Re: Baxendall Class-D oscillator with a big source inductor

On 30 dec, 16:39, MooseFET <kensm...@xxxxxxxxx> wrote:
On Dec 30, 5:20 am, bill.slo...@xxxxxxxx wrote:





On 30 dec, 02:29, MooseFET <kensm...@xxxxxxxxx> wrote:

On Dec 29, 4:37 pm, Rich Grise <r...@xxxxxxxxxxx> wrote:

On Fri, 26 Dec 2008 17:24:49 -0800, John Larkin wrote:
On Fri, 26 Dec 2008 12:22:15 -0800 (PST), MooseFET <kensm...@xxxxxxxxx>
On Dec 26, 10:58 am, "Bill Sloman" <bill.slo...@xxxxxxxx> wrote:
All the years that I've been fooling around with the Baxendall class-D
oscillator, I've believed the original paper, which said that if you
made the source inductor much bigger than the centre-tapped inductor in
the resonant tank, the circuit will "squeg", rather than settling down
to producing a nice steady sine wave. It certainly did when I wound
inductors that had something of the order of four or five times the
inductance of the inductor in the tank circuit.

I don't see any good reason for the topology you posted to want to squeg.
An increase in the center tap voltage biases the two MOSFETs on without
any delay.  Are you sure you didn't leave out some detail of the circuit.

Squegging is usually caused by some bypassed biasing network that has a
slow time constant, as the bipolar version may have had. I can't find any
not-for-pay schematics of the original Baxandall oscillator.

Is this close?http://home.planet.nl/~sloma000/Baxandall%20parallel-resonant%20Class...
(mind the wrap)

It looks to me like an astable MV inductively coupled to the output..

It basically is but note the capacitor across the transformer and the
inductor on the center tap.  It makes a more sine wave like signal on
the transformer and yet is fairly efficient.

If you do it right, the sine wave coming out of the transformer is
pretty good - around 1% third harmonic content and very little else -
until the load degrades the Q of the tank circuit below about 5 - and
the efficiency is usually better than 90%

That wouldn't be into the nasty nonlinear voltage multiplier circuit.

There are ways of getting the distortion down to the 0.1% level - my
favourite degraded the efficiency down to around 50% which didn't
matter in my application, while distortion did.

At 50% you could use a hifi amp to drive the transformer.  I think you
could get that low with a multiple tuned transformer section.  To keep
the capacitively coupled stuff from being a problem, you could use a
linked transformer pair.

I was using the oscillator to excite an LVDT in a precision weighing
system that monitored the weight of a single crystal of GaAs as it was
pulled from the melt in a Metals Research Czochralski crystal puller.

The original circuit that I was replacing had used a couple of parts
that had gone obsolete, so I had the fun of designing the replacement
into the few cubic inches of space directly above the LVDT (and below
the slip ring that carried the electrical signals in and out of the
rotating pulling head) that had accomodated the original electronics.
The replacement circuit was more complicated than the circuit it
replaced, but there really wasn't room for a hifi amplifier,

--
Bill Sloman, Nijmegen

.

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