Derivation of coupler and power sensing requirements for high linearity and efficiency tunable power amplifiers

In transmitter front-end modules (TX-FEMs) for mobile handsets, directional couplers are used to sample out the transmitting (forward traveling) radio signal to the antenna. In practice, antenna impedance is never ideal 50 Ohm. Its input impedance varies widely under varying user interface. This causes severe reflections from antenna port. In a system with a finite directivity coupler, the reflected signal is added or subtracted from the forward coupled signal at the coupled port. The magnitude of this reflected signal at the coupled port will depend on the input power to the coupler, coupling coefficient of coupler, reflection coefficient at the antenna port and directivity of the coupler. This paper describes derivation of directivity requirements of a TX-FEM coupler based on closed loop power out (PL) variation limitations dictated by the overall PA output power, linearity and efficiency requirements. Agilent ADS circuit simulations are performed to verify the derivations.