TABLE OF CONTENTS This chapter will consider the possibilities offered to the RFPA designer by switching mode circuits. Circuits of this kind have been used for many years in DC to DC converter applications and undoubtedly offer some possibilities for higher frequency use. This applies especially in the broader interpretation of RF ; high power applications in the MHz and tens of MHz frequency ranges have certainly benefited from infusions of techniques and practice from the switching power converter industry.
But at GHz frequencies there remains a stubborn and irrefutable central issue, which is that RF power transistors at these frequencies cannot realistically be modeled as simple switching elements. At these frequencies, and in any currently available power transistor technology, the device will not sweep through its linear region fast enough to behave like a switch, and the behavior in the linear region must be included in the circuit simulation, just as it is when simulating conventional PA circuits. For these reasons, RF designers most frequently move on and dismiss switching modes as being unrealizable for higher frequency applications. In fact, although the problem of slow switching speed can never be fully resolved at higher RF frequencies, it appears that it can sometimes be judiciously out-maneuvered and that useful RF applications do exist. This can lead to some useful applications in the gray area, when a very high frequency technology is being used at a low relative frequency; a GaAs PHEMT or HBT below 2 GHz would be such an example.
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