5. Millimeter-Wave Switching Mode Power Amplifiers

Switching mode circuits have been used in DC-to-DC converters and power supplies for many years, and do offer some interesting possibilities for high-frequency amplification. At the lower end of what is normally considered high-frequency operation (that is, somewhere in the tens of MHz range), high power circuits have been able to incorporate numerous techniques from switching power converters. At frequencies extending further into the GHz range, however, it becomes almost impossible to realistically model power transistors as simple switching elements. This is primarily due to the fact that the device is incapable of sweeping through its linear region at a high enough speed to imitate switching behavior, provided the frequency is high enough. This problem of slow (so to speak) switching speed cannot truly be overcome at frequencies in the high GHz range, but such limitations can often be solved with some sort of workaround. Perhaps the best known switching mode for higher frequency amplifiers is the Class E mode, which will be discussed in the context of its high-frequency characteristics. Furthermore, following the trend established thus far in this book, design issues and challenges of switching mode power amplifiers will be separately discussed for both CMOS and SiGe technologies.