Microwave Circuit Design: A Practical Approach Using ADS by Yeom Kyung-Whan

Author:Yeom, Kyung-Whan
Language: eng
Format: epub, mobi
Publisher: Pearson Education
Published: 2015-05-15T00:00:00+00:00

Figure 9.32 Class-B power amplifier with the third-order harmonic peaking circuit

Inserting an infinite number of odd harmonic parallel resonators in series, the collector voltage waveform becomes closer to a square wave and the efficiency becomes 100%. Instead of these multiple-harmonic resonators, Tyler presented a class-F power amplifier that uses a quarter-wavelength transmission line at the fundamental frequency, as shown in Figure 9.33. All of the amplifier’s harmonics in that figure can be ideally controlled and the class-F operation is achieved. The input drive circuit in Figure 9.33 acts as it does in a class-B power amplifier. The capacitor CB in the load side is a DC block capacitor. Inductor L1 and capacitor C1 form a parallel resonant circuit that resonates at the fundamental frequency. Thus, assuming a high Q, only the fundamental voltage appears across the load. The transmission line is one-quarter-wavelength long and the impedance seen from the collector toward the load at the fundamental frequency is (Zo)2/R. At even-order harmonics, since the transmission line becomes an integer multiple of a half wavelength, the impedance seen from the collector toward the load appears similar to that of a short circuit, while at odd-order harmonics, it appears similar to an open circuit due to the characteristic of the one-quarter-wavelength transmission line. Therefore, the one-quarter-wavelength transmission line provides a short-circuit impedance at the collector for even-order harmonics, and it provides an open-circuit impedance at odd-order harmonics. Thus, when the circuit in Figure 9.33 is compared to that shown in Figure 9.32, it acts as the infinite number of parallel resonators inserted in series to resonate at odd-order harmonics.

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