Hybrid recursive active filters for duplexing in RF transmitter front-ends

Duplex filters in modern base stations are designed as cavity filters, which are expensive and have big dimensions. Thanks to the emerging digital technology and fast digital converters, it is possible to transfer the efforts of designing analog duplex filters into digital numeric algorithms applied to feedback structures, operating on power amplifiers in transmitters. The design of a digital baseband feedback applied to the analog power RF amplifiers (hybrid filter) is presented and verified by measurements. The use of multiple digital feedback branches allows the closed loop system to show a periodic band-pass behavior at the output of the RF amplifier with a larger bandwidth up to 500 KHz. Moreover the band pass magnitude reaches up to 17 dB difference between maximum gain and attenuation.