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Old 30th October 2008, 11:44 AM   #1
Mooly is online now Mooly  United Kingdom
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Default Output stage device types. Yes or no question.

Quick question,
If a design used Lateral FET's would that put YOU off building it. Any reason why ?
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Old 30th October 2008, 11:50 AM   #2
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NO, no reason
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Old 30th October 2008, 11:51 AM   #3
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Not as easy to source laterals, laterals are more expensive.

Other than that? Not really had any experience with lateral mosfet outputs to know I'm afraid. Mosfets always strike me as the way to go when lots of power is the primary objective though...
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Old 30th October 2008, 02:46 PM   #4
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Quote:
Not as easy to source laterals, laterals are more expensive.
And you need a lot laterals, as their transconductance is a bit weak compared to verticals.

Personally I'm not very keen on laterals in power amps as verticals perform better at high bias and for low bias one gets better linearity from (high-beta) bipolars.

But in the end I would see no reason not build an amp because of its use of laterals in the output stage (and one can mod it to verticals anyway).

Have fun, Hannes
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Old 30th October 2008, 03:33 PM   #5
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i'm making my driver boards with jumper selectable bias ranges. that way output stages can be whatever is available/ desired/ cheapest/ most linear/ experimental/ etc...(choose one or more).....
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Old 30th October 2008, 04:48 PM   #6
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What? no pots? Just kidding, pots can fail, it sounds like a novel idea, especially to experiment and compare sound differences wrt bias current.

IMHO, vertical fets are better to my likening. Laterals cost too much. I can crank out 100Wrms from 2 pair of TO-220 hexfets and they cost 1 dollar each. If you overdrive or screw up your circuit and the transistor blows up, you’re not out that much. As stated by Hannes their transconductance is much higher. Verticals are faster too. As for linearity, with a good error correction scheme (EEK! complexity ) Verticals can match up with BJT's, and even better them on heavy reactive loads since they have no secondary breakdown.
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Old 30th October 2008, 05:13 PM   #7
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I'm listening I think they both have advantages and disadvantages.
If you want to know what I have been doing I have been seeing how my Lateral Mosfet design "takes" to vertical types. Very few changes seem to be needed, mainly the adition of a Vgs multiplier with good thermal tracking of the outputs. Slight change to some resistor values of the drivers but that's about it.
From a single pair of IRF9140/140 the performance on test was impressive. Particularly surprising was how well they coped with low impedance loads and most surprising of all -- stability seeing as it was a bit of a lash up.
thanks guys, keep the comments coming.
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Old 30th October 2008, 05:39 PM   #8
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Quote:
Originally posted by Mooly
I'm listening I think they both have advantages and disadvantages.
If you want to know what I have been doing I have been seeing how my Lateral Mosfet design "takes" to vertical types. Very few changes seem to be needed, mainly the adition of a Vgs multiplier with good thermal tracking of the outputs. Slight change to some resistor values of the drivers but that's about it.
From a single pair of IRF9140/140 the performance on test was impressive. Particularly surprising was how well they coped with low impedance loads and most surprising of all -- stability seeing as it was a bit of a lash up.
thanks guys, keep the comments coming.
I Mooly,
I use laterals almost exclusively in my designs nowadays. Granted they cost more. but I have no negative things to report about them (2SK1058/2SJ162). I never use a Vbe multiplier and I have never ran into thermal problems.
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Old 30th October 2008, 06:39 PM   #9
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Hi Nico,
"Nothing negative to report" only the tempco. And we like that . And they are tough, I once years ago accidently shorted the speaker output to one of the rails on an earlier amp. Flash/bang, but no harm done.
Just interested really in what I can do with the verticals, can I match the sonics of,
My MOSFET amplifier designed for music.

The verticals are just so cheap now, not that that's a reason in itself of course.
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Old 30th October 2008, 09:41 PM   #10
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Quote:
Originally posted by Mooly
Hi Nico,
"Nothing negative to report" only the tempco. And we like that . And they are tough, I once years ago accidently shorted the speaker output to one of the rails on an earlier amp. Flash/bang, but no harm done.
Just interested really in what I can do with the verticals, can I match the sonics of,
My MOSFET amplifier designed for music.

The verticals are just so cheap now, not that that's a reason in itself of course.

I prefer verticals, but one can build a very good amplifier out of either type.

I think it is fair to say that one can get into more trouble with verticals than with laterals, so they need a bit more care and feeding to be applied safely and optimally. Since they are much faster than laterals, the verticals are more prone to oscillation unless good measures are taken to avoid it. To get optimum stability with verticals while not throwing away all of their speed, sometimes a gate zobel network is needed.

While verticals can produce amazingly large currents (usually a desirable good thing), this can sometimes be their downfall if there is not some kind of short circuit protection. Laterals just seem to be naturally more robust to screwups.

Cheers,
Bob
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