An enhanced doherty amplifier design based on the derivative superposition method

An enhanced 20 W Doherty amplifier with highly optimized linearity using the derivative superposition method (DSM) is proposed. DSM is a linearization technique based on the device's transconductance characteristics. The gain compression of the main amplifier, biased in class AB, can be compensated by combining it with the gain, expansion of the peaking amplifier, biased in class C. The adjacent channel leakage power-ratio (ACLR) performance of the Doherty amplifier is optimized by adjusting the offset lines, shunt capacitors and gate biases. The shunt capacitors are an important parameter to achieve optimum linearization performance in the Doherty amplifier The results, measured on an optimized Doherty amplifier for a single-carrier WCDMA signal, achieved -45 dBc ACLR at a +/- 5 MHz offset frequency. This is an ACLR improvement of 16.7 dB in comparison to the Doherty amplifier before optimization. The digitally pre-distorted optimized Doherty amplifier achieved an ACPR of -52.5 dBc at a +/- 22.5 MHz offset frequency.