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Old 2nd October 2013, 04:03 PM   #1
dadod is offline dadod  Croatia
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Default 200W MOSFET CFA amp

Some people are interested in more powerful CFA amp, and this is 200W//8ohm and 400W//4ohm.
It uses lateral MOSFET output transistors like Hitachi 2SK134/135 and 2SJ49/50 or similar from Toshiba 2SK1056 2SJ162 or from other producers.
Interestin part of this amp is use of TPC compensation, but unusual one with one capacitance branch connected to the output instead to the VAS collectors.
That brings 20kHz distortion down significantly.
This thread is open to discussion and suggestions as itís tried in simulation only.
As CFA uses very low feedback resistance there is significant power dissipation on series resistor, and I show two schematic, one with lower FB resistors and other with double values. I case if Rf is 220 ohm at full power dissipation go up to 14 W and in case if Rf is 480 ohm it halved. Still this resistor should be parallel combination resistors to distribute power dissipation. first is low FB resistance, this gives lower distortion and higher slew rate.
Damir
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File Type: jpg CFA-200W-sch.jpg (135.1 KB, 3505 views)
File Type: jpg CFA-200W-CLG.jpg (243.3 KB, 3227 views)
File Type: jpg CFA-200W-LG.jpg (253.8 KB, 2524 views)
File Type: jpg CFA-200W-clipping-dip.jpg (155.1 KB, 2270 views)
File Type: jpg CFA-200W-clipping-start.jpg (154.4 KB, 2089 views)
File Type: jpg CFA-200W-PSRR-to-negative.jpg (243.3 KB, 265 views)
File Type: jpg CFA-200W-PSRR-to-positive.jpg (232.1 KB, 246 views)
File Type: jpg CFA-200W-square-10kHz 8_1n.jpg (148.2 KB, 242 views)
File Type: jpg CFA-200W-square-10kHz 8_.1u.jpg (139.6 KB, 230 views)
File Type: jpg CFA-200W-square-10kHz 8_1u.jpg (148.0 KB, 253 views)
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Old 2nd October 2013, 04:05 PM   #2
dadod is offline dadod  Croatia
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Here is schematic with higher FB resistance.
Damir
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Old 2nd October 2013, 04:12 PM   #3
mcd99uk is offline mcd99uk  United Kingdom
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Maybe add diode clamps across the VAS/TIS to clean up the clipping. Looks like saturation.

Do you need a Vbe multiplier with L-FETs? Maybe simple resistor biasing would work.

Last edited by mcd99uk; 2nd October 2013 at 04:20 PM.
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Old 2nd October 2013, 04:31 PM   #4
dadod is offline dadod  Croatia
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Quote:
Originally Posted by mcd99uk View Post
Maybe add diode clamps across the VAS/TIS to clean up the clipping. Looks like saturation.

Do you need a Vbe multiplier with L-FETs? Maybe simple resistor biasing would work.
This is typical TPC clipping, but here is no sticking, and the amp is going fast out of clipping.
Still I will try to find good way to make clean clipping, not sure what is the best way.
The Vbe multiplier is here to simplify Bias adjustement no for thermal control, as simple trimer can lose conntact and could provoke the bias go to high and burn mosfet. In the Vbe the resistor between base and collector is a trim pot and if loses conntact the bias is going down(but you know that).
I am not yet sure if some thermal compesation is needed for the drivers.
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Old 2nd October 2013, 04:50 PM   #5
mcd99uk is offline mcd99uk  United Kingdom
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Quote:
Originally Posted by dadod View Post
The Vbe multiplier is here to simplify Bias adjustement no for thermal control, as simple trimer can lose conntact and could provoke the bias go to high and burn mosfet. In the Vbe the resistor between base and collector is a trim pot and if loses conntact the bias is going down(but you know that).
Good point but a couple of resistors in series could be used instead of a trimmer in a simple resistive bias. L-Fets appear to be very easy to bias as you know. My preference is not to use a Vbe multiplier, I worry about the potential for oscillation.

Have you got a good source for the a1381/c3503 transistors where you can get the same Hfe Rank? Wonder if mismatched ranks would cause a problem when used as drivers?
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Old 2nd October 2013, 06:33 PM   #6
dadod is offline dadod  Croatia
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Quote:
Originally Posted by mcd99uk View Post
Good point but a couple of resistors in series could be used instead of a trimmer in a simple resistive bias. L-Fets appear to be very easy to bias as you know. My preference is not to use a Vbe multiplier, I worry about the potential for oscillation.

Have you got a good source for the a1381/c3503 transistors where you can get the same Hfe Rank? Wonder if mismatched ranks would cause a problem when used as drivers?
There is big problem with fakes, probably if you order from big sellers, but I can't recomand any, had bad experience.
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Old 2nd October 2013, 07:09 PM   #7
mlloyd1 is online now mlloyd1  United States
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just to eliminate any confusion (especially these days trying to stay ahead of the counterfeiters), Toshiba didn't (doesn't) make any lateral mosfets. that was hitachi, then that division became renesas who currently still provides the 2sk105x/2sj16x laterals. others (like profusion) provide lateral mosfets with similar dies (i.e. similar performance). i think someone even provides double dies in a single device for higher power (but also more capacitance).

the nicely complimentary mosfets toshiba provided a while back (2sk15xx/2sj20x) are "verticals" with low (almost lateral-like) threshhold voltage.

not interchangeable without suitable circuit modifications

ok, here's some links since it's never good to trust my memory:

for "hitachi-like" laterals:

MOSFETs for amplifier | Renesas Electronics America

Magnatec. ALFET Lateral MOSFETs

Lateral Mosfet

for the toshibas now EOL'd:
2SK1530 | Products | TOSHIBA Semiconductor & Storage Products Company
and
2SJ201 | Products | TOSHIBA Semiconductor & Storage Products Company

mlloyd1

Last edited by mlloyd1; 2nd October 2013 at 07:17 PM. Reason: added links
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Old 2nd October 2013, 07:56 PM   #8
Krisfr is offline Krisfr  United States
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Are LTSpice models available for all of these from Renesas or ?
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Old 2nd October 2013, 08:02 PM   #9
mcd99uk is offline mcd99uk  United Kingdom
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Quote:
Originally Posted by dadod View Post
There is big problem with fakes, probably if you order from big sellers, but I can't recomand any, had bad experience.
The only major supplier of these transistors I have used is Mouser and they have the current product KSA/KSC but they are of different hfe ranks.

Sometimes I wonder whether we should design these transistors out by using the dreaded 3 transistor VAS. Then with the cascoding we can use normal commonly available matched rank transistors. Regarding drivers I used bootstrapping and BC3x7-40s in my CFA which meant that the KSC/KSA could be avoided there. For drivers (if I am correct) the current gain of the transistors is more important.

Sorry for going slightly off topic.
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Old 2nd October 2013, 08:14 PM   #10
dadod is offline dadod  Croatia
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Quote:
Originally Posted by mlloyd1 View Post
just to eliminate any confusion (especially these days trying to stay ahead of the counterfeiters), Toshiba didn't (doesn't) make any lateral mosfets. that was hitachi, then that division became renesas who currently still provides the 2sk105x/2sj16x laterals. others (like profusion) provide lateral mosfets with similar dies (i.e. similar performance). i think someone even provides double dies in a single device for higher power (but also more capacitance).

the nicely complimentary mosfets toshiba provided a while back (2sk15xx/2sj20x) are "verticals" with low (almost lateral-like) threshhold voltage.

not interchangeable without suitable circuit modifications

ok, here's some links since it's never good to trust my memory:

for "hitachi-like" laterals:

MOSFETs for amplifier | Renesas Electronics America

Magnatec. ALFET Lateral MOSFETs

Lateral Mosfet

for the toshibas now EOL'd:
2SK1530 | Products | TOSHIBA Semiconductor & Storage Products Company
and
2SJ201 | Products | TOSHIBA Semiconductor & Storage Products Company

mlloyd1
Sorry, I was thinking that 2SK1056 2SJ162 where produced by Thoshiba too.
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