Tube DIY Asylum

RE: Class A distortion vs Class AB noise

4.159.80.74

In Reply to: RE: Class A distortion vs Class AB noise posted by Mark Kelly on July 06, 2008 at 03:24:19

Mark -
Imagine a transistor diff amp with zero emitter degeneration, and
variable tail current for gain control. The first IC OTA
(operational transconductance amplifier), the CA3080,
was little more than that. Noise and linearity were
major concerns. The CA 3080 came out in the late 60's,
a few years later, Solid State Music (great name!) came
up with the SSM 2010. It used class AB to improve dynamic
range, and a fwb(!) at the input to predistort for the 'S'
curve of the diff amp. It was a major improvement over the
3080. I used both of these ICs for VCAs and VCFs back then.

A tube diff amp doesn't have these shortcomings; the 6386
($$$) is the most famous example of a variable gain diff amp.
A few mV of input is all a 3080 could handle.

So yes, AB can increase dynamic range, but really only
for undegenerated transistors, when run open-loop; NFB
would tend to make the gain constant, defeating the whole purpose.

Thanks for the trip down memory lane!

Follow Ups:

Thanks - Mark Kelly 15:00:16 07/06/08 (0)

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