Switchmode RF Power Amplifiers
Switching mode RF amplifiers have become a hot area of RF/wireless design. This book explores both the theory behind switching mode RF amplifiers and design techniques for them.
TABLE OF CONTENTS Switchmode RF Power Amplifiers
Chapter 2: Class-D Power Amplifiers
In this chapter, the different configurations of Class-D power amplifiers are presented the increased efficiency of which is a result of employing the active device as switches. First, the switched-mode power amplifiers with resistive load of different configurations, which can be considered driver power amplifiers, are discussed. Then, the current switching and voltage switching configurations based on complementary and transformer-coupled topologies are analyzed. The effect of the saturation resistance, rectangular and sinusoidal driving signals, finite transition time, and parasitic shunt capacitance and series inductance is demonstrated. The practical design examples of voltage-switching and current switching Class D power amplifiers intended to operate at high frequencies and microwaves are described.
2.1 Switched Mode Power Amplifiers with Resistive Load
The efficiency of a power amplifier can be maximized if the active device is operated as a switch. When the transistor is turned on, the voltage is nearly zero and high current is flowing through the device; that is, the transistor acts as a low resistance (closed switch) during this part of a period. When the transistor is turned off, the current is zero and there is high voltage across the device; that is, the transistor acts as an open switch during the other part of a period.
Let us consider the three types of a switched-mode power amplifier with resistive load: a single-ended amplifier, a complementary voltage-switching push-pull amplifier, and a transformer-coupled current-switching push-pull amplifier [1]. Fig. 2.1(a) shows the simplified circuit schematic of a switched-mode single-ended bipolar power amplifier, where L ch is the RF choke...
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