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This paper presents a novel active capacitance circuit using BJT and RLC series-feedback. It is competitive in the noise performance and the stability by adjusting the feedback elements. We analyze its characteristics in detail and apply it to an RF application by designing a duplexer. The active capacitance circuit can replace the passive capacitors of resonators in Rx and Tx BPF, which provide negative resistance to compensate for parasitic resistance of inductors. With the detailed analysis, we can obtain the frequency range as well as the maximum value of negative resistance. The active duplexer is designed realized at cellular at Rx (824∼849 MHz) and Tx (869∼894 MHz) bands.