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1486 results in Circuits and systems

Page 10 of 75

  • 13 - Follow the Leader Feedback Filters

  • Muzaffer Ahmad Siddiqi
  • Book:
    Continuous Time Active Analog Filters
    Published online:
    24 December 2019
    Print publication:
    26 March 2020, pp 398-423

  • Summary

    Introduction

    One of the important issues in filter design is that parameter sensitivity has to be taken into consideration. A doubly terminated ladder is extensively used mainly because of the same reason, that is, parameters of the filter realized through it have low sensitivities. At the same time, an alternate synthesis method using the cascade approach has found favour because of its ability to tune specific pole–zeros in higher-order filters through the utilization of noninteractive second-order sections. However, in the cascade process, sensitivities increase, especially for high-Q filters. Since we know that negative feedback improves the performance of electronic circuits in a number of ways, the same have been applied to obtain what is known as multiple feedback (MF) topologies. An MF topology consists of a network with a single feed-forward path comprising unilateral (active structures are mostly unilateral unlike passive structures which are bilateral) second-order sections, having different kind of feedbacks. The nature of the feedback decides its final topology. One of the topologies is called leap frog and was discussed in Chapter 9. While performing operational simulation, the circuit had a topology shown in Figures 9.22 and 9.24, where it is easy to recognize the structure as a leap frog structure. Another topology under the broad area of MF topologies known as follow the leader feedback (FLF) is the subject of this chapter.

    In Section 13.2, the basic FLF structure and the kind of transfer functions obtained is included. Also included in this section is the derivation of the structure's transfer function when either lossless integrators or lossy integrators are used in the feedback paths. Use of only a feedback block could provide all pole functions. Hence, feed-forward is also included, as will be discussed in Section 13.3, to improve the versatility of the scheme. A slightly modified feed-forward scheme, called the shifted companion structure is discussed in Section 13.4. It was observed that all these schemes were special cases of a general FLF structure that is given in Section 13.5. Without sacrificing the generality, synthesis of the filter becomes a bit easier if the feedback blocks have BP structures with equal value of quality factor, instead of different quality factors. Such a scheme, known as the primary resonator block technique is also discussed in this chapter.

  • 14 - Switched Capacitor Circuits

  • Muzaffer Ahmad Siddiqi
  • Book:
    Continuous Time Active Analog Filters
    Published online:
    24 December 2019
    Print publication:
    26 March 2020, pp 424-452

  • Summary

    Introduction

    To meet the advances in technology, realization of monolithic electronic analog filters became an important issue. In the beginning, the problem of inductance realization was solved through its simulation using active RC circuits. However, integration needed the solution of a few more problems such that the realized filters perform as close to the design as possible. One of the main issues faced while integrating circuits was the amount of tolerance in passive elements; it was too large. This large tolerance resulted in large errors in the realized filter parameters. These errors could be reduced through electronic tuning of components or the tuning of filter parameters, with an obvious increase in circuit complexities. It was observed that the need of electronic tuning could be reduced through schemes in which filter parameters depend on the ratio of passive components, especially capacitors, as capacitor ratio tolerance is much better in integrated circuits. Switched capacitor filter (SCF) realization technique is one such scheme in which all filter parameters depend on capacitor ratios. Equally important is the fact that switched capacitor (SC) schemes do not use external resistors. This saved a large amount of chip area; a highly attractive feature for integrated circuits.

    In this chapter, the basic concepts of SCF realization will be discussed. One important approach is the simulation of grounded and floating resistors using capacitors, switches, and non-overlapping clock signals. Such a realization and its simple applications including integrators are shown in Sections 14.2 and 14.3. As switches and clocks are the basic components of integrated circuits, these are further studied in Section 14.4. The rest of the chapter covers the topic of first- and second-order SCF sections.

    Switched Capacitor Resistor

    Let us examine a simple circuit as shown in Figure 14.1(a), containing two switches and a capacitor CR. The port voltages are v1 and v2, and the two switches become ON and OFF continuously at a certain clock frequency fc. This means that each switch can remain on for a maximum time duration Tc = 1/2 fc. It is further assumed that either voltages v1 or v2 will remain constant during the small duration Tc when either of the switches is ON.

  • 7 - Single Amplifier Second-order Filters

  • Muzaffer Ahmad Siddiqi
  • Book:
    Continuous Time Active Analog Filters
    Published online:
    24 December 2019
    Print publication:
    26 March 2020, pp 170-212

  • Summary

    Introduction

    Depending on the specifications, the required order of the filter may be small or big. For less challenging requirements, a second-order or even a first-order section may be adequate. However, higher-order sections may become necessary in other situations. For higher-order filter requirements, either the direct form approach is used, or a combination of secondorder second sections is used which are connected in a cascade form or in a multiple-loop feedback form. Hence, the study of and realization methods of second-order sections assumes importance as they are used either stand-alone or form the building blocks for higher-order filters. Since the overall performance of the higher-order filter depends on the individual second-order building blocks, it is very important that these building blocks have desirable and attractive properties; the following are some such requirements.

  • (c) Low sensitivities of the two important filter parameters, pole frequency wo and pole-Q, with respect to the circuit elements are highly desirable.

  • (d) Utilization of a lesser number of passive elements (active elements as well, though in present discussion, it is assumed that only one active component is used) makes the realization economical in discrete, as well as in integrated form.

  • (e) Values of the passive components should be in the practical range of integrated circuit (IC) fabrication, and the component spread is preferred to be as small as possible in order to make them attractive in IC form.

  • (f) Independent tunability of the parameters wo and Q is important as analog filters require some tuning.

  • (g) To properly connect the second-order blocks in cascade or in multiple feedback system, their input impedance should be as high as possible and the output impedance as low as possible at all working frequencies. Otherwise, it can change the characteristics of the overall higher-order filter.

    • A large number of second-order active building blocks are available with each one claiming advantages over the others. Study of the development procedure of such sections might help the readers to gain more insight towards selecting the optimal section or help in designing new circuits/improving upon the existing ones.

    Hence, we begin the study of development of second-order sections using a single passive feedback in Section 7.2; the effect of using different types of passive structures in the feedback is also included.

  • 11 - Amplification and Filtering in Biomedical Applications

  • Muzaffer Ahmad Siddiqi
  • Book:
    Continuous Time Active Analog Filters
    Published online:
    24 December 2019
    Print publication:
    26 March 2020, pp 334-359

  • Summary

    Introduction

    There has been a lot of development in the use of biomedical equipment for diagnostic and treatment purposes in recent times. A good amount of literature has come from the interdisciplinary area of medical engineering, medical instrumentation, and so on, making medical electronics a specialized field of study. An important constituent of electronics engineering is the study of analog filters. Hence, the design and application of analog filters finds a place in the books and literature on medical engineering and medical instrumentation too. The design of analog filters especially the continuous-time types requires special attention. However, the available books on analog filter design do not adequately relate filter design with biomedical applications. A few simple examples included in this chapter will try to bridge the gap between theoretical design and the application of analog filters in this important field.

    We have already studied the basics of continuous-time analog filter in the previous chapters. In this chapter, we will first connect the design of a filter to its utilization in the field of biomedical electronics.

    It is a well-known fact that cells in the human body have different element (Na+, K+, Ca++, Cl-) ion concentrations inside and outside the membrane. This difference in ion concentration creates a small electric potential called biopotential. When there is a disturbance in a biopotential, an action potential is generated which is the result of depolarization and repolarization of the cells in the human body. It is the action potential at the location (nodes) on the body which is detectable and can be processed using biomedical circuits. When such signals generated by the heart are collected, it makes up the electrocardiogram (ECG). ECG detectors use electrodes to collect these signals, which are amplified, filtered and displayed for data analysis. ECG signals require filtering and amplification to produce high-quality signals. Not only are different stages of amplification used, specific signal processing is also required. Therefore, instead of a simple (DA) differential amplifier, instrumentation amplifiers (inst-amp) are used. Section 11.2 discusses the the necessity of converting DA into an inst-amp. Transformation of the inst-amp into a biopotential amplifier and as integrated circuit inst-amps especially suitable in medical instruments and devices are discussed in Section 11.3 along with a case study on a piezoelectric transducer.

  • 17 - Active R and Active C Filters

  • Muzaffer Ahmad Siddiqi
  • Book:
    Continuous Time Active Analog Filters
    Published online:
    24 December 2019
    Print publication:
    26 March 2020, pp 538-592

  • Summary

    Introduction

    Conventionally, the design of OA-RC (operational amplifiers with resistors and capacitors) circuits assumes that OAs are ideal, having a large frequency independent gain. However, a commercially integrated OA is a non-ideal device, whose gain is frequency dependent and exhibits a LP (low pass) characteristic. A simple model for OAs, given in Chapter 1, is repeated here depicting its frequency dependent gain nature.

    In equation (17.1), ωa is its first pole, Ao is the gain at dc and B = Aowa is gain bandwidth product. This effect of the gain bandwidth product, B, being finite, its effect on the magnitude and phase responses of the filter became important criterion for comparison of the performance of filters. Finite B restricts the frequency range of the operation of OA-RC filters to mostly the audio frequency range, particularly when commercially economical OAs, such as OA 741, are used. In fact, in some topologies, product of pole-Q and center frequency wo has to be much less than B to avoid undesirable enhancement in Q, and likely instability. Consequently, suitable compensating schemes were developed to design filters with lesser dependence on OA gain characteristics, like those shown for integrators in Chapter 2; in other cases, specially designed but costly OAs were used.

    In an alternative approach, instead of considering the frequency dependent gain of the OA as undesirable, it has been exploited by using it directly in the design itself. This approach increases the frequency range of operation, and also reduces or completely eliminates the requirement of external capacitors. In this context, networks which use only OAs and resistors, with no external capacitors in their implementation, and derive their response from the internal dynamics of the OAs, are known as active R circuits [17.1], [17.2].

    Similar to active R, active C networks have also been designed. They utilize the frequency dependent model of the OAs, but use only capacitances, mostly in ratio form. It results in an extended frequency range; the use of small value capacitors in ratio form provides related advantages.

    Basic Techniques in Active R Synthesis

    Active R synthesis is an off-shoot of the active RC synthesis, where external capacitors were eliminated. Hence, the techniques used in the active RC synthesis are only tailored to suit active R realizations; these are briefly discussed in this section.

  • 15 - Operational Transconductance Amplifier-C Filters

  • Muzaffer Ahmad Siddiqi
  • Book:
    Continuous Time Active Analog Filters
    Published online:
    24 December 2019
    Print publication:
    26 March 2020, pp 453-492

  • Summary

    Introduction

    Operational transconductance amplifiers (OTAs) have emerged as a powerful alternative to OAs because of their current mode (CM) nature. Their CM nature allows OTAs to be used for much higher frequencies. Another major advantage of using OTAs is the electronic control it has over trans-conductance, which allows much easier tuning of filter parameters. The fact that only OTAs and capacitors are needed also makes the filter attractive for monolithic integrated circuit fabrication.

    Since the development of OA-RC filter design is well established and extensively studied, it has been used to advantage while using OTAs. Filter design using OTAs follow a similar pattern and design procedures used in OA-RC synthesis; however, they are tailored to specific needs as will be shown in this chapter.

    A brief review of the basic building blocks using OTAs is provided in Section 15.2. Simulation of grounded and floating resistors, inverting and non-inverting integrators, addition of voltages (and currents) and voltage amplifiers, for their stand-alone use or their application for realizing filter sections is included. First-order LPFs (low pass filter) and HPF (high pass filter) design is shown in Section 15.3 using integrators. Because of the suitability of OTA based circuits for differential outputs, the filter circuits discussed in Section 15.3 are then converted to differential mode. Similar to OA-RC synthesis, second-order filters are realized using the two-integrator loop method. Higher-order filters are realized using the element substitution method through the simulation of inductances and capacitances in Section 15.6 and 15.7. Filter realization using operational simulation is also discussed in Section 15.8.

    Basic Building Blocks Using OTAs

    OAs have been used for inverting and non-inverting amplification, summation (subtraction), integration and differentiation. In addition, while designing filters, OAs have been extensively used for simulating inductors and FDNRs (frequency dependant negative resistors). OTAs have also been used for all such entities; in addition, they are used for simulating grounded resistances (GRs) and floating resistances (FRs) as well. Hence, the resulting OTA-C circuits are not only realizable, but preferred as it makes them easily integratable in the monolithic form with more precise filter parameters.

  • 6 - Sensitivity of Active Networks

  • Muzaffer Ahmad Siddiqi
  • Book:
    Continuous Time Active Analog Filters
    Published online:
    24 December 2019
    Print publication:
    26 March 2020, pp 149-169

  • Summary

    Introduction

    In the previous chapters, comparatively simple methods have been discussed for the realization of active OA (operational amplifier) RC filters. Many more methods of filter synthesis will be discussed later, providing many more filter circuit configurations. Among this large number of available circuits, the choice of a ‘best’ filter circuit may depend on the specific user's requirements. However, every application ideally requires a practical filter for which performance parameters, like ωo (center frequency or cut-off frequency) and quality (pole-Q), are needed exactly as designed, and expected to remain invariant with use in varying environment. However, in practice, the user is satisfied if the parameters remain within such limits that do not make the filter impractical. Though there are different reasons during fabrication which cause deviations in the performance parameters, there is one factor which is common to all circuits at design stage. This factor can be termed as the first consideration in connection with these deviations. It is studied under sensitivity and is due to the following reasons.

  • i. Design of a filter circuit assumes active and passive elements to be ideal, whereas in every practical fabrication process, the nominal value of the passive element has statistical variations around its mean value. In general, sophisticated, higher level fabrication processes reduce the parameter variations; elements are said to have smaller tolerance. However, for such advanced processes, filter fabrication cost will go up.

  • ii. The values of both active and passive components change with change in operating conditions like change in temperature, humidity and supply voltage. Some chemical changes due to aging also affect the element values.

    • Whatever be the reason for the difference between the practical element value(s) and the original design value(s), performance of the filter gets affected, and it is said that the filter performance parameters are sensitive to the elements used. Sensitivity studies of filter circuit parameters provide the information whether a particular circuit will meet the given specifications under likely tolerances of the elements or not. The studies also help in establishing filter stability and hence the filter's utility in the long term.

  • Preface

  • Muzaffer Ahmad Siddiqi
  • Book:
    Continuous Time Active Analog Filters
    Published online:
    24 December 2019
    Print publication:
    26 March 2020, pp xv-xviii

  • Summary

    Recent advances in solid-state technology and the well-known advantages of integrated circuits (ICs) have made microminiaturized filters and systems an important field of study. For a particular application, the main object of the designer is to select/design a circuit, which is optimized to maximize performance and minimize cost through economic integration. At present, several methods are available to adapt the design of linear networks. Among these, networks (filters) employing resistance, capacitance, and linear active devices, i.e., active RC filters, have become very popular because of their functional versatility, ease of IC implementation and cost reduction. Now, good quality active RC filters are being produced using integrated operational amplifiers (OAs), mainly because of the commercial availability of high quality OAs at low cost.

    It is for this reason that the majority of the, presently, available books on linear active filter design and the curriculum at almost all universities include OA-RC based circuit and systems design either as a full course or part of a course on linear integrated circuit design. However, it is only part of a story, which began with passive electric filter design. Approximation was used to develop filter structures, predominantly in the form of either singly or doubly terminated lossless ladders. Great strides were made in developing ladder structures and related information for different approximations, and related information like their transfer function in terms of ratio of polynomials or in terms of poles and zeros, and required values of elements used in the ladders. Not only are passive filter realizations still used today, but the large literature on it also finds application in active network synthesis. Hence, a discussion on passive filter design and properties of ladder structure finds place in books on network analysis/synthesis.

    Development of filter circuits from passive to active did not stop at the OA-RC case. The simultaneous development of digital filters and systems, and the replacement of analog sub-systems with digital sub-systems, motivated further advancements in continuous-time as well as discretetime analog filters. Employing the design procedures used with OA-RC filter synthesis, high speed sampling circuits such as the switched capacitor (SC) provided good alternatives to digital filters in many applications.

  • 8 - Multi Amplifier Second-order Filter Sections

  • Muzaffer Ahmad Siddiqi
  • Book:
    Continuous Time Active Analog Filters
    Published online:
    24 December 2019
    Print publication:
    26 March 2020, pp 213-243

  • Summary

    Introduction

    In Chapter 7, the basics of realizing second- (or third-) order filter sections using only one active device (OA) were explained. Such circuits are capable of providing any arbitrary secondorder function; they are also economical from the point of view of the use of active devices. However, depending on the filter specifications and the configuration chosen, the resulting circuit may not fulfil all the requirements like small number of passive components used and specific spread, sensitivity and variability. It is for this reason that many second-order filter sections use two, three or more OAs: multi amplifier biquads (MABs). Obviously, the intention is to overcome the mentioned limitations of the single amplifier biquad (SAB). In addition, a significant feature of multi amplifier biquadratic sections is their versatility in terms of providing more than one kind of response (like LP and BP) at the output terminals leading to general biquadratic structures.

    Almost all MABs use two integrators in a loop, a technique known as the state-variable approach. Based on this technique, an important practical circuit known as the KHN (Kerwin- Huelsman−Newcomb) biquad can be assembled. The scheme explained in detail in Section 8.2 realizes three types of output responses. A direct modification of the scheme, known as Tow–Thomas biquad is studied in Section 8.3. The schemes, being interesting and useful, are further studied while employing active compensation to inverting or non-inverting integrators used in the loop. Active compensation leads to another well-known biquad, the Ackerberg– Mossberg filter, which is studied in Section 8.5. Many schemes have been implemented to utilize these structures and obtain other types of responses as explained in Section 8.6. Another scheme for obtaining a multi-response configuration using a generalized impedance convertor (GIC) is explained at the end of the chapter.

    While designing a SAB, it was observed that a frequency-dependent finite gain of the OA results in a deviation in the performance parameters ωo and pole-Q. To compensate for these deviations, biquads using composite amplifiers are also used, in which instead of using only a passive negative feedback, an active feedback network is used. These amplifiers increase the number of OAs used, making it a MAB, though the design itself remains a SAB type.

  • 9 - Direct Form Synthesis: Element Substitution and Operational Simulation

  • Muzaffer Ahmad Siddiqi
  • Book:
    Continuous Time Active Analog Filters
    Published online:
    24 December 2019
    Print publication:
    26 March 2020, pp 244-298

  • Summary

    Introduction

    In the previous chapters, we studied realization of first-order and second-order filter sections. Though these filter sections are used as such, they are also used to generate higher-order filters employing different processes including the cascade process. However, a common alternate process for realizing second- or higher-order filter section is the direct form of synthesis. There are two broad categories in the direct form of synthesis: (i) element substitution method and (ii) operational simulation method. Though the filter realization procedures in the aforementioned categories differ, the starting point is the same. Initially, a passive structure with element values (mostly frequency and impedance normalized) is obtained. It is then converted into its active form. Although they have the same starting point, the construction and characteristics of the active circuit obtained through the direct form and that obtained through the cascade form differ on many counts, as shall be illustrated later.

    The most common passive structure that is used to realize passive filters is the doubly terminated lossless ladder. A typical lossless ladder is shown in Figure 9.1 where Rin and RL are the terminating resistors and the ladder contains only lossless elements, that is, inductors and capacitors; each series and shunt branch of the ladder can be any combination of inductors/capacitors.

    We will first discuss the element substitution type of direct form synthesis procedure, which is mainly the avoidance of the use of inductors. Therefore, simulation of inductors forms an important part of the chapter. Inductance simulation, configurations for inductance simulation and active filter realizations without using an inductor are discussed in Sections 9.2–9.5. Section 9.6 deals with the simulation of a floating inductance, mainly through using two circuit structures of grounded inductances. Another method in which the inductor can be eliminated from the general lossless ladder is through scaling of the structure by the complex frequency variable s. This method generates a new type of element called the frequency dependent negative resistance (FDNR). As simulation of inductors and FDNR requires impedance conversion configurations, it is important to study the basics of these concepts. The technique is included in Section 9.8.

  • 5 - Noise

  • Hooman Darabi
  • Book:
    Radio Frequency Integrated Circuits and Systems
    Published online:
    01 April 2021
    Print publication:
    12 March 2020, pp 278-330

  • Summary

    An unavoidable cause of electrical noise is the random thermal motion of electrons in conducting media such as wires or resistors. Active circuits comprising MOS transistors suffer from similar random motion of electrons leading to noise. As long as communication systems are constructed from such devices, the noise will be with us. Amplifying arbitrarily weak signals to make them detectable is unrealistic, as the presence of noise sets a lower limit to minimum detectable signal.1 This ultimately limits the range where the transmitter and receiver can communicate, yet maintaining a minimum signal-to-noise ratio at the detector output.

  • 12 - Transceiver Architectures

  • Hooman Darabi
  • Book:
    Radio Frequency Integrated Circuits and Systems
    Published online:
    01 April 2021
    Print publication:
    12 March 2020, pp 690-756

  • Summary

    By now we have a general idea of what the receivers and transmitter look like. Nonetheless, the exact arrangement of the blocks, the frequency planning involved, the capabilities of digital signal processing, and other related concerns result in several different choices that are mostly application dependent. While our general goal is to ultimately meet the standard requirements, arriving at the proper architecture is largely determined by the cost and power consumption concerns. The goal of this chapter is to highlight these trade-offs, and present the right architecture for a given application, considering the noise, linearity, and cost trade-offs, which were generally described in Chapters 5 and 6. Moreover, we will see that the proper arrangement of the building blocks is a direct function of the circuit capabilities that we presented in Chapters 7–11.

  • 7 - Low-Noise Amplifiers

  • Hooman Darabi
  • Book:
    Radio Frequency Integrated Circuits and Systems
    Published online:
    01 April 2021
    Print publication:
    12 March 2020, pp 386-436

  • Summary

    In Chapter 5 we showed that according to the Friis equation the lower limit to a receiver noise figure is set by the first block. Moreover, the gain of the first stage helps reduce the noise contribution of the subsequent stages. Hence, it is natural to consider a low-noise amplifier at the very input of a receiver. A first look then will tell us that for a given application, the higher the gain of the amplifier the better. However, the limit to that is typically imposed by the distortion caused by the blocks following the low-noise amplifier. As we stated earlier, there is a compromise between the required gain, noise, and linearity for a given application, and a certain cost budget. These requirements may be different for a different application or standard. In this chapter we assume that these requirements are given to us, and our goal is to understand the design trade-offs. In Chapter 12 we will study receiver architectures and various trade-offs associated with each choice of architecture.

Page 10 of 75