A new structure for current-mode continuous time Gm-C filters



In this paper a new structure for the MLF (Multi Loop Feedback) Gm-C group of filters is presented, granting the advantages of both current-mode and fully balanced topologies to the conventional structure of the group.
The ability of the structure to perform even more transfer functions (Low pass and Band Pass) than other members of the group is proved. Methods of enabling the proposed structure to perform other popular transfer functions are also presented. The favorite feature of systematical generation of the structure facilitates its arrangement for any order.      
        For practical comparison, a Butterworth 4th–order LP filter with a cut-off frequency of 10MHz is designed in three different structures viz; the proposed one, the single-ended current mode, and fully balanced voltage mode. Simulation results show that the PSRR+,PSRR-,CMRR, Noise, THD, DR, consumed power (P) and Figure of Merit (FOM) of the new structure compared to its voltage mode counterpart are improved at least by factors of 36643, 59841, 4.75, 76, 2, 2.45, 1.17 and 509500, respectively.Compared to single ended current-mode type they are improved by factors of 40, 73, not defined, 1.3, 7.8, 150, 0.68 and 1763000,respectively. Although the above mentioned comparison, due to both the similarity of the used technology and the completeness of the results, is the most equitable one for the most definite conclusion, to further widen the extent of the comparison, the proposed structure is also compared with some other works yet assumed as its closet counterparts. This latter comparison also proves the certain superiorities of the proposed structure such that its FOM is from 8500 to 4512740 times larger than those of others. Closer tracking of the input signal at pass-band and more attenuation at stop-band are also achieved by this structure. These results strongly support the theoretical suggestions. Most favorably the much higher PSRR of the new structure makes it an extremely suitable choice for Mix-Mode (System-On- a Chip, SOC/SOI) applications where power supplies (and analog blocks) suffer severely from digital noise.