Aktif ota-C filtrelerinde uygun oto problemi
The Suitable ota problem for second-order ota-c filters
- Tez No: 46286
- Danışmanlar: PROF.DR. HAKAN KUNTMAN
- Tez Türü: Yüksek Lisans
- Konular: Elektrik ve Elektronik Mühendisliği, Electrical and Electronics Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1995
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 85
Özet
ÖZET Levent Ö?DÜM özet; Aktif devre sentezi yöntemleriyle elde edilmiş aktif OTA-C filtre yapıları literatürde bol miktarda bulunmaktadır. Bu tezde, bu filtrelerde kullanılan ve analog tümdevre teknolojisi (CMOS) ile gerçeklenmiş OTA larm filtre karakteristiklerini nasıl etkilediği pratik olarak incelenmiştir. İnceleme, en basit OTA yapısı olarak kabul edilebilecek olan SİMETRİK OTA ile yapılmış, bu OTA dan kaynaklanan problemlerin, filtre karakteristiklerinin düzeltilmesine ışık tutacağı düşünülmüştür. Simetrik OTA nın kullanılması ile elde edilen karakteristiklerden yapılan değerlendirme sonucu, daha uygun bir OTA kullanma arayışına girişilmiştir. Literatürde CMOS OTA ların lineerleştirilmesi üzerine pek çok yayın bulunmaktadır. Bu OTA ların lineer çalışma bölgelerinin geniş olması, lineer çalışma bölgesinde mümkün olduğu kadar lineer bir doğrunun elde edilmesi mümkün olmaktadır. OTA lardaki bu lineerleşmenin filtre karakteristiklerinde iyileşme yapıp yapmadıkları incelenmiştir. Temel olarak ideal OTA dan sapma olarak kabul edilebilecek olan çıkış direnci, giriş direnci, band genişliği, lineer çalışma bölgesi, eğimin lineerliği gibi parametrelerin filtre özelliklerini nasıl etkilediği incelenmiştir. IV
Özet (Çeviri)
SUMMARY THE SUITABLE OTA PROBLEM FOR SECOND-ORDER OTA-C FILTERS Levent Ö?DÜM Abstract: The main purpose of the thesis is to examine and to improve a good performance for the active OTA-C filters. The procedure is designed to examine the practical behaviours of the active OTA-C filters and then to improve a good performance for the filters. The filters which are examined are the low-pass, high-pass, band-pass and ail-pass filter. All these filters have been examined in the literature. The conventional operational amplifier (op amp) is used as the active device in the vast majority of the active filter literature. For design purposes, the assumption that the op amp is ideal (A=°o, Rin=t*>» Ro=0) is generally made, and large amounts of feedback are used to make the filter gain essentially independent of the gain of the op amp. A host of practical filter designs hve evolved following this approach. It has also become apparent, however, that operational amplifier limitations preclude the use of these filters at high frequencies, attempts to integrate these filters have been unsuccesfull (with the exception of a few nondemanding applications), and convenient voltage or current control schemes for externally adjusting the filter characteristics do not exist. With the realization that the BJT and MOSFET are inherently current and transconductance amplifiers, respecctively, the following question naturally arises. Can any improvements in filter characteristics, performance, or flexibility be obtained by using one of the other basic types of amplifiers (e.g. transconductance, current, or transresi stance) in place of a voltage amplifier (or specificaly the operational amplifier) as the basic active device in a filter structure ? In the second section, basic first and second-order structures using the transconductance amplifier (often termed the operational transconductance amplifier : OTA ) are briefly discussed. Many of the basic OTA based structures use only OTAs and capacitors and, hence, are attractive for integration. Component count of these structures is often very low (e.g, second-orderbiquadratic filters can be constructed with two OTAs and two capacitors ). Convenient internal or external voltage or current control of the filter characteristics is attainable with these designs. They are attractive for frequency referenced applications. From a practical wiewpoint, the high-frequency performance of discrete bipolar OTAs, such as the CA 3080, is quite good. The transconductance gain, gm, can be varied over several decades by adjusting an external dc bias current Iabc- The major limitation of existing OTAs is the restricted differential input voltage swing required to maintain linearity. For the CA 3080, it is limited to about 30 mVp.p to maintain a reasonable degree of linearity. The OTAs in the thesis are assumed to be realized in CMOS technology. Prior to the mid-1970s, MOS technology was utilized primarily for memory and logic functions and the analog functions that were required in a given system were typically implemented by using bipolar integrated circuits such as operational amplifiers. However, in more recent years, the steady increases in chip complexity brought about by continuing improvements in lithographic resolution have created the economic incentive to implement subsystems containing both analog and digital functions on a single integrated circuit. Most often, the necessary analog functions are those associated with the conversion of signals from analog to digital form and vice versa, such as precision amplification, filtering, the sample-and- hold function, voltage comparison, generation of precision binary- weighted voltages and currents, and generation of precision reference potentials. The partitioning of subsystems into seperate bipolar analog and MOS digital portions is undesirable in many cases for reasons of package cost, physical space on printed circuit boards, and performance. Examples of such subsystems are analog-digital converters, and voice PCM encoder/decoders. Compared with bipolar technology, MOS technology has both advantages and disadvantages in implementing analog functions. MOS transistors inherently display lower transconductance than bipolar devices, leading to higher dc offsets in differantial amplifiers. However, the virtually infinite input resistance of the device when used as an amplifier and zero offset when used as a switch allow a signal voltage to be strored on a monolithic capacitor and sensed continuously and nondestructively. Thisresults in a precision on-chip analog sample/hold capability that does not exist in bipolar technology. This capability has been widely utilized to enhance the VIperformance of MOS analog circuits, performing functions such as sample-data analog filtering, offset storage and cancellation, precision amplification, and precision binary attenuation. It was known from the second-order filter equations that the design parameters of the filters wp and Q, depends on gm, the main parameter of the ota. Because of this, gm must have a reasonable linearty. It is clear that the linearly error will cause an error on the filter characteristics, especially at its frequency response. Morover the linearty error at gm will cause a harmonic distorsion at the output, because of these reasons, different kinds of linearization techniques have been developed widely for CMOS OTAs in the literature. The simplest CMOS OTA which is called 8 Symmetrical ota“ is based on the behaviour of the source- coupled differantial pair. As a starting point, it is worth considering the simple differantial pair integrator. In addition to its simplicity, tunability, and area efficiency, this integrator configuration potentially achieves the best high-frequency performance because it makes use of a single source-copied differantial pair to implement the voltage-to-current converter function. One major disadvantage of this elementary integrator, however, is that in order to maintain harmonic or intermodulation distortions at acceptable levels and to limit the unity-gain frequency shift due to the nonlinear behaviour of the source- coupled pair, the current modulation in the transistors must be kept low. Several circuit techniques for improving the linearty of MOS transconductance elements have been proposed in the literature. In one of them, linearization is achieved by simply adding an auxiliary cross-coupled differantial pair to the source- coupled pair and by properly scaling their W/L ratios and the tail currents, another possibility is to degenerate the source-coupled pair by means of a mos transistor operating in the triode region. Acombination of both techniques yields even better linearty performance. Other linearization methods use grounded- source triode-mode MOST's, cross-coupled quad configurations or class AB operation. All these OTAs are examined in the third section. This section begins with the discussion of the simple source-coupled differantial pair. By using this configuration, an improvement of ”symmetrical OTA“ is made. The performance of such an ota is discussed. It was seen that, linear range of such an ota depends vnon the W/L ratios of the driving transistors of the source- coupled differantial pair. Decreased W/L ratio of the driving transistorscauses a larger linear range but for keeping the tail current constant (channel-lenght modulation), it has a minimum value which can not be passed further. Also the transconductance parameter of the ota, gm depends on the ratio of W/L. A decreasing W/L ratio decreases the gm of the ota and visa versa. For to have a reasonable gm and linear range both, an optimum solution has to be found, the transconductance parameter gm of a symmetrical ota can be varied by adjusting the tail current of the source-coupled differantial pair, this current source is simply a single NMOS transistor, so in order to keep the tail current constant, the transistor must operate in the saturated region, as it is known well, this happens if the drain- source voltage of the transistor is greater or equal to the gate- source voltage of the transistor. This is why the W/L ratios of the driving transistors can not be choosen too small. Values less than a minimum will cause the current source to loose its stability. This will cause a transconductance error for the ota. From the theoritical calculations and the experimental data, the linear range for a symmetrical ota can not be changed independently. This is the important insufficiency of the ota. Second and the most important insufficiency for symmetrical ota is that the transconductance parameter gm does not have a sufficient linearty in the linear range. As it was said before, this will cause a wp and Q errors in the second-order filter applications. As it is clear, the main purpose for a analog integrated circuit designer, is to design an IC as close as possible to its ideal model. This is because of the active network synthesis methods that are used in order to get the configurations for various active OTA-C filters in which generally one assumes the active elements(OTA, OP AMP or etc.) ideal, but this is an Utopia of course. For a practical OTA, gm is not constant (linearty error), output resistance is not infinitive, input resistance is not infinitive and the frequency band is not infinitive too. All of these variations from the ideal model should be tested after the design. In the third section, another ota called ”Anti Phase Common Source Pair“ (ACSP)[1J is tested for the active ota-c filters, this ota has been designed for the improvement of gm linearty [2]. Also the dynamic range of the ota is larger than the symmetrical ota. In order to make comparison, transconductance parameter gm is kept the same for both symmetrical ota and ACSP ota. vmAlthough ACSP ota has some advantages as said above, it has a smaller frequency band when compared with the frequency band of the symmetrical ota. Because of this, the filter applications should have a frequency bandies s than the ACSP's frequency band. This means that larger capacitors or smaller gm should be selected in the design procedure. The main reason for the narrow band is the extra network in order to improve a compensation for the gm linearly. This kind of compensation is simply called feedforward compensation, which can be simply described as to provide a path from differantial input voltage to a certain node. The function that the feedforward network provides, depends on generally to the node that its output is connected. As it can be easily understood, the extra feedforward network will get extra parasitic capacitors in the configuration so, this will cause a narrow frequency band. Other kinds of otas are examined and tested for the active OTA-C filters in this section. All of them give different kinds of characteristics for the filters. For each OTA, all of the characteristics are given in this section too. In the fourth section, the characteristics of the filters for each ota are compared. This section has shown that active OTA- C filters with one of the OTAs do not have a certain advantage on others, it has some disadvantages also, so the election is left to the readers, for their purposes. In order to test the filter performances, the simulation program SPICE is used. The parameters used for the NMOS and PMOS transistors are the TÜBİTAK YlTAL's level 2 parameters, so, some parameters, something like mobility nonlinearty are not taken into account. In order to get the nonlinear DC characteristics of the otas and the filters, ”.DC“ analysis preferenced. As it is clear, for frequency analysis ”.AC“ analysis is preferenced. In order to get some theoritical results for the otas (their nonlinearity errors, DC bias, etc. ) simple square-law model for the transistors are used. In the fifth section, the results and the decisions are discussed briefly. The question of ”what can be done in the future ?" is tried to give an answer. This is because CMOS IXanalog IC technology is growing up recently, so more good performance otas will exist in the future.
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