Intermodulation distortion analysis of class-F and inverse class-F HBT amplifiers

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.