9.1 INTRODUCTION

In the preceding chapter, we intensively discussed the concept of negative feedback and its effects on amplifiers in detail. In the present chapter, the effect of feedback will be discussed in the context of working and analysis of oscillator circuits to generate sinusoidal and non-sinusoidal pulses and waveforms. The basic concept of oscillator differs from that of an amplifier. An amplifier amplifies a given AC input signal by using the DC power supply VCC and resistive networks to provide proper bias and stability with the aid of negative/degenerative feedback so as to stabilize gain and enhance bandwidth of operation. As completely opposed to this, an oscillator circuit needs no input AC signal. The power supply DC input VCC helps in picking up noise signals from its wide frequency spectrum, which due to regenerative/positive feedback networks and amplifier, gets sustained in the oscillatory circuit. Hence DC input gets converted into AC output. Thus, an oscillator can also be referred to as AC-DC converter. The working of an oscillator is exactly opposite to rectifier; hence it can be called an ‘inverter’. But it should be well understood that an oscillator does not create energy, instead it converts unidirectional current drawn from DC source into alternating current of desired frequency. Hence, the oscillator can be broadly defined as an amplifier with positive feedback. The difference between amplifier and oscillator is explained in the block diagram of Figure 9.1(a).

9.2 CLASSIFICATION OF OSCILLATORS

Oscillators can be broadly classified using following two aspects,

• a) Classification based on design principle

• b) Classification based on frequency of oscillation

a) Figure 9.2 shows the detailed family tree of various types of oscillators based on the design principle.

From Figure 9.2, based on the design principle, commonly used oscillators can be broadly classified as harmonic and relaxation oscillators. Harmonic oscillators generate sinusoidal waveforms, whereas relaxation oscillators generate pulse/rectangular/sawtooth waveforms. The basic tuning circuit for harmonic oscillators can be made of combination of L-C networks, which are suitable for generating RF (radio frequency) oscillations from 720 KHz up to hundreds of MHz, whereas R-C networks are suitable for generating AF (Audio Frequency) oscillations in the range of 20 Hz–20 KHz. The detailed description of each of the oscillator is performed in subsection 9.5 and 9.6.

Almost every undergraduate engineering discipline starting from electronics, electrical, computer science, instrumentation engineering, information technology, mechanical engineering, biotechnology etc. requires a solid foundation in basic electronics which comprises a study of fundamental devices like diodes, transistors and their applications in designing amplifiers, switching/logic circuits etc. While for some disciplines, a basic understanding might be sufficient, for other disciplines like electronics, electrical etc., this course acts as an important building block in the curriculum. Moreover, for various advanced courses like analog electronics, digital circuits and VLSI, in-depth understanding of basic electronics is a prerequisite. This book is written keeping both these aspects in mind and as a result it will serve undergraduate students of various engineering disciplines. In addition, it can also cater to the needs of college and university students belonging to disciplines like physics, electronics, and also in electronics courses in polytechnics. A competent electronics engineer, even in the modern era of integrated-circuit based design, needs a thorough and solid understanding of the basic concepts of semiconductors and a firm grasp of fundamental devices like diodes and various kind of transistors. This book attempts a perfect balance between housing all the basic concepts in accordance with the syllabi of leading institutes and universities and a thorough understanding required for future courses. In solid state electronics and analog electronics, which normally appears at a later stage of the undergraduate electronics curriculum, the approach is significantly different: while solid state electronics provides more emphasis on device physics and detailed understanding, analog electronics concentrates more on the applications, where the device is generally treated as a black box. Even though these two advanced courses are mutually dependent on each other, there exists a significant gap between the two approaches and undergraduate students face real difficulties in making a correlation and correspondence between them. This book attempts to minimize this gap between the different approaches to solid state devices and analog circuits. Special care has been taken to explain all the semiconductor devices in this book from both, physical and circuit approaches. To facilitate this, extra stress is given in chapters 2 and 3 while discussing crystal structures and the p-n junctions.

11.1 INTRODUCTION TO THYRISTOR

Thyristors are able to switch large levels of power and are used in a wide variety of different applications. Thyristors find use in low-power electronics, where they are used in many circuits ranging from light dimmers to power supply over voltage protection. The idea of thyristor was first described by William Shockley in 1950. It was referred to as a bipolar transistor with a p-n hook collector. The mechanism for the operation was analysed further in 1952 by Jewell James Ebers and in 1956 John L. Moll investigated the switching mechanism of the thyristor.

The first silicon-controlled rectifiers became available in the early 1960s when it started to gain a significant level of popularity for power switching. It is a multilayered semiconductor device which requires a gate signal to turn it ‘ON’. Once it is turned ‘ON’, it behaves like a rectifying diode. Thus, the name silicon-controlled rectifier is justified by its working principle. In fact, the circuit symbol for the thyristor suggests that this device acts like a controlled rectifying diode.

Four layers are formed by alternating n-type and p-type semiconductor materials. Consequently, there are three p-n junctions formed in the device. It is a bistable device. The three terminals of this device are anode (A), cathode (K) and gate (G). The gate (G) terminal is the control terminal of the device. The circuit symbol is shown in the Figure 11.1.The current flowing through the device is controlled by electrical signal applied to the gate (G) terminal. The anode (A) and cathode (K) are the power terminals of the device which can handle large applied voltage and conduct the major current through thyristor. For example, when the device is connected in series with load circuit, the load current will flow through the device from anode (A) to cathode (K) but this load current will be controlled by the gate (G) signal applied to the device externally.

11.2 CONSTRUCTION: REALIZATION USING TRANSISTORS

Unlike the normal p-n junction diode, which is a two-layer semiconductor device, or the transistor, a three layer (p-n-p or n-p-n) device, the Thyristor is a four-layer (p-n-p-n) semiconductor device that contains three p-n junctions in series, as shown in Figure 11.2.

A professor of mine once opined that the best working experimentalists tended to have a good grasp of basic electronics. Experimental data often come in the form of electronic signals, and one needs to understand how to acquire and manipulate such signals properly. Indeed, in graduate school, everyone had a story about a budding scientist who got very excited about some new result, only to later discover that the result was just an artifact of the electronics they were using (or misusing!). In addition, most research labs these days have at least a few homemade circuits, often because the desired electronic function is either not available commercially or is prohibitively expensive. Other anecdotes could be added, but these suffice to illustrate the utility of understanding basic electronics for the working scientist.

On the other hand, the sheer volume of information on electronics makes learning the subject a daunting task. Electronics is a multi-hundred billion dollar a year industry, and new products of ever-increasing specialization are developed regularly. Some introductory electronics texts are longer than introductory physics texts, and the print catalog for one national electronic parts distributor exceeds two thousand pages (with tiny fonts!).

Finally, the undergraduate curriculum for most science and engineering majors (excepting, of course, electrical engineering) does not have much space for the study of electronics. For many science students, formal study of electronics is limited to the coverage of voltage, current, and passive components (resistors, capacitors, and inductors) in introductory physics.

Introduction

In analog electronics, voltage is a continuous variable. This is useful because most physical quantities we encounter are continuous: sound levels, light intensity, temperature, pressure, etc. Digital electronics, in contrast, is characterized by only two distinguishable voltages. These two states are called by various names: on/off, true/false, high/low, and 1/0. In practice, these two states are defined by the circuit voltage being above or below a certain value. For example, in TTL logic circuits, a high state corresponds to a voltage above 2.0 V, while a low state is defined as a voltage below 0.8 V.

The virtue of this system is illustrated in Fig. 8.1. We plot the voltage level versus time for some electronic signal. If this was part of an analog circuit, we would say that the voltage was averaging about 3 V, but that it had, roughly, a 20% noise level, rather large for most applications and thus unacceptable. For a TTL digital circuit, however, this signal is always above 2.0 V and is thus always in the high state. There is no uncertainty about the digital state of this voltage, so the digital signal has zero noise. This is the primary advantage of digital electronics: it is relatively immune to the noise that is ubiquitous in electronic circuits. Of course, if the fluctuations in Fig. 8.1 became so large that the voltage dipped below 2.0 V, then even a digital circuit would have problems.

Introduction

The silicon controlled rectifier introduced in the last chapter was the first device we have seen that offered some measure of electronic control: the gate current determined the details of the I–V characteristic. This control, however, was fairly limited. In the examples we considered, the gate current determined the time at which the SCR switched to its on-state. Once the SCR was turned on, however, its behavior was no longer related to the magnitude of the gate current, and removing the gate current altogether would not return the SCR to its off-state.

We now turn to a device with a greater measure of electronic control: the transistor. Like the SCR, the transistor allows the user to control a large current through the device with a smaller control signal. But with the transistor, one can have proportional control; that is, the amount of current through the device is proportional to the control signal. This allows one to amplify signals, which is fundamental to all types of electronic communication.

Transistors come in two basic types: bipolar junction transistors (BJTs) and field-effect transistors (FETs). This chapter will cover the fundamentals of BJTs and also introduce some common terminology for transistor amplifiers. FETs are addressed in Chapter 5.

Bipolar transistor fundamentals

A bipolar transistor can be thought of as a sandwich of n-type and p-type semiconductors.

The band theory of solids

Introduction

A fundamental result from basic modern physics is that atoms are characterized by discrete energy levels. Each of these energy levels can accept up to two electrons. When “building” an atom, we start from the lowest level, fill in two electrons, and then move up to the next energy level and fill it with electrons. This continues until we have placed all the atom's electrons in energy levels. We also know that if an atom absorbs energy from the outside (for example, by absorbing a photon), an electron can be promoted to a higher energy level. Conversely, an electron that falls from a higher to a lower energy level emits a photon.

What happens to this energy level model when we assemble many atoms together into a solid? As the atoms get closer together, we must start to talk about the energy levels of the solid as a whole rather than those of the individual atoms. Rather than doing quantum mechanics for an isolated potential (the atom), we do it for a periodic array of atoms that exhibits a periodic potential. The net result of this is that, during the assembly of N atoms, the individual atomic levels split into N levels. This is shown schematically in Fig. 3.1. Thus when the solid is assembled and the atoms are at their final equilibrium spacing, the solid is characterized by a series of energy bands consisting of a large number of closely spaced allowed energy levels.

Chapter 1

1. 1. 1.27 Ω

2. 3a. 100 W

3. 5. I2 = 0.397 A

4. 7. 667 Ω

5. 9. 12.5 Ω

6. 11. I3 = 1.20A

7. 13. Vth = 0.025 V, with positive terminal on the right

8. 15. V2 = 15 V

9. 17. I1 = −1.096 A, I3 = 0.896 A, I5 = 0.086 A

Chapter 2

1. 1. 1.33 μF

2. 3. 8.3 μF

3. 6. magnitude 11.3 kΩ, phase −27.95 degrees

4. 8. 707 rad/s

5. 10. RC = 5.5 × 10−5 s

6. 12. ∣I∣ = 3.36 Arms, phase −9.89 degrees

7. 13. Turns ratio is 1/10, primary current is 0.06 Arms, secondary current is 0.6 Arms

8. 15. Output voltage will be 24 Vrms

9. 16. The output is a 90 Hz sine wave with peak amplitude 0.946V0