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rtarbell 19th November 2005 06:14 AM

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?

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.


djk 19th November 2005 11:51 AM

Just what I always wanted.

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

mikeks 20th November 2005 12:26 AM


Originally posted by djk
...............output phase reversal on overdrive.....
Vanishingly unlikely in an audio amp....

djk 20th November 2005 01:48 AM


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


Originally posted by djk 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


Originally posted by djk

Do you have any real experience?

I didn't think so.

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