Simplified Doherty Power Amplifier With Harmonic Suppression

Communication radio need back-off power efficient amplifier with harmonic suppression. Efficiency and harmonic suppression are two contradicting requirements. While Doherty’s architecture improves back-off efficiency, the cascaded harmonic suppression filter nullifies the benefits. Moreover, a UHF Doherty would be space constrained by the matching network each at carrier (to $100 \Omega$) and peaking amplifiers. This paper demonstrates a simplified design methodology for UHF Doherty amplifier with harmonic suppression features. The impedance inverter is implemented at transistor’s load line impedance and the matching network at carrier and peaking amplifier’s drain are eliminated. After load modulation, an impedance transforming filter, embedded with harmonic suppression features, performs the final translation to $50 \Omega$. Similarly at the input, an impedance transforming wilkinson power divider reduces gate matching networks. The simplifications have resulted in a compact Doherty amplifier which is a form-fit replacement for class AB modules while significant back-off efficiency improvement of $15 \%$ and $18 \%$ at $\mathrm{6 d B}$ and 10 dB back-off have been achieved. Besides, harmonic suppression levels as low as $\mathrm{- 7 3 d B c}$ have been recorded in the $\mathrm{2 2 0 - 3 0 0 M H z}$ fabricated demonstrator circuit. The module delivers $49.4-50 \mathrm{dBm}$ of CW power with $\mathrm{6 5 \%}$ efficiency at peak output power, $47 \%$ efficiency at $\mathrm{6 d B}$ back-off and $\mathrm{3 8 \%}$ efficiency at 10 dB back-off output power.