Butler input pair

[rtarbell]

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]

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]

Just what I always wanted.

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

http://patft.uspto.gov/netacgi/nph-...0&s1=5101126.WKU.&OS=PN/5101126&RS=PN/5101126

 

[mikeks]

...............output phase reversal on overdrive.....

Vanishingly unlikely in an audio amp....

 

[djk]

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]

.....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]

Right.

Do you have any real experience?

I didn't think so.

http://www.diyaudio.com/forums/showthread.php?postid=198393#post198393