Eşikaltı çalışan otaların iyileştirilmesi ve tip elektroniği alanına uygulanması
Başlık çevirisi mevcut değil.
- Tez No: 55758
- Danışmanlar: PROF.DR. H. HAKAN KUNTMAN
- Tez Türü: Yüksek Lisans
- Konular: Elektrik ve Elektronik Mühendisliği, Electrical and Electronics Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1996
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 154
Özet
ÖZET Bu çalışmada, eşikaltında çalışan geçiş iletkenliği kuvvetlendiricisinin (OTA), kuvvetli evirtime göre, akım geçiş eğrisinin simetriliğinin bozulma sebepleri ve OTA üzerinde yapılan düzeltmeler ile akım geçiş eğrisinin simetriliğinin nasıl düzeltilebileceği araştırıldı. Akım geçiş eğrisi düzeltilmiş OTA ve kapasite elemanları kullanılarak dördüncü dereceden süzgeçlerin tasannu yapıldı. Bu tür süzgeçlerin tıp elektroniği uygulamalarında özellikle EEG işaretlerinin filtre edilmesinde elverişli olabileceği görülmüştür. Eşikaltı çalışmadan dolayı akımlar çok küçük olduğundan, OTA'ların geçiş iletkenlikleri de çok küçük olmaktadır. Bu da a, |3, 0 ve 5 bandlarının yer aldığı lHz-40Hz aralığında gerçekleştirilecek süzgeç yapılan için, 30-250pF mertebelerindeki küçük kapasite değerlerinin kullanılması olanağını vermektedir. Böylece, dört süzgeci kapsayan bir yapının tümleştirilebilecek boyutlarda gerçekleştirilebileceği görülmüştür. Tasarlanan bu OTA'ların tıp elektroniğine uygun olabilmesi için OTA'ların düşük besleme gerilimli olarak tasarlanması, tasarlanan düşük beslemeli OTA'nın diğer OTA'ya göre daha elverişli olduğu simülasyon sonuçlarında görülmüştür. Ayrıca simetrik OTA'ya göre daha geniş bir lineer çalışma bölgesine sahip ve harmonik distorsiyonu daha düşük olan Antiphase Common Source Pair (ACSP) OTA'nın EEG işaretlerinin elde edilmesi için daha elverişli olduğu görülmüştür. Bilindiği gibi, OTA'nın gerçekleştirilmesinde kullanılan tranzistorların ve sistemlerde kullanılan OTA sayısının artması ile simülasyon süreleride artmaktadır. Simülasyon süresini azaltmak için, OTA'nın tüm karakteristiği ile uyumlu olan bir makromodel kullanılmış ve bu makromodelin simetrik OTA'yı tam olarak temsil edebileceği simülasyon sonuçlarından görülmüştür.
Özet (Çeviri)
Prior to the mid-1970's, MOS technology was utilised 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 voltage and currents and generation of precision reference potentials. The partitioning of subsystems into separate bipolar analog and MOS digital portions are 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 differential 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 stored on a monolithic capacitor and sensed continuously and nondestructively. This results in a precision on-chip analog sample/hold capability that does not exist in bipolar technology. This capability has been widely utilised to enhance the performance of MOS analog circuits, performing functions such as sample-data analog filtering, offset storage and cancellation, precision amplification and precision binary attenuation. Today, amplifiers in which transistors operate in subthreshold region (weak inversion region) become more important from day to day. The reason for this is the need for low-power and low-voltage devices that can operate for a long time with battery-power. However, for human body implantation of biomedical viinstruments it is very important and also for movable instruments that must work as long as possible with battery-power such as Notebooks, wireless and pocket phones. In this thesis the main thought was to find out why the output current-differential input voltage transfer characteristic in subthreshold region is not symmetrical but in the strong inversion it is symmetrical with the same operational transconductance amplifier (OTA). The main purpose was to examine and to improve a good performance OTA that operates in subthreshold region. As a starting point for all these things, second sections include the definition of the subthreshold region and SPICE's small signal model equations for MOS transistors. Derivatives of current and voltage equations are needed in the electrical circuit simulation programs,. However, in SPICE model, the derivative of drain current with respect to gate voltage is discontinuous along the transition region between the subthreshold and strong inversion regions. However, in the SGS version of SPICE a more precise model for subthreshold and transition regions was proposed to eliminate this problem. This model is based on the separate calculation of the contributions of the diffusion and drift currents between the regions of weak and strong inversion. Furthermore, continues current-voltage equations exist for the MOS transistors three distinct operation regions. In subsection 2.2 some OTA structures current-voltage equations are defined for the transistors who operated in subthreshold region. In subsection 2.3 is briefly explained the way to find out the maximum input signal level without to cause a clipping and slew-rate limitations at the output. It is known that OTA itself doesn't behave linearly if the output voltage goes in saturation. As a result of such a saturation a clipped signal at the output appears. If the output current goes in saturation, saw-tooth wave forms at the output appear because of the slew-rate problem. The last subsection is about the application to medical science electronic where the use of EEG signals is important for some illness diagnosis. EEG frequency bands are normally classified into four categories as shown below. These EEG signals are send by small surfaced electrodes that are placed at specific points on the head. The magnitude, phase and frequency of these signals depend also on the electrode's placements. The meanings of these different frequencies are not completely known. However, alpha waves are less than İO^V from peak-to-peak and this signal can be perceived from the brains front region when the person is awake but his eyes are closed. The vumagnitude of the alpha signal increases when the person's eyes are opened and it gives a warning. Beta waves are less than 20u, V from peak-to-peak and the stimulation comes from whole Brian. If the person is in relaxation the stimulation comes much more from the central region of the Brian. Theta and Delta waves are less than IOOjiV from peak-to-peak and they are around the central region of the Brian and they indicated the sleeping of the person. In the third section, the main reason for the anti symmetrical output current is presented. The reason for the anti symmetrical output current in subthreshold region is found as channel length modulation and new designs techniques are improved for the symmetrical CMOS OTA to realisation a symmetrical output current. Another part of this section is about the examine of the cascade symmetrical OTA's behaviour in subthreshold region because this OTA has a very good symmetrical output current in all three MOS transistor operation regions. This type of OTA has nearly double MOS transistors as much as the symmetrical OTA has. It is clear that the main purpose for an 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, OPAMP,...) ideal, but this is an Utopia of course. For a practicable OTA, the gm is not constant (linearity error), the output and the input resistance are 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 section four, fourth order filter structures are designed for a, P, 0 and 5 frequency bands. In various applications OTA-C filters that are mostly used for operating in strong inversion region. However, OTA-C filters that are here used operate in subthreshold region so that they are suitable for EEG signals. The frequency bands that are here used are so low that large valued of capacity elements are required if the classical approach are used for OTA-C filter designs. But here, the OTA-C filters are operating in subthreshold region so the capacity values are small and these gives us the possibility to integrated all the filter in one IC. The OTA-C filter topology that is used here was proposed by C. Acar, F.Anday and H.H.Kuntman. To provide a simplicity in the realisation of the filters which are obtained by connecting second order low pass and high pass filters in cascade, the transconductances Gm of the OTA's in each cell, were set equal to each other. Designed band-pass filters were simulated with PSPICE. Simulation results of these filters have been compared with theoretical results. It can be easily observed that each of the characteristics fulfils the main requirements in the pass band. The viiideviations from theoretical results in the stop-band are caused by the OTA non- idealişe which can be easily neglected since they arise in frequency regions far from the pass-band where the input signal is attenuated more then 60 dB. The maximum input signal level of an OTA-C filter is limited dominantly by the slew-rate arising from the current saturation at the output of the the OTA's. In this work a simple formula was derived for the input signal amplitude not causing any clipping and slew-rate limiting problems. However, applying the derived formula to the designed active filter structures, the maximum input signal level of the filters SPICE simulation has performing the transient analyses. This is caused by the low level of the dc operating currents, which results in slew-rate at the output signal even though at low load capacitance values of the order of several ten pico farads. Fortunately, the EEG signals are small enough and therefore there is no need of taking any measure to over come this limiting problem. The output signals of the filters must be applied to following amplifier stages to obtain meaningful levels for signal processing. In section five a new design are made for the symmetrical CMOS OTA and symmetrical Kaskod OTA that operate in subthreshold region from section four so that they can be used with a 3V battery-power. Another part of this section is to examine the suitable of OTA-C filters which were also designed in section four. Furthermore, the comparison of both the symmetrical CMOS OTA and Kaskod OTA that operate with low-voltage and ±5 V were made. Low-noise opapms are important in several respects. A large percentage of applications for analog CMOS circuits is in the area of telecommunications were the signal -to- noise ratio (S/N) is important so the lower the noise, the better the value of (S/N) for a given signal. Another way of looking at this characteristic is from the viewpoint of dynamic range. The dynamic range of a circuit is the ratio of the largest -to- smallest signal that can be processed without distortion. The upper level is typically established by the power supplies and the large-signal swing limits. The lower level is established by the noise or the ripple injected by the power supply. In section six, another OTA called“Anti Phase Common Source Pair”(ACSP) is used for subthreshold region operating design. This OTA has been designed for improvement of g“, linearity. Also the dynamic range of the OTA is larger than the symmetrical OTA. In order to make comparison, transconductance parameter gn, is kept the same for all OTA's. In this section, two types of OTA's, ACSP OTA and Cascade ACSP OTA, are designed for subthreshold region operating. Also a comparison are made with the other OTA's. In section seven, ACSP and Cascade ACSP OTA's are used for OTA-C filter designs as in section 4 and 5 was made. IXThe last section is about a simple and accurate makromodel for symmetrical CMOS OTA. The derived macromodel is especially suitable for SPICE simulation of OTA-C active filters realised in CMOS technology. The OTA characteristics are simulated by using this macromodel and compared with the simulation results obtained by SPICE device level models. The results show that the proposed OTA macromodel represents the symmetrical CMOS OTA approximately with the same accuracy of semiconductor SPICE device models, but with a significantly reduced computer time. In conclusion, the Cascade OTA's are more general for use because the symmetrical CMOS OTA and ACSP OTA have only a narrow g”, range in subthreshold region around a specific gm in which we will a symmetrical output current after the correction for a symmetrical output current. However, symmetrical CMOS OTA with low-voltage is therefore suitable for medical applications for human body implantation of biomedical instruments. Although ACSP OTA's have a better characteristic than symmetrical CMOS OTA's its disadvantage is it has five times more MOS transistors than the symmetrical CMOS OTA. In order to test the OTA and filter performances, the simulation program SPICE is used. The parameters used for the NMOS and PMOS transistors are the TÜBİTAK YİTAL's level 2 parameters.
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