Chapter 11RF Power Amplifiers with Dynamic Power Supply

11.1 Introduction

This chapter studies the pulse-width modulated (PWM) buck switching-mode converter for applications in radio transmitters as a dynamic power supply or an amplitude modulator. In these applications, the converter output voltage is variable, and the dc input voltage is usually constant [1–61]. This circuit is also called Class S amplifier. Analysis is given for the buck converter in the continuous conduction mode (CCM) and for the synchronous buck converter. Current and voltage waveforms for all the components of the converter are derived. The dc voltage function is derived for CCM. Voltage and current stresses of the components are found. The boundary between CCM and discontinuous conduction mode (DCM) is determined. An expression for the output voltage ripple is derived. The power losses in all the components and the transistor gate drive power are estimated. The overall efficiency of the converter is determined. A design example is also given.

11.2 Dynamic Power Supply

In RF power amplifiers, the transistors are operated either as a dependent current source or as a switch. The drain efficiency of RF power amplifiers in which transistors are operated as dependent current sources is given by

11.1 equation

where is a constant dependent on the type of amplifier for the Class A RF power amplifier and for Class ...

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