Intermodulation Distortion Analysis of Class-F and Inverse Class-F HBT Amplifiers

摘要

The third-order intermodulation distortions (IM3s) of class-F and inverse class-F heterojunction bipolar transistor amplifiers were compared experimentally. It was revealed that the IM3 of inverse class F is lower than that of class F at high input power (P_(in)), although the better class for IM3 at low P_(in) changes from class F to inverse class F according to the increase of the quiescent current (I_(q)). The different IM3 behaviors are mainly caused by the different gain variations in both amplifiers. At a low P_(in), the gain of class F is larger than that of inverse class F. The larger gain in class F causes a gradual gain decrease and a steep gain rise at high and low I_(q), respectively. On the other hand, the gain of inverse class F is larger than that of class F at a high P_(in). The larger gain in inverse class F causes a gradual gain decrease at all I_(q) values. These gradual gain decreases are one of main causes of a lower IM3. These phenomena can be explained by the output current (I_(out)) and voltage (V_(out)) waveforms, which differ according to the harmonic loads. The I_(out) and V_(out) positive half-sinusoidal waveforms, which consist of in-phase second harmonics, are effective for large-gain operations. The positive half-sinusoidal I_(out) waveform of the class-F amplifier prevents a gain decrease at a low P_(in) and high I_(q), and the similar V_(out) waveform in inverse class F prevents a gain decrease at a high P_(in).