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1562 results in RF and microwave engineering

Page 11 of 79

  • 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.

  • 4 - RF and IF Filters

  • Hooman Darabi
  • Book:
    Radio Frequency Integrated Circuits and Systems
    Published online:
    01 April 2021
    Print publication:
    12 March 2020, pp 206-277

  • Summary

    Almost every radio includes one or more filters for the general purpose of separating a useful information-bearing signal from unwanted signals such as noise or interferers. We will discuss the role of filters and their requirements in Chapter 6 when we talk about distortion. Here we discuss their properties from a circuit point of view and offer general design guidelines.

  • 1 - RF Components

  • Hooman Darabi
  • Book:
    Radio Frequency Integrated Circuits and Systems
    Published online:
    01 April 2021
    Print publication:
    12 March 2020, pp 1-78

  • Summary

    In this chapter basic components used in RF design are discussed. Detailed modeling and analysis of MOS transistors at high frequency can be found already in many analog books [1], [2]. Although mainly offered for analog and high-speed circuits, the model is good enough for most RF applications operating at several GHz and beyond, especially for nanometer CMOS processes used today. Thus, instead we will have a more detailed look at inductors, capacitors, and LC resonators in this chapter. We will also briefly discuss the fundamental operation of distributed circuits and transmission lines and follow up with more in Chapter 3. In Chapters 5 and 7 we will discuss some of the RF aspects of the transistors, including a more detailed noise analysis as well as substrate and gate resistance. New to this edition are Sections 1.8 and 1.9, which cover the fundamentals of wave propagation and antennas.

  • 10 - PLLs and Synthesizers

  • Hooman Darabi
  • Book:
    Radio Frequency Integrated Circuits and Systems
    Published online:
    01 April 2021
    Print publication:
    12 March 2020, pp 595-650

  • Summary

    In this chapter we present the phase-locked loops and synthesizers, built upon the discussions of the previous chapter, VCOs and crystal oscillators. It is a new chapter in this edition, although some pieces of it existed in the previous edition under Chapter 8. A detailed discussion of PLLs and synthesizers would perhaps require an entire book of its own, and our objective here is only to establish enough background to allow design and analysis of synthesizers for typical radio applications.

  • 6 - Distortion

  • Hooman Darabi
  • Book:
    Radio Frequency Integrated Circuits and Systems
    Published online:
    01 April 2021
    Print publication:
    12 March 2020, pp 331-385

  • Summary

    In Chapter 5 we showed that the thermal noise of a receiver sets a lower limit on the signal detectable at the receiver input. The upper limit to the maximum signal a receiver can handle is set by the distortion, arisen from nonlinearity present in the active circuits making up the receiver. However, handling a desired large input is generally not an issue as it is typically managed by the proper gain control in the receiver. On the other hand, we will show in this chapter that the distortion has a far more detrimental impact on the receiver, when subject to large unwanted signals, known as blockers. Similar to noise, the blockers will also set a lower limit on the detectable signal.

Page 11 of 79