To All from PA0SE

The following subject may have been discussed on the reflector before but I can't remember it.

Class D and E final amplifiers have high efficiency because they produce square waves. When the voltage between source and drain of the FETs is high, current is zero; when current flows voltage is almost zero…

Dear Dick, LF Group,

The first thing is that class E refers to a very specific circuit configuration – see Alan G3NYK’s web pages at http://www.alan.melia.btinternet.co.uk/classepa.htm for a discussion of the concept.

The basic Class D amplifier configuration comes in two flavours; current-fed and voltage-fed. The voltage fed type drives a load resistor with a square-wave voltage, through a series-tuned tank circuit (ideally with infinite loaded Q), resulting in a sinusoidal current in the load. The current-fed type drives a load resistor shunted with a parallel tuned tank circuit (again, ideally with infinite loaded Q) with a square wave current, resulting in a sinusoidal voltage across the load. In both cases, the switching devices produce a square wave by alternately reversing the direction of the DC supply voltage or current at the desired output frequency. The voltage-fed type uses a conventional DC supply, decoupled to ground in the normal way with low impedance capacitors. A constant-current supply is not a convenient thing to produce, so in practice the current-fed type uses a conventional low impedance DC supply with a high-impedance series choke, which is a satisfactory approximation.

As Dick points out, one could not apply a square-wave voltage to a tank circuit with a shunt capacitor without having huge (theoretically infinite for ideal components) peak currents and power losses, as you tried to charge and discharge the capacitor instantaneously (I = C.dv/dt). Likewise, if you tried to apply a square-wave current to a series-tuned tank circuit, you would have huge voltage spikes from trying to reverse the tank inductor current instantaneously (V=L.di/dt), with similar disastrous results. So one cannot pick and choose – a voltage switching circuit must drive a load with series inductance, to keep the current to finite levels, whilst the current switching version must have shunt capacitance to avoid the voltage spikes.

Note that the function of the resonant tank circuit is NOT primarily to “filter out harmonics”, but to act as an energy storage “flywheel”, which allows the conversion of the square wave drive into a sinusoidal output without loss of power. It also achieves desirable switching conditions for the power transistors – in the voltage fed case, the load current is zero when the voltage reversal occurs, whilst with the current fed circuit, the voltage across the load is zero when current switching occurs. This means that, even with fairly slow switching of the transistors, transient power dissipation is low.

The “Decca” circuit is a clear example of a voltage-fed class D amplifier with a series L and C tank circuit. But with the push-pull designs typified by the G3YXM and G0MRF circuits, things are not so clear. For a start, neither has a resonant tank circuit as such. However, the output low pass filter is “Quasi-parallel-resonant”, in that it has resistive impedance at the output frequency (or almost so in the G0MRF case), and a capacitive impedance at higher frequencies, so it acts in a somewhat similar way to a very low Q parallel tuned tank circuit. This suggests a current-fed configuration, and indeed the ‘YXM design has the DC supply fed to the PA transformer primary via a choke, as one would expect. However, the ‘MRF has the transformer centre tap decoupled to ground via a (fairly) low impedance capacitor. So at first sight, it would appear that each time switching occurs, one MOSFET or the other must try to transfer charge instantaneously from this capacitor to the filter input capacitor through the transformer, resulting in a large transient current spike. However, in reality, life is more complicated. The transformer has substantial leakage inductance, which prevents a very rapid rise in current – so the circuit is somewhat in a grey area between voltage- and current-fed class D. The trouble is that the leakage inductance resonates with the various capacitors, and the other inductors in the circuit, to produce several different resonant frequencies in the output network, all of which are driven by the switching transients. This results in a very complicated waveform with high amplitude ringing at high frequencies. Although the circuit “works” in the sense that it produces substantial 136kHz output, it results in high power losses in the PA components, if you are unlucky. I suspect that the reason some people have had problems with the MRF design, while for others it works well, is that the overall circuit behaviour is quite sensitive to the exact amount of leakage inductance, and so on the exact physical construction of the transformer. When the ringing occurs at just the right frequency, the circuit could act as a class ”F” amplifier, but that is another story…

The mods I came up with for the ‘MRF circuit aim to convert it into something nearer the “textbook” current fed configuration. Particularly, I tried to reduce the leakage inductance of the transformer – the original design had a leakage inductance of about 2.5uH, which has a reactance of the same order as the load impedance of a few ohms on the primary side of the transformer, so is bound to substantially affect circuit behaviour. The new transformer I described reduced this to about 0.25uH. Eliminating the “decoupling” capacitor and increasing the feed choke inductance makes the drain current a fair approximation to a square wave as desired. The reduced leakage inductance and the introduction of the “snubber” RC networks get rid of a lot of the HF ringing, and “simplify” the circuit behaviour. The low-pass, Low-Q output filter network means that the PA voltage waveform is not the ideal sinusoidal form, but seems near enough not to make a lot of difference. I have experimented with higher-Q tank circuits, and also with voltage-fed arrangements with a series LC tank, which do improve the waveform. However, since the circuit is about 90% efficient anyway, there is not a great deal of improvement to be had, plus the higher loaded Q circuits have the disadvantage of needing bigger, lower loss, output Ls and Cs which must be more accurately adjusted in value. Also, the low-pass configuration gives better reduction of harmonics; even if a higher Q tank circuit is used, an additional LPF will probably still be needed. Although the antenna itself will give substantial reduction of lower order harmonics, the switching transients inevitably mean that significant harmonics are generated throughout the MF and HF range at 136kHz intervals. It is unlikely that the antenna-plus-loading coil will give good rejection throughout that range.

Cheers, Jim Moritz

73 de M0BMU