terribly important), and have a parallel higher value trimpot that’s used to set the actual bias. An advantage of this scheme is that you don’t need to use multiple turn pots, though I generally do anyway.
 
#33
Re source resistors:
If you really are going to bias them 500mA per device, they are so far into PTC region. Looking up the transconductance of such a MOSFET, best case is 2 Siemens or 0.5 Ohms per device. And in that region, it is even higher than 2 Ohms, according to datasheet.
Match them well, and you are fine without source resistance. If any, I wouldn't add more than 0.1 Ohm.


If you bias them below 100 mA per device, things are a bit different. I'd go with 150mA per device (or complementary pair, to be precise).
 
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#34
synonymous said:
Yeah thanks for the article...

The zener protection circuit is unnecessary in my design because the exicon devices already incorporate them into the devices. I thought I read that in the exicon site.

Thanks
Click to expand...

The Exicon Lat Fets have protection zeners for the gates, but the additional zener protection added to some amplifiers, stop the amplifier from overdriving the mosfets if there is very low impedance load.
As such you might not need them - But they are a useful addition especially for PA amps as they increase the survival rate of the FETs.
 
#35
Re zeners for the gates:

I suppose it couldn't hurt to fit them in or leave some space, however, this amp is going to work along with a raspberrry pi and I have both the DAC and a clip monitoring circuit built onto the amp board. The clip monitoring is actually going to signal to the RPI, therefore, I can have it do anything I wish in response. My use will simply reduce the volume until clipping is no longer present, whether that be from a short or naturally overdriven. The only mis-wiring possible will be on the output.


Suzyj,

I notice in some of your designs that you use mica caps. I've got one 10p cap in the feedback loop. Do you find the mica cap to be an improvement in that space? Is there a real difference using those over ceramic in the bypassing to ground as well?

Re bias

I plan to experiment some with the bias. I have had great results in the past using lower bias, although, I've never used distortion meters or actually performed tests relating to, aside of listening and general observations. I realize that the distortions are of types perhaps undetectable to my ears, trained or not, so using a higher bias as per recommendations is most likely in the cards. I love the fact that there are so many different views on subjects like it. I doubt that I'll eclipse the 150mA/pr for more than testing and experimentation. Never kno tho, it's winter time here! Maybe program the PI to do that for me as seasonal climate control! lol

Thanks again all
 
#36
I've gone away from mica caps of late in favour of NP0 ceramics since my ability to measure distortion has improved, as I just can't see any difference in terms of distortion, and NP0 ceramics are a factor of ten cheaper, plus generally smaller to boot, not to mention being much more readily available.

An exception is for the VAS compensation on higher power amps, as these see >100V, and it's a good match for the typically 500V rated mica caps.
 
#37
Thanks Suzy

I've also seen in your designs, 100nF on each output device. Rod Elliot seems to have found they're inconsequential to larger bypass electrolytic caps. Any particular use you have for them? I've 10nF and looking to add most likely a few electrolytic caps on the rails.
I've also read some that there's an option to add some 33pF caps across gate to drain each lmosfet.

Thoughts?

Thank you
 
#38
Bob C. suggests to add R/C stubber across G,D to prevent oscillations. that and the series damper R.
I made a OPC wire amp using the LME14830, I set the bias at about 75mA using dual die types, I measured THD, not much improvement above 75-100mA vs extra heat not worthit to me.
 
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