A CMOS TRANSCONDUCTANCE-C FILTER TECHNIQUE FOR VERY HIGH-FREQUENCIES

This paper presents CMOS circuits for integrated analog filters at very high frequencies, based on transconductance-C integrators. First a differential transconductance element based on CMOS inverters is described. With this circuit a linear, tunable integrator for very-high-frequency integrated filters can be made. This integrator has good linearity properties (1% relative gm error for 2-V(p-p) input signals, V(dd) = 10 V) and nondominant poles in the gigahertz range owing to the absence of internal nodes. The integrator has a tunable dc gain, resulting in a controllable integrator quality factor. Experimental results of a VHF CMOS transconductance-C low-pass filter realized in a 3-mu-m CMOS process are given. Both the cut-off frequency and the quality factors can be tuned. The cut-off frequency was tuned from 22 to 98 MHz and the measured filter response is very close to the ideal response of the passive prototype filter. Furthermore, a novel circuit for automatically tuning the quality factors of integrated filters built with these transconductors is described. The Q-tuning circuit itself has no signal-carrying nodes and is therefore extremely suitable for these filters at very high frequencies.