适用于OTA-C滤波器的高线性OTA的设计

Design of new highly linear OTA for OTA-C filters

设计了一种适用于OTA-C滤波器的高线性运算跨导放大器(OTA).该OTA采用新型的乘法器输入级,以获得大的线性跨导输入范围;采用一种新的共模负反馈(CMFB)策略,将主放大器输出电压线性压缩后再引入CMFB电路,以改善传统CMFB结构对OTA输出线性范围的限制.在SMIC.35μm标准CMOS工艺下仿真,结果显示:输入级的线性跨导差分输入电压范围达到了-2～2V,等效跨导在1μS时,直流(DC)开环增益达到了76dB,共模抑制比(CMRR)为140dB,电源抑制比(PSRR)为144dB.基于这种OTA设计了OTA-C二阶低通巴特沃斯滤波器.通过调节OTA的跨导,滤波器在1pf的负载电容下的截止频率从11kHz变化至419kHz;当截止频率为100kHz时输出为3Vp-p@100kHz时的总谐波失真(THD)为-47dB.

A highly linear operational trausonductance amphfier （OTA） was designed for OTA-C fil- ters, in which a new multiplier input stage was adopted to get a high linear trans-conductance input range. Structure limitations in conventional common mode feed bark （CMFB） were addressed with a new common mode feedback strategy, in which the outputs of the OTA linearly were compressed be- fore putting them into CMFB circuits. The simulation result shows that the linear divided input volt- age is --2-2 V, and with 1μs trans-conduetance, the OTA gets a DC-gain of 76 dB, common mode rejection ratio （CMRR） of 140 dB, power suppy rejection ratio（PSRR） of 144 dB, under the SMIC. 35μm standard complementary metal-oxide semicondouctor （CMOS） process. Based on such OTA, an OTA-C low-pass butterworth filter was designed. The filter＇s cut-off frequency can alternate from 11 kHz to 419 kHz with 1 --47 dB total harmonic pf capacitance load, and when the cut-off frequency is 100 kHz, the filter gets distortions with 3 Vp--p@ 100 kHz output.