高阶 (2n) VSVC单位增益巴特沃斯低通滤波器设计,可分解为 n 个二阶低通,通过对这多个二阶低通的组合优化,可提高滤波器的低通特性和稳定性。
串联的传递函数是各个二阶滤波器传递函数的乘积:({{rm{H}}_{2n}}(s) = prodnolimits_{i - 1}^n {{H_2}^{(i)}(s)});
二阶压控电压源低通滤波器电路图:

由“虚短-虚断”得到,传输函数:(H(s) = {{mathop Vnolimits_o } over {mathop Vnolimits_i }} = {{mathop Anolimits_F /mathop Rnolimits_1 mathop Rnolimits_2 mathop Cnolimits_1 mathop Cnolimits_2 } over {mathop snolimits^2 + s({1 over {mathop Rnolimits_1 mathop Cnolimits_1 }} + {1 over {mathop Rnolimits_2 mathop Cnolimits_1 }} + {{1 - mathop Anolimits_F } over {mathop Rnolimits_2 mathop Cnolimits_2 }}) + {1 over {mathop Rnolimits_1 mathop Cnolimits_1 mathop Rnolimits_2 mathop Cnolimits_2 }}}});
其中(s = jomega),(mathop Anolimits_F = 1 + {{mathop Rnolimits_f } over {mathop Rnolimits_r }});
去归一化低通滤波器的传递函数:(H(s) = {{mathop Hnolimits_0 mathop omega nolimits_0^2 } over {mathop Snolimits^2 + alpha mathop omega nolimits_0 S + beta mathop omega nolimits_0^2 }});
其中(beta mathop omega nolimits_0^2 = {1 over {mathop Rnolimits_1 mathop Rnolimits_2 mathop Cnolimits_1 mathop Cnolimits_2 }}),(mathop Hnolimits_0 mathop omega nolimits_0^2 = {{mathop Anolimits_F } over {mathop Rnolimits_1 mathop Rnolimits_2 mathop Cnolimits_1 mathop Cnolimits_2 }}),(alpha mathop omega nolimits_0 = {1 over {mathop Rnolimits_1 mathop Cnolimits_1 }} + {1 over {mathop Rnolimits_2 mathop Cnolimits_1 }} + {{1 - mathop Anolimits_F } over {mathop Rnolimits_2 mathop Cnolimits_2 }});
({omega _0})是截止角频率,(alpha)、(beta)是二项式系数,代表不同的滤波特性。
设定(mathop Cnolimits_2 = kmathop Cnolimits_1),那么(mathop Hnolimits_0 = beta mathop Anolimits_F),(beta mathop knolimits^2 mathop omega nolimits_0^2 mathop Cnolimits_1^2 mathop Rnolimits_2^2 - alpha kmathop omega nolimits_0 mathop Cnolimits_1 mathop Rnolimits_2 + (1 + k - mathop Anolimits_F ) = 0)(关于({R_2})的二次方程),由于({R_2})存在实数解,则 k 必满足(k le {{mathop alpha nolimits^2 } over {4beta }} + mathop Anolimits_F - 1);
求解可得:(mathop Rnolimits_1 = {{alpha mp sqrt {{alpha ^2} - 4beta (1 + k - {A_F})} } over {2beta (1 + kappa - {{rm A}_F}){omega _0}{C_1}}}),(mathop Rnolimits_2 = {{alpha pm sqrt {{alpha ^2} - 4beta (1 + k - {A_F})} } over {2beta k{omega _0}{C_1}}})
选定({C_1}),k后根据计算公式设计任意特性的VSVC低通滤波器。
归一化的巴特沃斯多项式:

对于单位增益(mathop Anolimits_F = 1),二阶低通,多项式系数(beta=1);
那么(mathop Hnolimits_0 = 1),(k le 0.25{alpha ^2})(k取值为(0.25{alpha ^2})时,VCVS二阶单位增益低通同时具有方便、低成本和稳定的优势)并且(mathop Rnolimits_1 = {{alpha mp sqrt {{alpha ^2} - 4k} } over {2k{omega _0}{C_1}}}),(mathop Rnolimits_2 = {{alpha pm sqrt {{alpha ^2} - 4k} } over {2k{omega _0}{C_1}}})。
通常情况下,为设计硬件电路方便,使得({R_1} = {R_2})。({C_1})的选取一般根据经验公式({C_1} approx {10^{ - 3 sim - 5}}{f_0}^{ - 1})得出。
这样进一步简化为:({C_2} = 0.25{alpha ^2}{C_1}),({R_1} = {R_2} = {2 over {alpha {omega _0}{C_1}}} = {1 over {pi alpha {f_0}{C_1}}})。
另外为运放正端提供回路补偿失调,取定({R_f} ll {R_r},{R_f}//{R_r} approx {R_f} = {R_1} + {R_2} = {2 over {pi alpha {f_0}{C_1}}}),到此完成了低通二阶巴特沃斯低通滤波器的参数配置。
对于高阶LPF设计,参照多项式系数和设定的截止频率即可完成。
实例仿真设计:以截止频率为100khz,增益为1,设计四阶巴特沃斯低通滤波器:
四阶低通存在参数:({alpha _1} = 0.7654,{alpha _2} = 1.8478),f=100khz,取第一级第二级({C_1} = 4.7nF);
得到:
第一级({C_2} = 0.68nF),({R_1} = {R_2} = 884.8Ω),({R_f} = 1769.6Ω);
第二级({C_2} = 4.02nF),({R_1} = {R_2} = 366.5Ω),({R_f} = 733Ω),
({R_r})取定1MΩ。Multisim仿真如下:
