A new design technique of CMOS continuous-time VHF current-mode bandpass ladder filters using VHF bandpass biquads

In this paper, a new design technique to implement VHF current-mode bandpass ladder filters directly by using the VHF current-mode bandpass biquads is proposed and analyzed. The VHF current-mode bandpass biquads are implemented by the transresistance (Rm) amplifier-capacitor structure. Moreover, the special Q-enhancement technique is applied to the biquad to obtain the tunable and high-quality factor Q. Starting from the prototype of LC ladder bandpass filter and assigning the proper state variables, the state equations of the LC ladder bandpass filter are rewritten in terms of the current-mode state variables and current-mode high-Q bandpass biquadratic functions. Thus, the current-mode ladder filter can be constructed by directly using the current-mode high-Q bandpass biquads as basic cells. A 6th order current-mode bandpass ladder filter with the center frequency up to 50 MHz has been successfully designed by using the current-mode high-Q biquadratic bandpass filters. It has the advantageous features of low component sensitivity and simple structure. It is also free from DC instability problems.