Traditional filters usually have low Q and gain values and it is difficult to adjust their center frequencies. Moreover, it is very complicated to analyze their transmission charateristics through conventional methods. Therefore, in this paper, a tunable differential N-path bandpass filter that uses a new adjoint network method to analyze the transmission characteristics of the differential N-path structure is proposed. The filter circuit adopts a novel circuit structure consisting of two differential N-path structures, two transconductance amplifiers and an off-chip transformer. The differential structure eliminates even harmonics, the transconductance amplifier increases the circuit gain and the off-chip transformer acts as a balun, improving the filter’s Q value and achieving impedance matching. Unlike the traditional switching capacitance method used for analyzing the differential circuit structure, the method proposed in this paper does not involve complicated calculus operations. In fact, the method greatly simplifies these complex operations, and the transmission function of the circuit can be obtained through simple algebraic operations. The proposed filter was designed using TSMC 180nm CMOS process. Simulation results for a differential four-path bandpass filter formed under 1.2V supply voltage show that the gain of the filter is greater than 8.5 dB, the center frequency can be adjusted from 0.1GHz to 1GHz, the in-band insertion loss S11 is greater than 10 dB, the out-of-band IIP3 is greater than 10 dBm, the out-of-band rejection is 28 dB and the noise figure is less than 2.2 dB at fs=300MHz.

中文翻译：

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可调谐差分N通带通滤波器的研究与分析

传统滤波器通常具有较低的 Q 值和增益值，并且难以调整其中心频率。而且，通过常规方法分析它们的传输特性非常复杂。因此，本文提出了一种可调谐差分 N 路径带通滤波器，该滤波器采用一种新的伴随网络方法来分析差分 N 路径结构的传输特性。滤波电路采用了一种新颖的电路结构，由两个差分N通路结构、两个跨导放大器和一个片外变压器组成。差分结构消除偶次谐波，跨导放大器增加电路增益，片外变压器充当巴伦，提高滤波器的Q值，实现阻抗匹配。与传统的用于分析微分电路结构的开关电容方法不同，本文提出的方法不涉及复杂的微积分运算。实际上，该方法大大简化了这些复杂的运算，通过简单的代数运算就可以得到电路的传递函数。建议的滤波器是使用 TSMC 180 设计的纳米 CMOS 工艺。1.2下形成的差分四通带通滤波器的仿真结果V电源电压表明滤波器的增益大于8.5 dB，中心频率可从0.1调整GHz 至 1GHz，带内插入损耗 S11 大于 10 dB，带外 IIP3 大于 10 dBm，带外抑制为 28 dB，噪声系数小于 2.2 dBFs=300兆赫。