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,, input signals, V , , = 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-pm CMOS process are given. Both the cutoff frequency and the quality factors can be tuned. The cutoff 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.