Single or dual differential? - Page 9

hitsware · 16th November 2003, 07:11 PM

somebody's doin' it ......

Fred Dieckmann · 16th November 2003, 08:00 PM

DO NOT RUN K389 AND J109 AT 32 VOLTS.

There also needs to be gate resitors on the mosfets and preferably the jfets as well I would cascode or series zener drop the drain voltages for the JFETs. I wonder if an amp from this schematic was actually built......

Fred Dieckmann · 16th November 2003, 08:20 PM

Quote:

Originally posted by john curl
Fred's biasing has problems. Not in concept, but in execution. The resistor values are too large, and the differential gain is asymmetrical

The point was to make the gain asymmetrical to compensate for the the N channel mosfet having a greater transconductance. I believe you would want indentical open loop gains for each half of the circuit for lowest 2nd harmonic distortion. I believe the transconductance of the J109 is also a little greater than the K389 at the same current if I remember correctly. I don't remeber seeing this done before but I am sure it has been, perhaps even by Mr. Curl. The bias resisitors value will depend on the Idss of the Jfets and the value of the drain load resistors. I believe I stated that the would probably not be 75 ohms. I would try for at least 2 or 3 mA of bias current for each jfet and pick ones with Idss several mA above the bias current that they will be used at.

hitsware · 16th November 2003, 08:22 PM

http://perso.wanadoo.fr/jm.plantefeve/2SK389.pdf

Fred,

looks like they're good for 50V

Fred Dieckmann · 16th November 2003, 08:28 PM

The J109s are good for 30 Volts....... There are reasons for a cascode circuit besides just the drain to source voltage being too high.

Elso Kwak · 16th November 2003, 08:32 PM

Quote:

Originally posted by hitsware
http://perso.wanadoo.fr/jm.plantefeve/2SK389.pdf

Fred,

looks like they're good for 50V

Hi hitsware,
JFets get noisy at high supply voltage because of leakeage, though not destructed. See the datasheet of f.a 2N5912 at www.vishay.com and Horowitz.
I learned it the hard way. Some form of cascoding will prevent this from happening.

hitsware · 16th November 2003, 08:37 PM

http://www.ne.jp/asahi/evo/amp/device/2sj109.pdf

Oops.......See what you mean.......Strange they call them complementary? I like super simple so don't do cascodes.
.........mike

Fred Dieckmann · 16th November 2003, 08:38 PM

Give that man a gold star. I was also considering power dissipation which is about 200 mW for these devices.

john curl · 16th November 2003, 09:47 PM

The circuit should work, just as it is. Gate resistors are most probably optional on this design. Why? I don't know why you like gate resistors of jfets. All it does, in this case, is to make the input more noisy.

PMA · 16th November 2003, 09:57 PM

Simple is good, but not the best. This amp (post 81) will have pretty high distortion, driving gate-source capacitance of the Mosfets.

16th November 2003, 08:00 PM	#82
Fred Dieckmann diyAudio Retiree Join Date: Oct 2002 Location: Spain or the pueblo of Los Angeles	KaBoom......... DO NOT RUN K389 AND J109 AT 32 VOLTS. There also needs to be gate resitors on the mosfets and preferably the jfets as well I would cascode or series zener drop the drain voltages for the JFETs. I wonder if an amp from this schematic was actually built......

16th November 2003, 08:28 PM	#85
Fred Dieckmann diyAudio Retiree Join Date: Oct 2002 Location: Spain or the pueblo of Los Angeles	Wrong Data Sheet...... The J109s are good for 30 Volts....... There are reasons for a cascode circuit besides just the drain to source voltage being too high.

16th November 2003, 08:38 PM	#88
Fred Dieckmann diyAudio Retiree Join Date: Oct 2002 Location: Spain or the pueblo of Los Angeles	One reason...... Give that man a gold star. I was also considering power dissipation which is about 200 mW for these devices.

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16th November 2003, 08:22 PM	#84
hitsware diyAudio Member Join Date: Mar 2002	http://perso.wanadoo.fr/jm.plantefeve/2SK389.pdf Fred, looks like they're good for 50V

16th November 2003, 08:37 PM	#87
hitsware diyAudio Member Join Date: Mar 2002	http://www.ne.jp/asahi/evo/amp/device/2sj109.pdf Oops.......See what you mean.......Strange they call them complementary? I like super simple so don't do cascodes. .........mike

16th November 2003, 09:47 PM	#89
john curl diyAudio Member Join Date: Jul 2003 Location: berkeley ca	The circuit should work, just as it is. Gate resistors are most probably optional on this design. Why? I don't know why you like gate resistors of jfets. All it does, in this case, is to make the input more noisy.

16th November 2003, 09:57 PM	#90
PMA Design engineer, consultant diyAudio Member Join Date: Apr 2002 Location: Prague, Kitakyushu, Fukuoka	Simple is good, but not the best. This amp (post 81) will have pretty high distortion, driving gate-source capacitance of the Mosfets.

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