EC312 Lesson 1: Transistors Objectives: a) Define the term transistor and state two uses. b) Identify two different types of transistors by their symbology and alphanumeric designations. c) Properly bias an NPN transistor in a circuit configuration. d) List the precautions to be taken when working with transistors and describe ways to test them. e) Explain how the transistor can be used in digital logic. f) Explain the purpose and function of various hardware components found in a computer system: CPU, Hard drive, Ram.* g) Review digital data and its binary representation.* h) Convert between binary and decimal notation.* i) Convert between hexadecimal and decimal notation.* j) Convert between hexadecimal and binary notation.* k) Evaluate how characters are stored using ASCII notation.* [*Note: objectives (f)-(k) are review objectives that cover material which students have already learned in SI110 and ES200/202. Assigned questions in the first two homework sets will assist students in reviewing this material.] I. INTRODUCTION TO TRANSISTORS 1 The discovery of the first transistor in 1948 by a team of physicists at the Bell Telephone Laboratories sparked an interest in solid-state research that spread quickly. The transistor, which began as a simple laboratory oddity, was rapidly developed into a semiconductor device of major importance. The transistor demonstrated for the first time in history that amplification in solids was possible. Before the transistor, amplification was achieved only with electron tubes. Transistors now perform numerous electronic tasks with new and improved transistor designs being continually put on the market. In many cases, transistors are more desirable than tubes because they are small, rugged, require no filament power, and operate at low voltages with comparatively high efficiency. The development of a family of transistors has even made possible the miniaturization of electronic circuits. Figure 1 shows a sample of the many different types of transistors you may encounter when working with electronic equipment. 1 These lesson notes were adapted from the Navy Electricity and Electronics Training Series (NEETS) course notes, Module 7, Chapter 2, September 1998. 1
Figure 1: An assortment of different types of transistors. Transistors have infiltrated virtually every area of since and industry, from the family car to satellites. Even the military depends heavily on transistors. The ever increasing uses for transistors have created an urgent need for sound and basic information regarding their operation. Transistors are a semiconductor device, similar to diodes which you have already studied. Semiconductors are a group of materials that have different resistances based on what the material is doped with when it is made. From your study of the PN-junction diode, you should have a basic knowledge that they are created with a junction between a P-type material and a N-type material. II. TRANSISTOR FUNDAMENTALS Semiconductor devices that have three or more elements are called TRANSISTORS. The term transistor was derived from the words TRANSfer and resistor. This term was adopted because it best describes the operation of the transistor the transfer of an input signal current from a low-resistance circuit to a highresistance circuit. Transistors were first used as amplifiers, but their use in digital circuits to create logic gates has driven the computing age. There are many different types of transistors, for this class we will look at NPN and PNP Bipolar Junction Transistors. Figure 2 shows the diagrams for these transistors. These transistors are similar and each has a collector, emitter and base but they can be distinguished by the arrow on the emitter in the diagram. If the arrow points in, (Pointing in) (as shown in Figure 2), the transistor is a PNP. On the other hand, if the arrow points out, the transistor is an NPN (Not Pointing in) (as shown in Figure 2). 2
Figure 2: Transistor representations III. TRANSISTOR BIASING 1. NPN Transistor Operation In this class, we are concerned with transistor s application to computers. When implementing transistors in computers, they are utilized as switches for logic. To use an NPN transistor as a switch, a positive voltage must be applied between the collector and emitter. The on/off control of the circuit is performed by applying or taking away the base to emitter voltage. For a positive base to emitter voltage there is a current from the collector to emitter, and a zero or negative base to emitter voltage results in no collector current. A properly biased NPN transistor is shown in Figure 3 and typically the base-emitter voltage is less than the collectoremitter voltage. Figure 3: Properly biased NPN transistor 3
2. PNP Transistor Operation The PNP transistor works essentially the same as the NPN transistor. However, since the emitter, base, and collector in the PNP transistor are made of materials that are different from those used in the NPN transistor, current flows in the opposite direction. Therefore, the reference voltages applied to a PNP transistor are reversed in polarity to those applied to the NPN transistor as shown in Figure 4. Figure 4: A properly biased PNP transistor. IV. TRANSISTORS USE IN DIGITAL CIRCUITS The main electronic function of a transistor is amplification, however, its logical operation makes them useful for digital circuits, particularly in digital logic. Digital circuits capitalize on the transistor s characteristic of acting like a switch to control the output signal. By configuring multiple transistors, we can produce Logic Gates which are the basic building blocks of digital circuitry. We will use our understanding of the NPN transistor (also called a Bipolar Junction Transistor or BJT) to understand how Logic Gates operate. This analysis can be extended to other transistors used in the industry, such as the Metal-Oxide-Semiconductor Field- Effect Transistor (MOSFET) and the Complementary Metal-Oxide-Semiconductor (CMOS). 1. NPN BJT LOGIC Recalling our knowledge of the operation of an NPN transistor, it can be said that: A positive voltage applied to the base-emitter (V BE ) will turn ON the transistor and allow current to flow from the collector to emitter. A zero voltage applied to the base-emitter (V BE ) will turn OFF the transistor and prevent current from flowing from the collector to emitter. Let s review this on a circuit with a variant. The voltage for the logic High or 1 and Low or 0 is dependent many factors such as the logic family being used (i.e. TTL, CMOS, BTL, GTL). For this course we will use 0V for a logical 0 and 5V for a logical 1. In practice the actual voltage needed for V BE and V CE is much smaller than 5V (Common ranges are 0.2 to 0.6V based on the makeup of the transistor). To accomplish this on a board or chip the logic voltage is chosen (in our case 0V and 5V) and then resistors are used to limit the current and reduce the voltage seen by the transistor. By inserting one or more resistors in a circuit, different methods of biasing may be achieved and the emitterbase battery eliminated. Some of these biasing methods compensate for slight variations in transistor 4
characteristics and changes in transistor conduction resulting from temperature irregularities. With that, let s analyze the logic in the circuit on Figure 5. Figure 5: BJT schematic If a logical high (5V) is applied to V IN, the state of the transistor would be ON, and current would flow from the collector to emitter. Thus the voltage measured at V 0 would be 0V, as depicted in Figure 6. Figure 6: ON State (V IN is High) Likewise, if a logical low (0V) is applied to V IN, the state of the transistor would be OFF and current would not flow from the collector to emitter. Thus the voltage measured at V 0 would be 5V, as depicted in Figure 7. 5
Figure 7: OFF State (V IN is Low) Thus, this circuit behaves logically as an inverter gate. A gate is a collection of components (transistor, resistors, etc.) that do a particular function. If we relabeled the input (V "# ) as A and the output (V $ ) as X we can construct a Truth Table. A Truth Table is a tabular list to illustrate all the possible combinations of digital inputs to a gate and the outputs that will result. A logic operation can be looked at as a mathematical expression. This mathematical expression is a Boolean Expression because the inputs and outputs can only have only one of two different possibilities. Lesson 2 will cover these topics more in depth. The Truth Table, gate symbol and Boolean Expression for the inverter can be seen Figure 8. Figure 8. Truth Table, Symbol and Boolean Expression for an Inverter Gate. 2. Building logic gates from transistors Not only the NOT gate can be built from transistors, but in fact all logic gates are. For example, consider the circuit diagram in below, which contains two transistors. Figure 9: Circuit to analyze Suppose we apply a logical High or 1 (i.e. 5V) to A and a logical Low or 0 (i.e. 0V) to B, illustrated on the left in Figure 10. Then transistor A will be in the on state (which we can regard as a short circuit) and transistor B 6
will be in the off state (which we can regard as an open circuit), and the end result is that the output will read 0V (i.e. Low). Now suppose we apply a logical High (i.e. 5V) at both A and B. Then both transistors will be in the on state (as in the figure on the right in Figure 10), and the output will read 5V, or a logical High (i.e. 1). In fact, this is the only case of which we get a High output; in the next lesson we will learn that this combination is an AND gate. Figure 10: Circuit with A=1 B=0 (Left) and with A=1 B=1 (Right) You don t have to know how to design these circuits, but you should know that all of our logic gates can be built from transistor circuits. And that proved to be pretty significant in the development of much of our technology. As Wikipedia puts it, the transistor revolutionized the field of electronics, and paved the way for smaller and cheaper radios, calculators, and computers, among other things. 7