Low intermodulation amplifiers for RF and microwave wireless systems

A novel linearisation technique for reduction in the third-order intermodulation distortion products, with injection of second harmonics through a feedback loop of a power amplifier, was applied in this paper. The power amplifier including the feedback loop components (bandpass filter, phase shifter, attenuator) was designed as a hybrid microwave integrated circuit by using the program Libra. The adjustable parameters are the phase and amplitude of the loop signals. Therefore, a voltage that controls a phase shift of the phase shifter and a control current of a PIN diode in the attenuator circuit were optimised to obtain a reduction in the third-order intermodulation distortion. The achievable improvement was found to be 21 dB for the case of two fundamental signals at the power amplifier input.