Op amp questions for Mr. Wurcer

[Fred Dieckmann]

Mr. Wurcer,

There has been some interest here in composite op amp-buffer gain blocks. Walt Jung has been a proponent of this approach for audio circuits for several years. My questions are on biasing the op amp output stage with a constant current source, often in conjunction with an in loop buffer. Are improvements in sonics due to conversion of the op amp's complementary follower output to an effectively single ended follower with the CCS (biased greater than the original quiescent current)? What are the advantages and disadvantages of increasing the current source value further above the original complementary follower quiescent current? What would a typical output stage bias be for something like the AD825 or AD8610?

[scott wurcer]

Fred,
This biasing technique can just put the crossover issue out of the picture. There is a measurable
reduction in distortion by doing this. Power is a big concern in most commercial applications
so the output stage is usually biased at a few hundred micro-Amps to a couple of milli-Amps
depending on the intended application. If you bias the output so it is true class-A you are
ahead of the game. Most of the modern amplifiers are rated to drive a pretty hefty load, so
you can go beyond a little but there will be diminishing gain. Stay well within the rated drive
current and you will be OK.

The question remains as to N-ness or P-ness in the direction of the CCS, you will have to
experiment and determine for yourself. I did this experiment years ago on a classic Bipolar
process (bad pnp) and the pnp (pull up) was better, go figure.

[Fred Dieckmann]

Thanks Scott! You are a gentleman and a scholar.