MMIC feedforward linearizers for microwave power amplifier

The application of the feed forward method of amplifier linearization potentially enables deep distortion cancellation to be achieved over large operational bandwidths. In order to achieve high linearity, a transmitter output power amplifier must either be considerably backed-off from saturation or some form of linearization must be employed. Several linearization techniques are known but the feed forward method is preferred for large bandwidths and is capable of high distortion cancellation provided the amplitude and phase are carefully controlled in the cancellation loops. This thesis addresses the means to implement broadband feed forward linearization and in so doing establishes a method of implementing the technique using MMIC (monolithic microwave integrated circuit) technology. The distributed amplifier circuit is capable of providing broadband multi-octave gain at microwave and millimetre-wave frequencies and is amenable to implementation in MMIC form. The basis of this thesis is the application of feed forward linearization to a MMIC distributed amplifier whereby broadband linearization is demonstrated and the need to back-off the amplifier is reduced. Feed forward systems have hitherto been implemented using discrete components such as waveguide or MIC couplers, combiners, delay lines and phase shifters. Work has been undertaken which establishes the design methodology for broadband active couplers based on a distributed amplifier type of circuit. These couplers have been fabricated using a GaAs pHEMT (gallium arsenide high electron mobility transistor) process for operation over 2-20 GHz and a new method of setting the coupling factor has been determined. The design of broadband active combiners has also been undertaken. A 2 - 30 GHz active combiner with 8 dB gain has been demonstrated for the first time using a GaAs pHEMT process. The concept of artificial transmission lines on GaAs has been used to develop delay lines which are shorter than the equivalent micro-strip lines and miniature phase shifters that are electronically tuneable. It has been shown that such delay lines save considerable GaAs space and cost over their micro-strip counterpart. These novel components have been utilized to demonstrate an all-MMIC feed forward linearized amplifier with greater than 30 dB third harmonic suppression, capable of operation over 2-20 GHz and suitable for future adaptive control.