3. The Baxandall Class-D Oscillator with a Pulse-Width Modulated Voltage Drive


It is more interesting to used pulse width modulation to generate a more or less crude approximation to the raised half-sine wave at the free end of L3 to create the same effect.


Because the PWM waveform can’t rise above the power supply rail, we lose a third of our output amplitude, but that can be dealt with in the output windings (L4 and L5).


We have the same problem with the uncertainty in the resonant frequency of the tank circuit, but in this case we can use a phase-locked loop to force a voltage controlled oscillator to run at some convenient multiple (at least twelve times) of the resonant frequency – the CMOS 4046 or one of its variants could be used to do the job.


The circuit simulated doesn’t include the 4046, and was tuned by adjusting the period of V2 by hand to 5.32usec, longer than the 5.236usec  that one would expect from the resonant frequency of the tank circuit (L1 plus L2 in parallel with C1).



Even with the snubber in parallel with L3 and L6, there is very nasty 10MHz ringing on the switcher-overs between M3 and M4 – non-over-lapping drive waveforms would help.



The current swing in the inductor L3 is now +/-7mA (ignoring the switching spikes to +25mA and –15mA).



This gives the third harmonic distortion some 48dB below the fundamental – not as good as the resonant trap at 55dB down, but better than the classic circuit at 40dB.



The circuit still looks good with a 100R at R3, rather than the 10k used above



though the third harmonic is a lot higher, only 37dB below the fundamental.


Shifting the phase of the drive waveform to bring the drive current more or less in-phase with the output voltage



doesn’t make much difference to the distortion, though the frequency moves a bit higher.


Interestingly, the voltage at the centre tap moves from peaking about 1% too early to peaking about 2.5% too late (15.9usec to 16.3usec).


Obviously I’d like to try a slightly smaller phase advance to get the drive current (though L3) a little more nearly in-phase with the voltage across the tank circuit (essentially Vct), but that would involve a much longer counter which would be difficult to manage in LTSpice.


In real life, a longer counter wouldn’t be a problem.