A Novel SFG Structure for C-T Highpass Filters

This paper presents the design of a sixth order elliptic highpass filter having a passband frequency of 3.0KHz, a passband ripple of 1.0dB and a stopband attenuation of 50dB. The filter is based on a novel integrator based SFG describing a passive prototype filter; this SFG is simulated using MOSFET-C building blocks. The noise performance is considerably enhanced when compared to other highpass filter structures. Well within the stopband the output noise is dominated by amplifier noise, but only one amplifier contributes and its equivalent input noise is simply copied to the output (not amplified). Around the passband frequency MOSFET-resistor noise is the dominant noise source, but the number of MOSFET-resistors is kept at a minimum, thus reducing this noise source. The above mentioned noise properties are directly related to the new filter structure, which in addition to this has the advantage of being quite simple when compared to other integrator based highpass filter structures.