具有最少元件之電流式OTA-C萬用二階濾波電路於高頻操作時非理想效應之研究與改進

Non-Ideal Effect and Improvement of The Current-Mode High-Frequency OTA-C Universal Biquad Filter with The Minimum Components

詹智源(Chih-Yuan Chan)

指導教授 ： 張俊明

中原大學/工學院/電機工程研究所/碩士(2007年)

https://doi.org/10.6840/cycu200700406

摘要

摘 要
在運算轉導放大器（OTA）及電容器（C）的電路中，會產生許多的寄生電容，包括:輸入端寄生電容，輸出端的寄生電容，以及節點上的寄生電容。在設計電路時，為了要讓外加電容的位置與所有寄生電容的位置相同，所以要避免使用雙輸入端的OTA及浮接電容，如此寄生電容值才可以被外加電容值吸收，在高頻時，才可得到較精準輸出訊號。
最近提出的分析合成法(Analytical Synthesis)，所設計的電路同時符合三項運算轉導放大器（OTA）及電容器（C）的電路設計準則：(1)使用單端輸入的OTA(以免除寄生電容的Feedthrough效應)，(2)使用接地電容(可直接吸收寄生電容)，(3)使用較過去為少的主被動元件(使得具有總體較低的寄生電容效應)符合這三項準則的電路，所有外加電容的位置才會與所有寄生電容的位置相同。
OTA內部的非理想效應來自其內部諸電晶體的寄生電容及寄生電導。至於OTA外部的非理想效應則由OTA的輸入端、輸出端及內部節點的寄生電容與輸出端的寄生電導所產生。當模擬的諧振頻率較理想值為低時，顯示寄生電容的存在使電路的諧振頻率降低。以往運算轉導放大器（OTA）及電容器（C）的主動濾波電路操作頻率範圍在1MHz範圍以內，而本文將以電流式二階運算轉導放大器（OTA）及電容器（C）萬用濾波電路為例，探討此電路在高頻及超高頻以上的問題探討與改進。主要方法為計算出寄生電容值大小，來適當調整外加電容值，當調整至外加電容值近似於寄生電容值時，此時將外加電容移除，並搭配OTA轉導值的調整，可讓此電路在高頻無外加電容時有更精準的輸出。
本論文所提出之以吸收寄生電容或調整轉導值來修正外加元件大小的改進法，經H-SPICE 0.35 m製程的模擬結果，證實可將運算轉導放大器（OTA）及電容器（C）濾波電路在高頻操作時的非理想效應降低，為一具有極大實用價值之電路改進法。

關鍵字

非理想效應 ； 電流式 ； 具有最少元件

並列摘要

Abstract
There are several kinds of parasitic capacitances, including input and output parasitic capacitances of an OTA and the nodal parasitic capacitance at the internal node in an OTA-C (Operational Transconductance Amplifier and Capacitor) circuit. In the analogy circuit design ,in order to have the same place for both given capacitor and all the parasitic capacitances, it must use for differential-input OTA and floating capacitance. An improvement approach is then proposed by the absorption of parasitic capacitances from the given capacitors to obtain a precise high-frequency circuit.
Recently, the “Analytical Synthesis Method” has been proposed to realize the high-order OTA-C circuits which achieving the following three important criteria simultaneously for the design of OTA-C filters: (1) using single-ended-input OTAs (overcoming the feedthrough effect due to the use of differential-input OTAs), (2) using grounded capacitors (absorbing the shunt parasitic capacitance), and (3) using the least number of component counts (reducing the total parasitic effects). Note that all the parasitic capacitances have the same places as those of all the given capacitors in the realized circuits achieving the above three important criteria.
The non-ideal effect in an OTA is resulted from the parasitic capacitances spreaded among the MOS transistors which is called the frequency dependent transconduce, namely, the ratio between the output current phasor and the input voltage phasor, of an OTA. The non-ideal effect out of the OTA includes the input and output parasitic capacitance and the output parasitic conductance of an OTA and the nodal parasitic capacitance at each internal node. If the simulation resonance frequency is lower than the theoretical value, it means that the additional parasitic capacitance makes a total capacitance larger than the exact value. The resonance frequency of the OTA-C is usually operation within 1MHz. In this thesis, a current-mode second-order OTA-C universal filter structure is used for example to demonstrate the non-ideal effect improvement for a H.F.(High Frequency) and more than V.H.F(Very High Frequency) circuit. Main of the method is reduction given capacitor to absorption of parasitic capacitance from the given capacitor. After several reductions of absorptions of capacitances and transconductances tunned can enter the very precise range with the error rate very low for the simulation resonance frequency.
Finally, The above thesis was verified by TSMC 0.35u H-Spice simulation and improve the non-ideal effect for filter circuit with OTA-C at a high-frequency.

並列關鍵字

with The Minimum Components ； Non-Ideal Effect ； Current-Mode

參考文獻

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