A novel Doherty amplifier for enhanced load modulation and higher bandwidth

We are reporting a new topology for the Doherty amplifier to increase its bandwidth and enhance the load modulation. A varactor-based impedance transformer has been employed to replace the bulky and narrowband quarter-wave impedance inverter. Load modulation is carried out adaptively using the proposed varactor-based structure based on the input power level. An envelope detector is employed for adaptive impedance transformation of the carrier amplifier as well as bias adaptation of the peak amplifier. Based on the proposed topology, a 2W Doherty amplifier has been fabricated using discrete pHEMT transistors and low loss varactors. In order to evaluate the broad-band/multi-band performance of the proposed topology, measurements have been carried out at three sample frequencies (1.8 GHZ, 2 GHz and 2.2 GHz) over a 400 MHz bandwidth. Power added efficiency of better than 45.3% has been achieved at maximum power level and 6-dB power back-off and maintained over the entire bandwidth. Measured IM3 is better than -42.2 dBc at P1 dB of 33 dBm for all design frequencies.