電流式OTA-C二階高通濾波電路於高頻操作時之研究

Current-Mode Second-Order OTA-C High Pass Filter Structure with High-Frequency Investigation

林炳成(Ping-Cheng Lin)

指導教授 ： 張俊明

中原大學/工學院/電機工程研究所/碩士(2008年)

https://doi.org/10.6840/CYCU.2008.00066

摘要

中文摘要
電流式電路的設計在近幾年來蓬勃的發展;在這些年之中，也逐一證實電流式比電壓式電路有較好的精準度。由於算轉導放大器OTA (Operational Transconductance Amplifier)與第二代電流控制傳輸器CCCII(Second-generation Current Controlled Conveyor)可藉由一偏壓電流改變其內部轉導值或本質電阻(intrinsic resistance) ，所以在設計電路時無需考慮外加電阻及電阻接地問題。本論文所研究的電路為什麼是以OTA為主的電路，是因為CCCII有額外的缺點:1. 電壓追蹤誤差2. 電流追蹤誤差。

使用運算轉導放大器(OTA)與電容器(C)做類比電路設計是一個值得研究的方向，其主要原因為運算轉導放大器(OTA)不僅具備主動元件特性並且可藉由等效電路代替電阻器。

於OTA-C的電路中，由於寄生電容的種類繁多(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)，使得當電路設計時，如果使用雙輸入端的OTA及浮接電容，便無法讓外加電容的位置與所有寄生電容的位置相同；使得在過去的文獻中，無法找到以吸收寄生電容值的大小，來給予恰當的外加電容值，而得到具有精準輸出訊號的高頻電路的構想。

OTA-C為主要架構之濾波電路同時具備了以下三項優點：(1)使用單端輸入的OTA(可免除寄生電容的Feed-through效應)，(2)使用接地電容(可直接吸收寄生電容)，(3)使用最少的主動元件，且所有外加電容位置恰好與所有寄生電容的位置相同，吸收了寄生電容，並減少寄生效應對整體電路的影響。

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。由OTA輸出電流與輸入電壓的相量比值，吾人可求得OTA與頻率變化有關的轉導函數。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主導。本論文以電流式二階OTA-C濾波電路為例，説明如何以吸收寄生效應，調整外加電容值或OTA的轉導值以得到精準高頻電路的方法。當模擬的諧振頻率比理想值高時，顯示所給的OTA的轉導值略大吾人可適當減少OTA電導值，使模擬結果更接近理想值。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了。最後，本論文使用TSMC035的製程參數進行實際的模擬條件做模擬驗證。

關鍵字

高通濾波電路 ； 高頻操作

並列摘要

Abstract
In the last few years, the current-mode circuit is proved to be much more precise than its voltage-mode counter part. Since the transconductance of an Operational Transconductance Amplifier (OTA) and the inner intrinsic resistance at terminal x of a second-generation current controlled conveyor (CCCII) are electronically adjusted by the internal bias current,no resistors in the OTA-C or CCCII circuit become an important advantage in the active filter design. The thesis studies why is the circuit by the OTA,because CCCII has other shortcomings:1. Voltage tracking error。2. Current tracking error。

It is worthy of research to do the analogue circuit using Operational Transconductance Ampplifiers (OTA) and Capacitors (C), because an OTA with the property of an active element can replace a resistor such that no resistors will be needed in the integrated circuit。

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. This leads to the difficulty to have the same places for both given capacitors and all the parasitic capacitances. When the differential-input OTA and floating capacitances are employed in the circuit structure, and the impossibility to obtain a high–frequency circuit with precise output responses by giving a proper capacitor value after absorbing the parasitic capacitance。
OTA-C elliptic filters has achieved the three following important advantages: (i) using single-ended-input OTAs (overcoming the feed-through effect due to the use of differential-input OTAs), (ii) using grouded capacitors (absorbing the shunt parasitic capacitance), and (iii) using the minimum components and all the parasitic capacitances have the same places as those of all the given capacitors in the realized circuits。

The non-ideal effect in an OTA is resulted from the parasitic capacitances spreaded among the MOS transistors which are 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。 In this thesis, a current-mode second-order OTA-C universal filter structure is used for example to demonstrate this new improvement for a high-frequency circuit. When the simulation resonance frequency is lower than the theoretical value, , this implies that the given transconductance is also higher than the exact value. The reduction of the given transconductance leads to approach the theoretical prediction. On the contrary, 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.
Finally, The above proposed improvement was verified by UMC035 H-spice simulation。

並列關鍵字

Current-Mode Second-Order OTA-C ； High Pass Filter Structure

參考文獻

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