A New Design for a Class E Driver

I have come up with a new class E driver design that appears to greatly improve the performance (balance, stability, proper drive level) of my 2x2 balanced Class E rig. I think that this design can easily be applied to a 3x3 or larger Class E rig.

The schematic is shown above.

The r.f. input drive power to the driver transistor, in my case, is 1 watt. I use an inductor [L1] in series with the primary of the 5:1 driver input transformer to resonate with the (approximately) 300 ohms of capacitive reactance looking into the transformer from that point. [Note: the capacitive reactance from gate-to-source of a single QFET is ~ 12 ohms at 3.885 MHz. The 5:1 input transformer converts this to 25 x 12 ohms = 300 ohms]. At this same point, there is also approximately 50 ohms of series resistance. [Note: the series resistance from gate to source of a single QFET is around 2 ohms. The 5:1 input transformer converts this to 25 x 2 ohms = 50 ohms]. With about 15 turns of insulated hookup wire on a 1.25” outer diameter PVC coil form … I obtain very close to 1:1 SWR at resonance… and less than 2:1 SWR from about 3.8 – 4 MHz. [Start with 15 turns, and remove a turn at a time until you achieve resonance at your desired center frequency]

I have wired the two output module gate-drive transformers (both 4:1) in series (and, of course, out of phase). This causes the currents flowing into both output module gate buses to be the same. If the total capacitance associated with each output module gate bus as about the same... then the gate drive voltage on each bus will be about the same.

For my 2 x 2 design, the 4:1 turns ratio produces an impedance, looking into the primary of each transformer, of approximately 16 – j96 ohms (i.e., 16 ohms of resistance and 96 ohms of capacitive reactance). The two primaries, placed in series, result in a load on the driver of approximately 32 – j192 ohms. It makes no difference if one uses a single transformer to drive each gate bus, or a pair of transformers in parallel (one at each end of the bus) to drive each gate bus. Two N:1 transformers with their primaries in parallel and their secondaries in parallel will be electrically equivalent (for this application) to a single transformer with the same turns ratio.

For a 3 x 3 design, use gate bus driver transformers with a turns ratio of 5:1. For a 5 x 5 design, use a turns ratio of 6:1. The objective is to adjust the turns ratio of the gate driver transformers to keep the inpedance looking into the primary of each transformer about the same, as one adds more FETs.  

The purpose of L2 is to form a near-resonant (but not exactly resonant) circuit with the total capacitive reactance (192 ohms) looking into the primaries (in series) of the gate driver transformers... and also to block the 2^nd and higher harmonics of the driver transistor's drain current. In my case, using 7 turns of #18 insulated hookup wire (close wound) on a piece of standard PVC pipe (2 1/4 " outer diameter) produced the following nice results:

1. The voltage on each gate bus is reasonably sinusoidal, and about 14 volts peak

2. The driver transistor has a drain voltage waveform that is "Class E" in shape, with no strange artifacts on that waveform.

3. In my case, the output current on each 2-FET module is approximately equal (2.5 amps).

Start with about 10 turns on the inductor, and reduce the number of turns until you get the desired gate drive voltage of around 12 volts (peak).  CAUTION: the driver will attempt to (briefly) produce much more gate drive voltage than you need if you adjust L2 to resonance… so bring down the turns on L2 slowly… until you achieve the desired gate drive voltage.

The purpose of the 6:6 transformer on the output of the driver FET is to:

1. decouple the DC from the interstage wiring and produce a balanced interstage wiring method to avoid r.f. pickup
2.  to add some additional inductive load (from the leakage inductance of the 6:6 transformer) to coax the driver transistor into class E operation.