27 MHz High Efficiency Variable Gain Amplifier for Shortwave Diathermy

This paper proposed a 27 MHz high-efficiency variable gain amplifier (VGA) that could be applied to short-wave diathermy, as a way to improve the efficiency for long-term operation of such amplifier in the therapy. We used a class-B amplifier architecture for design, which allowed the variable gain amplifier to be extremely efficient, both at high and low power output. In addition, we also effectively suppressed the second and third harmonics, further permitting this variable gain amplifier to be practical for commercial use. We adopted this high-efficiency variable gain amplifier with a hybrid circuit at an adjustable gain range of 5.8 dB to 31 dB under an input power of 0 dBm. Within the gain adjustment range, the power additive efficiency (PAE) of the VGA was almost always maintained above 50% with the suppression of the second harmonics at more than 65 dB.