Narrowband high-selectivity active bandpass filters using open-loop multiple-ring resonators
This article presents a narrowband high-selectivity active bandpass filter that is constructed by two passive bandpass filters and one amplifier. The bandpass filters with two transmission zeros provide a sharp cutoff frequency response to suppress adjacent channel interferences. The amplifier coupling to the passive bandpass filters not only compensates for the insertion loss of the passive filters, but also provides gain for the active filter. Since the amplifier uses only one transistor, the active filter can lower the cost, minimize the noise, and be easily designed to operate in a stable region. Moreover, the complete active filter consisting of a filter placed in front of the amplifier can suppress unwanted signals in order to reduce the noise floor of the active filter and achieve a wide dynamic range. The active filter has a 6.8-dB gain at 2 GHz, a 3-dB fractional bandwidth of 2.4%, and stopband rejection better than 40 dB within 1-3 GHz. The minimal noise figure of the active filter is 2.6 dB at the passband. This high-performance active filter can be used in many wireless communication systems. (C) 2004 Wiley Periodicals, Inc.
narrowband bandpass transmission zeros active filter ring resonator high selectivity