Feedforward Power Amplifier with a New Adaptive Control Method

1. Analysis and Experiments for High-Efficiency Class-F and Inverse Class-F Power Amplifiers

The class-F amplifier, which has short load termination at even-order harmonics(current peaking) and open load termination at odd-order harmonics(voltage peaking), has become a representative of the high efficiency amplifier. Recently, the inverse class-F amplifier has started to draw an attention for its superior performance. It is commonly known that the inverse class-F amplifiers, which have open load at even-order harmonics(voltage peaking) and short load at odd-order harmonics(current peaking), can deliver higher efficiency than class-F operation. Some papers with partial analyses, simulations, or experiments, which demonstrated advantages of the inverse class-F amplifier, have been reported. But there have been no reports treating fully analytic and experimental comparisons for the clear explanation of the better efficiency of the inverse class-F amplifier.

The purpose of this work is to provide the quantitative and clear explanation of better efficiency, and the design guide for optimizing the output power or efficiency of the inverse class-F amplifiers in comparison to the class-F amplifiers. For that purpose, the efficiency equations of the inverse class-F and conventional class-F amplifiers are derived using ideal time-domain waveforms. Then, the analytic comparisons are carried out using the equations under the condition that the fundamental output power of both amplifiers are identical, around where they have maximum PAE.

For an experimental comparison, the class-F and inverse class-F amplifiers at 1GHz band are designed and implemented. The analysis and experimental results clearly show why the inverse class-F amplifier has higher efficiency than the class-F amplifier.

Figure 1

Fig. 1 shows the ideal time domain current and voltage waveforms of the class-F and inverse class-F amplifiers, when they have the same fundamental output power under the same drain biases. The comparison results of performances between the class-F and inverse class-F amplifiers are presented in Fig. 2. For the calculation, we assumed V_dc supply of 5 V and i_d,peak of 1 A with a uniform transconductance. The efficiency of inverse class-F amplifier is better than that of class-F amplifier with increasing R_on due to the higher V_peak to V_K ratio to maintain an identical P_fund, as shown in Fig. 2(a). Fig. 2(b) shows a significantly decreasing DC power consumption of the inverse class-F amplifier for the same RF output power, as R_on increases. From the analysis results and Fig. 1, we can expect that the inverse class-F amplifier delivers superior efficiency when the amplifiers are not limited by the breakdown voltage but by the bias voltage, which is the normal operation condition of handset power amplifiers.

Figure 2

The photographs of the implemented class-F and inverse class-F amplifiers are shown in Fig. 3((a) class-F, (b) inverse class-F). We have measured output powers and PAE's of the two amplifiers using 1GHz one-tone signal. Fig. 4 shows the simulated and measured power responses and PAE's of the class-F and inverse class-F amplifiers. As shown in fig. 4(a), the inverse class-F amplifier has about 1dB lower gain because square-wave drain current needs more input drive than half-sinusoidal drain current for same output power level. The maximum PAE of the class-F amplifier is about 64% at an output power of 22.5dBm. The maximum PAE of the inverse class-F amplifier is about 74% at the same output power level, which is 10% higher than that of the class-F's. The superior PAE performance of the inverse class-F amplifier at the same output power level supports the analysis results.

Figure 3

Figure 4