A MMIC active bandpass filter with FET negative resistance elements

Abstract

Many researchers have investigated microwave active filters, and many new circuit ideas have been presented. However, little more than the basic filter frequency response measurements is presented. Unfortunately, impressive small-signal frequency responses do little to establish whether or not these active filters have any real practical application. For this purpose, linearity results and the noise figure of a MMIC active filter, which employs a negative resistance technique, are presented. It is shown that the FET device is suitable within this application as it can have a varactor-controlled negative resistance component. A twostage monolithic varactor-tuned bandpass filter has been demonstrated using this technique. The measured response of the filter exhibits a 400 MHz 3 dB bandwidth centered on 4.7 GHz, with tunable insertion gain and ±0.05 dB ripple in the pass band.