Butler input pair

rtarbell · 19th November 2005, 06:14 AM

I know that some audio-grade opamps use Butler front ends (a pair of BJT and FET transistors in parallel as the input transistors, to try and achieve the best of both worlds), but has anyone tried this for a discrete power amp? What where your results?

amplifierguru · 19th November 2005, 06:26 AM

Hi rtarbel,

What problem to solve? This is probably the question to ask. BJT stages have 'bias cancellation' and FET chips have adequate monolithic matching to suit audio. Could simply be introducing the weaknesses of both.

I don't see a need so wouldn't be looking for this.

Cheers,
greg

djk · 19th November 2005, 11:51 AM

Just what I always wanted.

High input bias (BJT) and output phase reversal on overdrive (FET).

http://patft.uspto.gov/netacgi/nph-P...&RS=PN/5101126

mikeks · 20th November 2005, 12:26 AM

Quote:

Originally posted by djk
...............output phase reversal on overdrive.....

Vanishingly unlikely in an audio amp....

djk · 20th November 2005, 01:48 AM

Right.

Do you have any real experience?

I didn't think so.

Having built an amp with this (and other FET opamps) with this problem is why I bring it up.

The data sheet from Analog Devices (OP275) even talks ways to minimize the problem.

Of course, what do they know? They just designed it, built it, and got a patent on it.

Maybe to be of help you should write us an application note, AD may even pay you for it.

mikeks · 20th November 2005, 02:33 AM

Quote:

Originally posted by djk
.....got a patent on it.....

...patents can be obtained for virtually anything....

...usefull...or otherwise...

...phase reversal due to overdrive in an audio power amp specifically is a non-issue....

mikeks · 20th November 2005, 02:42 AM

Quote:

Originally posted by djk
Right.

Do you have any real experience?

I didn't think so.

http://www.diyaudio.com/forums/showt...393#post198393

19th November 2005, 06:14 AM	#1
rtarbell diyAudio Member Join Date: Sep 2005	Butler input pair I know that some audio-grade opamps use Butler front ends (a pair of BJT and FET transistors in parallel as the input transistors, to try and achieve the best of both worlds), but has anyone tried this for a discrete power amp? What where your results?

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19th November 2005, 06:26 AM	#2
amplifierguru Account Disabled Join Date: Aug 2004 Location: Earth	Hi rtarbel, What problem to solve? This is probably the question to ask. BJT stages have 'bias cancellation' and FET chips have adequate monolithic matching to suit audio. Could simply be introducing the weaknesses of both. I don't see a need so wouldn't be looking for this. Cheers, greg

19th November 2005, 11:51 AM	#3
djk diyAudio Member Join Date: Feb 2001 Location: USA	Just what I always wanted. High input bias (BJT) and output phase reversal on overdrive (FET). http://patft.uspto.gov/netacgi/nph-P...&RS=PN/5101126

20th November 2005, 01:48 AM	#5
djk diyAudio Member Join Date: Feb 2001 Location: USA	Right. Do you have any real experience? I didn't think so. Having built an amp with this (and other FET opamps) with this problem is why I bring it up. The data sheet from Analog Devices (OP275) even talks ways to minimize the problem. Of course, what do they know? They just designed it, built it, and got a patent on it. Maybe to be of help you should write us an application note, AD may even pay you for it.