In order to satisfy the applications requirement in fiber-radio links and phased array antennas,a novel microwave photonic filter with passband and stopband interchanged is proposed and demonstrated. The filter is based on a recirculating delay line loop with a semiconductor optical amplifier (SOA) followed by a tunable narrowband optical filter. The negative tap is generated by using the wavelength conversion employing the cross-gain modulation of the amplified spontaneous emission spectrum of the SOA. When the central wavelength of the optical filter is aligned with the laser wavelength, a microwave bandpass filter is obtained. On the other hand, when the optical filter detunes from the laser wavelength a little, a narrow notch filter with flat passband is realized. The structure can be operated as a high Q or notch filter with flat passband by tuning the central wavelength of the optical filter.