Yet another amp - simple inverted with 10x gain, mosfet output stage - Page 10 - diyAudio
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Old 14th April 2012, 10:21 PM   #91
sonnya is offline sonnya  Denmark
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You have to draw this output stage. I cannot remember i have seen it.

The reason for the low distortion is the use of a driver stage for the mosfets.

You need to get rid of the gate charge and build it up fast. That requires a lot of current which the VAS stage cannot deliver.

I also discovered that standard current mirrors and folded cascode generates slewing related distortion. They both has to supply the compensation cap. And the output stage or the driver stage. The solution was to ADD current gain to the mirrors.

On the same time we need to raise the linearity of the mirror. That is the reason i have choosen Wilson current mirror.

The next problem was the fact that the current mirror with gain reducere the influence of the compensation cap by the gain factor.. We dont need more bandwitdh, just reducing the slewing distortion. So by applying the compensation cap across the current mirror fixed this problem.

I hope it explains how i Got there.
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Old 14th April 2012, 11:04 PM   #92
mikelm is offline mikelm  England
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My o/p darlington pairs are shown in this thread here:

Yet another amp - simple inverted with 10x gain, mosfet output stage

Only I use zvn & zvp3310a instead of 4424a

Better than VAS driving lateral mosfets direct - now VAS sees very small i/p capacitance of 3310a . . . .

Perhaps 10mA still too little current to drive laterals ?

but much lower op impedance than VAS
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Old 22nd April 2012, 07:31 AM   #93
hahfran is offline hahfran  Germany
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Quote:
Originally Posted by WuYit View Post
Without increasing costs...
"laws of semiconductor physics" forever forbid complimentary jFETs.
Forget such designs.
Forget complementary Wilson current mirrors. Yo cannot get that working in practice.
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Old 22nd April 2012, 07:58 AM   #94
hahfran is offline hahfran  Germany
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This inverting needs no compensation and is insensitive to "lack of complimentarity".
However a second PS is inevitable. D1 and D4 resp are thermal track diodes. Capacitors not shown here.
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Old 22nd April 2012, 08:02 AM   #95
hahfran is offline hahfran  Germany
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Not Q1 throu Q6 sit on a thermally conductive plate held at constant temp. That is n-times better than "thermal compensation". Those who understand semiconductor physics understand immediately why this must be so.
Further Q7 Q10 must be mounted on a heatsink separate from the heatsink Q8 Q9.
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Old 22nd April 2012, 08:24 AM   #96
hahfran is offline hahfran  Germany
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Power supply double star grounding
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Old 22nd April 2012, 08:42 AM   #97
sonnya is offline sonnya  Denmark
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Quote:
Originally Posted by hahfran View Post
"laws of semiconductor physics" forever forbid complimentary jFETs.
Forget such designs.
Forget complementary Wilson current mirrors. Yo cannot get that working in practice.
I do not fully follow you.

I think i have never spoken about complimentary JFET'S !? You can make a buffer with two N or P channel Jfets that will act like a complementary, but i don't think that this is what you mean.

For the record: Wilson current mirrors does work in practice.
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Old 22nd April 2012, 09:16 AM   #98
hahfran is offline hahfran  Germany
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But not complementary Wilson mirrors. of course one could select from a big bunch of devices those which yield a DYNAMICALLY (!!) complemnetry pair of Wilson mirrors but why? It is far more clever to follow a design rule that yields topologies insensitive to
such inevitable differences . Only ring-emitter or multi-emitter BJTs can be made dynamically complements, not simple Epi-planars. its not possible.
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Old 22nd April 2012, 10:21 AM   #99
mikelm is offline mikelm  England
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Quote:
Originally Posted by hahfran View Post
Power supply double star grounding
Hi Hafran,

You have not yet realised the full potential of having two separate secondaries & bridges in your PSU diagram.

Whether you use cheap noisy diodes of nice schhotky ones . . . when those diodes turn off a resonance will be excited in the secondary windings of the transformer - usually in the 1meg hz region.

In your diagram you are injecting that resonance directly into the amps earth - sonic disaster !

You need to either damp that resonance with a snubber cct or filter the earth lines before you make your star earth point - or do both !
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Old 22nd April 2012, 10:32 AM   #100
PMA is offline PMA  Europe
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Even a foil cap of some 47nF across secondary may be a total cure. BUT - in any case - it is necessary to MEASURE the effect, there is no general rule, as parasitic impedances of the transformer and switching properties of the diodes used count.
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