S-Band Monolithic Active Filter using Actively-Coupled Resonator Technique

A highly-selective bandpass active filter design technique is presented which uses a monolithic actively-coupled resonator cell combined with additional transmission zeros implemented with off-chip microstrip line resonators. The technique gives the advantages of light-weight and small-size, inherent in active filters, but achieves greatly improved selectivity and high tolerance to GaAs process variations. A prototype filter consisting of one MMIC actively coupled resonator chip has been designed and tested. The filter operates at 3 GHz, with a 3-dB bandwidth of 200 MHz and 10dB of gain. 40dB of rejection is achieved at 400 Mhz from the center frequency. The gain can be controlled by the bias voltage, while the Q factor and the center frequency remain unaffected by these changes.