A push-pull RF power amplifier with capacitive gate-drain cross-coupling

摘要

Gate-to-drain cross-coupling capacitors 50 and 52 are added to the common-source stage of a cascode RF amplifier in order to reduce the variation of the stages input capacitance as a function of the signal swing. AM-PM distortion at higher signal levels may be reduced by counteracting the phase lag (602, figure 6) in the MOS transistors M11 and M12 with a phase lead (600, figure 6) in the driver stage. The leading phase characteristic of the driver stage is obtained by controlling the variable capacitor 18 (such as a CDAC (capacitor digital-to-analogue converter)) so that the LC resonant frequency of the driver stage is slightly below the signal frequency. The bias currents 20 and 24 respectively increase and decrease with temperature so that the gain and the drain voltage of the devices M11 and M12 do not vary much over the working temperature range.