An improved source-follower based Sallen-Key continuous-time biquadratic cell with auxiliary path

An improved Source Follower based low-pass Sallen-Key continuous time biquadratic cell is presented. An innovative structure with auxiliary path is proposed in order to increase the out-of-band attenuation of Sallen-Key filter. The auxiliary path eliminates one out-of-band parasitic zero in the transfer function. A prototype of Sallen-Key second order filter with a bandwidth of 120MHz and a quality factor of 1.19 has been designed in a standard 0.35μιη technology occupying 390μιη X 222μm area. A 75dB HD3 for a IV input signal and 50dB out-of-band attenuation at 3.2GHz are the filter peculiarity. The cutoff frequency deviation due to technological spread, aging and temperature variation is adjusted by using 4-bits capacitors arrays.