Introduction

In the early days, an operational amplifier (op-amp) was the only linear integrated circuit (IC) that was used in the design of linear IC circuits and systems. Typical applications of the op-amps were mathematical operations, such as summation, subtraction, integration, small signal amplification, and generating oscillations. Over the years, other devices, such as operational transconductance amplifiers, current conveyors, and so on, have also come into common use; still, it has not reduced the importance and areas of application of op-amps. Rather, it became possible to realize many more advanced functions with linear ICs and many applications coming under the domain of nonlinear applications with advances in the process technology and increased level of integration. Some of the more common nonlinear applications are precision rectifiers, voltage-level detectors, and Schmitt trigger circuits. The Schmitt trigger circuit itself is very popular in generating varieties of pulses and other waveforms like triangular waveforms. Some other nonlinear applications such as log and antilog amplifiers, analog multiplier, charge amplifier, and isolation amplifiers are discussed in brief; phase lock loop and its basic function are also included.

Precision Rectifiers

Conventional rectifiers work well for converting alternating supply to a pulsating one. Filters are normally used to remove ripples in the pulsating voltage to obtain dc. It is observed that these rectifiers have some limitations. One of the main limitations is that when a diode conducts during rectification, it has a voltage drop across its terminals, which is approximately 0.7 V. Hence, the ac voltage available for conversion to dc is reduced by that amount.

Operational Amplifiers: Introduction

An operational amplifier (op-amp) is a very prominent active device used in analog integrated circuit (IC) design. Prominence is due to the widespread and diverse areas of applications of the op-amps as its parameters are very close to ideal in a certain range of operating frequencies. Apart from basic arithmetic operations such as addition, multiplication, and integration, op-amps are also widely employed as amplifiers, wave shaping circuits, active filters, log/anti-logarithmic amplifiers, nonlinear function generators, and in analog-to digital and digital-to-analog conversion, and so on.

Figure 8.1(a) shows a pin connection diagram of the most commonly used type-741 op-amp; it needs a dual power supply, has two terminals for inverting and non-inverting inputs, one terminal for the output, and three terminals without any connections for simple applications. Dual op-amps and quad op-amp ICs with matching characteristics are also available.

Op-amp is essentially a high-gain differential amplifier (DA) that can be shown in its simplest form as represented in Figure 8.1(b). The output voltage of the op-amp is the difference between the two input voltages multiplied by the high-gain factor A, so the output voltage is expressed as:

The differential gain A is frequency dependent in a practical op-amp. Therefore, as a first approximation, it is represented by a single-pole roll-off model given below.

Semiconductor Diodes

A semiconductor diode is a two-terminal device. Ideally, the diode behaves as a short circuit in one direction for current flow; it is called the forward direction, and the same diode behaves as an open circuit for the current flow in the opposite direction, which is called the reverse direction.

Ideal Silicon p–n Junction Diode

One of the most important characteristics of an ideal diode is that it behaves as an ideal switch, and the switching action is controlled by the direction of the current that flows in one direction only. Figure 1.1(a) shows a symbol for an ideal diode as a switch, where the arrow is in the forward direction. It also shows the positive direction of the current and voltage drop across the diode. Figure 1.1(b) shows the v, i characteristics of the ideal diode, which conducts only in one direction. The names assigned to the diode terminals are anode and cathode. The direction of the forward current in the diode is from the anode to the cathode inside the diode.

Fabricating an ideal diode in practice is not possible. Still, its idealized model in Figure 1.1(b) serves as a good approximation of a practical diode for basic analysis purposes.

At an early stage, diodes were realized using vacuum tubes with filament inside. However, now solid-state diodes are fabricated using semiconductors.

Introduction

Figure 10.1 shows a simple block diagram of a typical analog signal processing system. The first step in the development of an analog signal processing system is to divide the given specifications into analog and digital parts. Based on the advantages in VLSI technology, a variety of signal processing systems have been developed and will continue to be developed with the format of Figure 10.1. One such example is the advent of sampled data technique and MOS technology, which enabled the fabrication of a general signal processor. However, analog-sampled techniques moved onward from MOS technology toward CMOS technology, as it was highly suitable for combining analog and digital systems.

In most of the practical cases, input signals are of analog types like speech signals, sensor output, radar signals, and so on. The first block in Figure 10.1 is a pre-processing block, which usually consists of analog filters, sample and hold process, and an analog-to-digital converter (ADC). Depending on the nature of the input signal, the input block may require signal processing with strict speed and accurate specifications. After the conversion of analog signals to digital signals, the next block is mostly a microprocessor. The advantage of using a microprocessor is that its function can easily be controlled and modified. Post-processing is done in the final stage, wherein the signal is converted back to the analog form in most cases, and this stage uses digital-to-analog converter (DAC) and some filtering. Proper interfacing is required between the three building blocks of Figure 10.1; placement of the the inter facing is indicated symbolically by the arrow heads.

Frequency Response

Some amplifier circuits employing either BJT or FET were analyzed in Chapter 2. Almost all the circuits contained blocking and bypass capacitors in addition to the biasing components, load resistance, and so on. However, these external capacitors and the internal parasitic capacitors of the transistor were considered absent during the analysis because either dc analysis was done or for the simplification in the analysis. There was no mention of the practical limit on the device parameters or on the components used. It does not mean that the obtained expressions for the voltage gain, current gain, and input and output resistance were wrong. These expressions are very important and relevant, and for most of the operations of amplifiers, these expressions are to be employed. However, there is something more, which is also very important, and that remains to be studied.

When the internal device capacitances or the intentionally connected capacitance or load impedance having reactive components are considered, the amplifier gain becomes a complex number, A–θ, instead of a real number. A significant point is that both the gain A and phase angle –θ depend on the input signal frequency, and the gain magnitude decreases at low and at comparatively high signal frequencies; amplification remains almost constant in the mid-frequency range. The frequency response characteristics of an amplifier are the plot of gain and phase with frequency.

Sinusoidal Oscillations

The feedback amplifiers having negative feedback are likely to become unstable because of some unavoidable phase lag at the output due to the frequency-dependent nature of the primary amplifier gain or feedback network, or due to both. The feedback becomes positive if the additional phase lag becomes 180o at a certain frequency. If the loop gain becomes unity at any frequency, the amplifier starts oscillating at this frequency. This instability and consequent oscillations are undesirable in circuits used as amplifiers. However, practically, such oscillations are generated intentionally for widespread applications also. Hence, many schemes and approaches are in use to obtain oscillations for different frequency ranges. Such circuits are known as oscillators, or signal generators.

To understand this instability and the development of oscillations, intended or otherwise, consider the basic block diagram of a negative feedback amplifier shown in Figure 6.1. Here, vin is the input signal, the net input to the amplifier is ve, the voltage gain of the amplifier is (Av) , βff is the feedback factor, and vout is the output voltage.

The voltage gain of the feedback amplifier is (vout/vin ) = Av/(1 + βf Av). When the voltage gain Av and feedback factor βf are negative (or positive) and real constants, the feedback becomes negative. However, because of the frequency-dependent nature of the primary (and/or in feedback circuit) amplifier, the loop gain has an additional phase shift of 180o at some critical frequency ωo.

Classification of Large Signal Amplifiers

The amplifiers studied so far are small signal amplifiers, where the magnitude of the input signal is small, and the main aim is to amplify either voltage or current with minimum distortion. However, in many applications like control, communication, and power conversion, a large amount of power, sometimes exceeding tens of kW, is to be handled by transistors and other semiconductor devices. In that case, the employed amplifiers are called power amplifiers or large signal amplifiers, where output signals, voltage and current, are large in magnitude.

Based on the type of circuit configuration like CE, CB, and CC, and the location of the quiescent point on the output characteristics, power amplifiers are classified as class A, class B, class AB, class C, and D, E, and F. Each class has its advantages and limitations, which will be discussed along with their circuits and operation. Class D is used very little, and classes E and F are rarely used, so only A, B, and C types of amplifiers will form part of this study, and their classification criterion is mentioned next.

Class A Amplifier: In class A operation, an amplifier is so biased that its operating point is almost in the middle of the output characteristics. The magnitude of the input signal is such that the amplifier operates over its full linear region of the characteristics, but without any clipping of the input signal. So, the output is the amplified replica of the input signal with minimal distortion. However, class A operation works with poor power conversion efficiency; the theoretical maximum power conversion efficiency from DC input to AC output is from 25–50%.

Concept of Negative Feedback

In a feedback system, a signal that is proportional to the output is fed back to the input. It may happen unintentionally or be done intentionally. When the feedback signal adds to the input signal, it is called positive feedback, and when the input signal gets subtracted from the feedback signal, it becomes negative feedback.

Positive feedback is mostly used for the realization of oscillators, whereas negative feedback is used to stabilize the gain of amplifiers against a variation in transistor parameters, supply voltage, and temperature etc.The study in this chapter is limited to negative feedback only, which is primarily used to improve any one of the four types of amplifiers given in the next section, such that the amplifiers become as close to ideal as possible. However, certain conditions are required that help achieve the objective. For example, a primary amplifier is needed to have a very high gain in the forward direction, minimum reverse transmission, which normally happens as a property of the transistors used. Appropriate negative feedback connection and minimum effect of loading due to the feedback network on the main amplifier circuit are also very important.

The above mentioned term appropriate negative feedback needs a bit of explanation. In the voltage and current amplifiers, variables at the input and output are the same, hence there is no problem as such while feeding a part of the output to input.

Introduction

Classification, characteristics, and basic design methods of certain types of networks that perform filtering action on the basis of the frequency of signals are briefly discussed in this chapter. The filters, which used only passive elements, and known as passive filters, were the only kind of filters in earlier days. Passive filters are still in use in many specific cases but have been replaced by active filters (using at least one active device) in a majority of applications. One essential reason for the changeover from the passive filters to the active filters was the inability of the realization of practically feasible inductors in integrated circuit (IC) form over a large frequency range of operation. Hence, structures that replaced (simulated) inductors employing resistance, capacitance, and op-amp were synonymous with the active filters, and these were called active RC filters. The usage of op-amps is still dominant, but other active devices are also used in a big way.

Another important approach to analog filter realization has emerged in the form of switched capacitor (SC) circuits. An important feature of the SC circuits is that it uses only capacitors, op-amps, and electronic switches. Consequently, performance parameters of the circuit depend on capacitor ratios and switching frequency. It is to be noted that very small value capacitances can be used, resulting in consuming less chip area, and better practical results as capacitors in ratio form can be fabricated with much less tolerance.

Appendix A

Analog electronic circuits are generally offered as a core subject during the third or fourth semester of the second year in a four-year course in the electronics and communication, instrumentation and control, and computer engineering branches. It is an important subject and may be slightly toned down in the electrical, civil, chemical, and information technology branches of engineering. Design of discrete and linear integrated circuits (ICs), digital electronic modules, and electronic instrumentation are some of the obvious areas where knowledge of microelectronic circuits becomes essential. Therefore, it becomes important for students at this level of study to be proficient in electronic circuit analysis and their usage in relevant areas.

There are many fine books on electronic (devices and) circuits. Many of them have combined “devices” and “circuits”; a good practice, but sometimes resulting in the book becoming bulky. The idea here is to provide a text that deals with the fundamentals of analog electronic circuits for those who already have a basic knowledge of electronic devices, like semiconductor diodes, bipolar junction transistors (BJTs), and metal oxide semiconductor transistors (MOSFETs), either as full subject or as an introductory subject. Serious effort has been made in preparing the text so that it is not only as study material for examinations but also emphasizes fundamental concepts without being overly voluminous.

Appendix B

Transistor Biasing

When transistors are used as switches, they operate either in cut off or in saturation mode. Whereas, when transistors are used to amplify small signals, a quiescent operating point is selected somewhere in the middle of the conduction range. The region of the location of a quiescent point depends on the kind of amplifier. For example, an amplifier may be used for maximum voltage and/or current gain, or high input resistance, or power gain. In some applications, an amplifier ought to consume minimum power, especially when it is used with a battery-operated device. After selecting the quiescent operating point, it is also required that it remains stable. If there is some change in the operating temperature or variation in supply voltage, the operating point may change its location. Variations due to the manufacturing tolerance in component values and in transistor parameters also affect the quiescent point. Irrespective of the reason, it is required that the quiescent point should remain located within specified limits.

Three amplifier configurations are commonly used while employing either BJT or FET amplification. The configuration depends on the terminals, out of the three, that is common to the input and the output of the amplifier. These configurations are studied on the basis of their characteristics, such as voltage gain, current gain, input and output resistance, and bandwidth, i.e., the frequency range within which the amplifier operates without any significant reduction in the output waveform. The operating frequency range becomes limited as the voltage gain drops at low and high operating frequencies. Hence, the study of frequency response becomes important.

Introduction

A current mirror is a transistor-based circuit that the current level is controlled in an adjacent transistor, and the adjacent transistor essentially acts as a current source. Such circuits are now considered a commonly used building block in a number of analog integrated circuits (IC). Operational amplifiers, operational transconductance amplifiers, and biasing networks are examples of such circuits that essentially use current mirrors. Analog IC implementation techniques such as current-mode and switched-current circuits use current mirrors as basic circuit elements.

A significant advantage associated with the current mirrors is that they act as a near-ideal current source while fabricated using transistors and can replace large-value passive resistances in analog circuits, saving large chip area.

The later part of the chapter discusses another important analog circuit, namely, differential amplifier. As the name suggests, differential amplifiers amplify the difference between two signals that are applied to their two inputs. In addition to the differential amplification, it is also required that differential amplifiers suppress unwanted signal, which is present on the two input signals in the form of a common-mode signal. A differential amplifier is a particularly very useful and essential part of operational amplifiers. A differential pair is the basic building block of a differential amplifier that comprises of two transistors in a special form of connection.