An active bandpass filter is disclosed herein. The active bandpass filter has N transmission lines, N negative resistant circuits, a DC circuit, and at least (N−1) coupling circuit. Each transmission line has a first end and a second end. Each negative resistant circuit has a third end and a fourth end and is electrically coupled with a related transmission line, wherein the third end and the fourth end are electrically coupled with the first end and second end, respectively. The DC circuit provides a bias voltage for N negative resistant circuits, wherein the DC circuit electrically couples with N transmission lines via N coupling elements. Each coupling circuit has a fifth end and a sixth end and is electrically coupled with any two transmission lines, wherein the fifth end and sixth end are electrically coupled with the second end and the first end, respectively.