20.9 A 1.92mW filtering transimpedance amplifier for RF current passive mixers

Abstract

Nowadays, current passive mixers represent the state of the art for signal down-conversion in wireless receivers. In such kind of structures, noise, distortions and losses are strictly correlated to the performance of the stage following the mixer. The most common solution adopted to sense the down-converted current is a transimpedance amplifier (TIA) in shunt with a capacitance to ground that assures a low input impedance when the loop gain of the amplifier decreases (Fig. 20.9.1a). A low input impedance is necessary to have a small voltage swing at the output of the mixer (typically few hundreds mV) to minimize the modulation of the switch resistance and with it the distortion produced during the downconversion. The input capacitance can also be used to filter the majority of the out-of-band interferers by transforming the TIA into a filter [1,2] (Fig. 20.9.1b). This reduces the dynamic range required by the TIA and its power consumption. This advantage comes at a cost of area, since the limited voltage swing tolerable at the input of the TIA demands a large capacitor to absorb the downconverted interferers. This trade-off is relaxed with the proposed solution (Fig. 20.9.2), where the input capacitance (C1) is partially boosted by a feedback network minimizing the swing required at the input of the TIA. This idea, originally proposed in [3] only to improve the 1dB compression point, is now used to maximize the spurious-free dynamic range of the TIA, exploiting an intrinsic in-band highpass shaping of noise and distortion. Furthermore, an adaptive transfer function, which improves its filtering action in presence of large out-of-band interferers, is realized.