A comprehensive study of a new planar-type balun prototype is presented in this paper for application in the design of balun bandpass filters (BPFs) with widely used opentype resonators. The proposed balun is in general composed of two identical back-to-back quarter-wavelength (λ/4) coupled line sections. The design concept originates from the analysis of the standing wave pattern on a half-wavelength (λ/2) opencircuited transmission line. After an intuitive illustration is given, its working principle is presented to validate the feasibility of the proposed balun. Afterward, two types of the coupled line prototypes are analyzed to implement the proposed balun with filtering response, whereas a basic rule is given to realize the transmission zeros. To further demonstrate the validity of the proposed balun prototype, two practical balun BPFs with common dual-mode resonators are designed and fabricated. As expected, transmission zeros are generated near the desired passband of our presented balun filters, thereby improving their frequency selectivity. Both simulated and measured results exhibit good filtering and balun performance.