Total posts : 45366
I hope that Wilcom Labs will, indeed, develop the Part 15 AM tube transmitter he proposed in his thread. I wish him great commercial success.
I am planning one more post about tubes. I stated in a previous post in the Wilcom Labs thread that a really efficient tube transmitter requires a powerful narrow-pulsewidth generator to drive the grid. I am now working on a MOSFET driver for the grid. If it works, I will report the results.
Another open question in Part 15 AM that I am working on is whether or not a power MOSFET can be used as the active component of the final stage of an efficient transmitter. In 1978, Frederick H. Raab published an article about a 1.8 to 54 MHz 16 watt broadband amplifier in “Ham Radio” that used power MOSFETs. He claimed 78.5% efficiency. My own experience with power MOSFETs in Part 15 AM designs, however, indicates that the parasitic diode between the drain and source of the MOSFET severely reduces the efficiency. I already mentioned the disadvantages of this parasitic diode in the Wilcom Labs thread. The problem is that the diode dissipates much of the energy that is stored in the drain inductor during the “on” state of gate drive cycle of the MOSFET. Dissipating the drain circuit energy severely reduces the overall transmitter efficiency. Sometimes a Schottky rectifier diode is connected between drain and source (a “snubber” diode) to prevent current from flowing through the parasitic diode. The snubber diode does not help efficiency, however, because the snubber dissipates the drain circuit energy just as well as the parasitic diode in the MOSFET. I will be looking into the issue of the MOSFET parasitic diode carefully and will post in the Wilcom Labs thread if I come up with a definite conclusion.