An Active Polyphase Filter with Image Rejection Enhancement by Compensating for Nonideal Effects

In this paper, a four-stage polyphase filter (PPF) with an image rejection enhancement technique is proposed for broadband applications. The proposed image rejection enhancement technique adjusts the gain of the low-pass unit with current compensation to counteract the effect of the nonidealities on the image rejection ratio (IRR) of the PPF. A generalized first-order PPF model is established to describe the nonideal effects, and a calibration condition for the case of equal real and imaginary cutoff frequencies is derived for most PPFs. In order to adapt to the process and temperature variations, an adaptive bias is designed to implement the image rejection enhancement technique. Simulations show that the IRR of the single-stage PPF is improved by more than 20 dB by reducing the effects of the output conductance of the tail current source in the circuit. Realized in a broadband low intermediate frequency (IF) receiver in 0.18-μm CMOS process, the designed four-stage PPF achieves over − 52 dB IRR from 20 to 65 MHz. The output intercept point (OIP3) and the imputed input referred noise (IRN) are 3.8 dBm and 92.7 μV, respectively. The core of the PPF consumes 2.78 mA from a 1.8 V supply while occupying a chip area of 0.56 mm².