Dynamically Biased Radio Frequency Power Amplifier

"With the current trend in mobile communications towards more complex modulation schemes to enhance spectral efficiency, the design of RF power amplifiers is the subject of considerable interest and research. The newer more spectrally efficient modulation schemes require a much higher level of linearity due to the non-constant level of their signal envelope. A variable envelope signal would in non-linear amplifiers lead to unacceptable levels of distortion. Linear amplifiers solve this problem at the cost of significant loss of efficiency. With the evolution towards more spectrally efficient modulation schemes, the trade off between linearity and power efficiency has become more challenging. Linear power amplifiers operated below their maximum power rating suffer even greater loss of operational efficiency. This occurs in the case of modulation schemes, which possess non-constant amplitude envelopes and also in urban networks where power output from each handset is minimised to prevent distorting signals on adjacent channels. The RF power amplifier consumes up to 80% of the power used in a handset during talk mode, whereas in standby mode power usage drops to around 5-10% of that used in talk mode. Clearly, PA efficiency is crucial to battery life and charge frequency. Dynamic bias has been adopted as one technique for achieving greater efficiency from linear amplifiers. This research work describes a novel digital transmitter implementation of a dynamic bias scheme. The theory of the technique has been investigated. A class S modulator has been designed and tested. Operational efficiency achieved for the modulator was 75% plus operating at or above 10% maximum output power. The linearity of the modulator - Vin versus Vout - was less than 1% rms deviation from a straight line. A transmitter containing a dynamically biased RF amplifier was developed and tested. Improvements of operating efficiency of more than 2x - compared to conventional bias - were achieved over a Gaussian distribution for PA power levels. This increased to greater than 3x for power levels of 10-15dBm at the PA output. Increases in spectral re-growth were measured for the dynamically biased RF PA."