使用負回授的理論可以消除混波器輸出端所造成的直流飄移. 濾波器中包含了兩個操作放大器以及一個主動偏壓的濾波器, 與其他的理論相較之下, 負回授架構的優點除了可以壓縮直流飄移的現象外, 也可以將訊號放大並且送入基頻. 負回授消除直流飄移濾波器所呈現的是一個帶通的濾波器轉換函數, 從濾波器的頻率響應, 我們可以觀察到電路設定一個靠近直流的通帶, 與直流壓縮的特性, 本論文中將呈現模擬與量測的結果並討論,

　　在主動偏壓的低通濾波器中, 我們藉著P型電晶體產生一個浮接的大電阻, 這方法可以同時減低電路的面積以及所需的花費. 在單端輸出的濾波器結構中, 直流壓縮量為46dB, 頻寬為20MHz, 中心頻率為10MHz, 品質因素為0.5, 直流消耗為0.4mA, 直流功率消耗為1mW. 在完全差動的濾波器結構中, 直流壓縮量優於23dB, 頻寬為19.15MHz, 頻寬範圍微1MHz至20.15MHz, 頂峰頻率為5MHz, 品質因素為0.26, 直流消耗為0.841mA, 直流功率消耗為2.1mW. 兩個結構都很適合應用於低功率系統中. 電路採用台積電0.25um 1P5M的製程來實現.

　　Feedback theory can suppress DC offset voltage at the mixer output. This filter includes two operation amplifiers and an active bias low-pass filter. The advantages of feedback structure not only cancel DC offset phenomenon but also amplify signals passed to base-band. Feedback DC offset cancellation filter can carry on band-pass filter characteristics. From the frequency response, the filter not only sets a channel near zero frequency but also shows dc stress performance. This thesis presents simulations and implementations.

　　In the active bias low-pass filter, we realize the floating resistor by a PMOS. The method can minimize both the chip area and the cost. The single-ended filter exhibits DC compression -46dB, bandwidth 20MHz, center frequency 10MHz, quality factor 0.5, DC current consumption around 0.4mA, and DC power consumption around 1mW. The fully-differential filter exhibits DC compression better than 23dB, bandwidth 19.15MHz, channel range 1MHz~20.15MHz, peak frequency 5MHz, quality factor 0.26, DC current consumption 0.841mA, and DC power consumption 2.1mW. Both single-ended and fully-differential filters are suited for low power systems. We realize the circuit by TSMC 0.25um 1P5M technology.

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