2. The Baxandall Class-D Oscillator with a resonant trap

The distortion in the sine wave output for lightly loaded inverters can be reduced by putting a an extra LC filter in series with basic inductor LC to block the second harmonic.

Adding the inductor L8 and the capacitor C2 appreciably reduces the third harmonic spike

for a lightly loaded tank circuit – there is now some second harmonic distortion, but it is 56dB down on the fundamental, while the third harmonic component is also 55dB down, as opposed to 40dB down in the classic circuit.

The current swing in L3 is now reduced from around +/-17mA to around +/-3mA and the fourth harmonic is now dominant.

L3 no longer sees all the voltage difference between the power supply and the centre tap – the filter inductor L8 now deals with all most all the second harmonic component, and the second harmonic current is now routed to ground through C2, rather than being fed through the tank circuit. Comparing the voltage at the “filter node” – the common point of L8, L3 and C2 – one can see how L3 has been unloaded.

This isn’t a particularly practical circuit. L8/C2 has to be tuned to be quite close to the second harmonic of the tank circuit, and while we can buy 1% tolerance capacitors in the values used in the examples, the sort of ungapped core usually used the transformer/tank circuit (L1,L2,L4,L5) has a typical tolerance around +/-25%. We could used gapped cores with adjustors, but this takes us a long way away from the elegant simplicity of the original Baxandall circuit.