cmos power amplifier design issues
Low breakdown voltage of deep sub-micron technologies. This limits the maximum gate-drain voltage since the output voltage at the transistor’s drain normally reaches 2 times the supply for classes B, and F, and around 3 times the supply for class E operation. Thus, transistors have to operate at a lower supply voltage, delivering lower power. Additionally CMOS tech- nology has lower current drive; i.e. the gain provided by the single stage is very low. Either multiple stages would be used or new design techniques that would reduce the number of stages by decreasing the input drive requirements of the large transistors in the PA, are employed .
2 In contrast to semi-insulating substrates, a highly doped substrate is common in CMOS technology. This results in substrate interaction in a highly integrated CMOS IC. The leakage from an integrated power amplifier might affect the stability of, for example the VCO in a transceiver chain.
3 Conventional transistor models for CMOS devices have been found to be moderately accurate for RFICs, and need to be improved for analog operation at radio frequencies. Large signal CMOS RF models and substrate modeling are critical to the successful design and operation of integrated CMOS radio frequency power amplifiers, owing to the large currents and voltage changes that the output transistors experience. As a result, traditional PA design relies heavily on data measured from single transistors.
4 Since the inherent output device impedance in the power amplifier case is very low, impedance matching becomes very difficult, requiring higher impedance transformation ratios. Additionally, the output matching elements require lower loss, and good thermal properties since there are usually significant RF currents flowing in these elements. If CMOS technology is used, the losses in the substrate will decrease the quality factor of the passive elements in the matching network. Usually the output- matching network is implemented off chip as the antenna itself is off chip.
5 The power amplifier delivers large output current in order to achieve the required power at the load. This current can be high enough that electromigration and parasitic in the circuit may cause performance degradation