Switch-Mode MOSFETs for Class A Radio Frequency Amplification

Abstract

Vertical double diffused MOSFETs (“VDMOSFETs”) developed for switching power supplies were successfully used in high voltage, high frequency linear amplifier circuits. These high voltage power switches are surprisingly useful for linear RF power amplifiers and transmitters. In most of their intended applications such as in switching power supplies or switched digital amplifiers, VDMOSFETs are pulsed by a high voltage and the pulsed output is heavily filtered. This avoids the linear region of the VDMOSFET, which normally limits the efficiency to a theoretical maximum of 25%. However, in a search of MOSFETs for high voltage linear amplification using Class A bias without heavy use of filtering, these low-cost switching MOSFETs were found to work best, if limited to radio frequencies above 1.5 MHz. Conditions, and in particular, very low impedance driving of the MOSFET gate, were discovered that gave efficiencies of 70%, rivaling that of non-linear pulse techniques such as Class C or Class D operation. The beneficial effect of linear operation combined with high efficiency at a high voltage is suggested to arise from Miller capacitance. This was only seen at radio frequencies and with the larger current, higher gate capacitance devices. Switching MOSFETs as described here should be considered alongside dedicated RF MOSFETs such as the more expensive LD (lateral diffused) MOSFET when working with RF power.