Tunable active bandpass filters using three-terminal MESFET varactors

The concept of three-terminal MESFET varactors is presented. Based on this concept, two types of tunable active bandpass filters working at the X-band were developed. The first circuit used two MESFETs, of which one was used as a MESFET varactor and the other was used to generate a negative resistance. The second circuit used only one MESFET, which had both the functions of the two MESFETs in the first circuit. The experimental results showed the possibility of using the MESFET in the active mode to generate both a negative resistance and a variable reactance.<<ETX>>