Bias control of Class E amplfiers

Previously undocumented phenomenon

Posted By: Robert Nickels (ranickels)

Micro SDR innovator Guido PE1NNZ has implemented polar modulation using an Arduino MCU and a class E PA.   For more informartion on this fascinating project, join the discussion group at https://groups.io/g/ucx

Initially, Guido's design implemented the polar or EER modulation scheme using modifications to the QCX CW transceiver hardware in the traditional way as described by Leonard Kahn, that is by using a PWM analog voltage to control the drain voltage to the class E PA.    However in his experiments he found that envelope control could also be achieved by controlling the DC bias applied to the MOSFET gate.    A key characteristic of class E is that the output device operate in a switching mode, i.e. either off or fully saturated.    Guido's design did this, but claimed that superimposing a DC bias resulted in envelope control as effective as drain voltage control.    Since very little power is required to control the gate bias compared to the full current of the device, the heat and reliability are improved by the elimination of the pass element, and more voltage is available to the PA resulting in more power output.

To convince myself this was true,  I set up a test using one of my 2 watt BS-170 class E amplifier board.    This amplifier runs right at 80% efficiency.  The charts below show the result - varying the bias from zero to 2 volts DC gave control of the ouptut from zero to 2 watts.    The drain voltage transfer function is nearly linear, which results in the approximately square root curve when translated to power.