Re: Crossover distortion and NFB

From: Mac (foo_at_bar.net)
Date: 10/24/04 

Date: Sun, 24 Oct 2004 04:20:28 GMT

On Sun, 24 Oct 2004 01:00:56 +0000, Joerg wrote:

[snip]

> This is why all linear audio amps
> need a certain amount of quiescent current. Class D doesn't, of course,
> but that's a whole other topic.

Have you ever read Douglas Self's book, _Audio Power Amplifier Design
Handbook_?

In it he argues that if the output transistors are not conducting half the
time for a sine wave, then it shouldn't be called "class B." He reserves
the term "class B" for when there is an essentially seamless transition
from the upper transistor to the lower one, but no period where they
both conduct. So if the bases of the output transistors are connected
together, it is NOT class B, according to Self. This makes some sense.

But the interesting thing is that he measures the THD and residuals for
class B (his definition) and Class AB open loop output stages, and
the class B stage has lower THD than class AB. This makes a certain amount
of sense, too, because when both output transistors with their finite
betas are on, the driving stage (op-amp or other) experiences lower load
impedance than when one of the transistors is off. This non-linearity is
bound to have repercussions. But if you can get the bias just exactly
right, you would never have both transistors on, and you wouldn't have a
dead zone, either.

Anyway, he concludes that class AB is not a good idea. Either go fully
class A, or just go class B (again, with his definition of class B).

[snip]
>
> Regards, Joerg
>
> http://www.analogconsultants.com

--Mac

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