INTRODUCTION TO BIPOLAR JUNCTION TRANSISTORS

Transcription

1 INTRODUCTION TO BIPOLAR JUNCTION TRANSISTORS s are three port devices used in most integrated circuits, such as amplifiers s are active components (recall that resistors, capacitors and inductors are passive) There are two types of transistor: 1. The unipolar or field effect transistor (FET), its operation is due to the flow of majority carriers only (either electrons or holes) 2. The bipolar junction transistor, its operation depends on the flow of majority and minority carriers The bipolar junction transistor is one of the most important and widely used semiconductor devices Its principal applications are as an amplifier or as a switch Generally transistors are highly efficient, robust, incredibly reliable and usually inexpensive 1

2 GENERAL CONSTRUCTION A bipolar junction transistor consists of two pn junctions in close proximity to each other Two arrangements are possible according to whether the middle region is n- or p-type material npn or pnp Leads are attached to the three regions, known as emitter, base, and collector Emitter p n p Collector Base Emitter n p n Collector Base 2

3 THEORY OF OPERATION (1) The majority of current in a pnp transistor is carried by holes which, being positively charged particles, move in the same direction as conventional current flow In an npn transistor, the majority of current is carried by electrons The theory of operation of an npn transistor is in all respects the same as that of a pnp device The theory of operation for pnp transistors will be considered here circuit symbols are shown below The arrow on the emitter indicates the direction of conventional current flow under normal bias conditions npn pnp base base Collector Emitter Collector Emitter 3

4 THEORY OF OPERATION (2) An amplifier is a device that boosts the power of a signal, whilst keeping its waveform the same s are used in amplification circuits In normal use as an amplifier, the pn junction between collector and base is reversed biased The junction between emitter and base is forward biased The operation of the transistor is also dependent on the width of the material used to construct the base region The effect or reducing the base width is to increase the collector current, I C, whilst correspondingly reducing the base current, I B The emitter current, I E, is highly dependent on the base-emitter voltage, V BE The collector current, I C, is more or less independent of the collector-base voltage V CB 4

5 THEORY OF OPERATION (3) A transistor may be thought of as an electronic tap that is able to control a large flow of electrons with only small variations of the handle The handle in the case of a transistor is called the base The in and out pipes are called the emitter and collector Voltage changes at the base of the transistor result in changes to the flow of current through the transistor In practical terms, a small base current, I B, flows and turns on a much larger collector current, I C I C is zero until I B flows A junction transistor is thus a current operated device 5

6 THE TRANSISTOR AS AN AMPLIFIER (1) There are three ways to connect a transistor for amplification purposes in practice In each case one terminal is connected to signal common (ground) though not always directly As a result, the input and output signals are taken between common and the other two terminals The three amplifier configurations are: common emitter, common collector and common base Here we shall only consider the common emitter Typical circuit (npn) and values shown below 6

7 THE TRANSISTOR AS AN AMPLIFIER (2) Collector typically fed from approx. 12V DC supply Fairly high resistor, R C, placed in series (4.7kΩ) A further DC supply is also connected between base and emitter in series with input signal V in DC base bias supply is arranged to make I C about 1mA The collector-emitter voltage, V CE, is then the collector supply voltage less the voltage drop across R C For silicon transistors, the base bias voltage is about 0.6V, with I B about 10µA 7

8 TYPICAL OPERATION OF JUNCTION TRANSISTOR AMPLIFIER Typically the collector current, I C, is 100 to 1000 times greater than the base current I B, depending on the type of transistor used If we take the base current I B as an input and the collector current I C as an output, then we can say that the transistor amplifies current, i.e. it is a current amplifier The current gain is denoted by h FE F indicates forward current and E denotes the connection type which is common emitter h FE = I C / I B For the circuit shown on the previous slide, I C = 1mA and I B = 10µA Therefore h FE = 1mA/10µA = 100 The input current is amplified by a 100 times in this example Also note that since the current flowing out of the transistor must equal the current flowing into it (Kirchhoff s Current Law) then I B + I C = I E Also I C >> I B and I E I C 8

9 CONVENTIONAL AMPLIFIER CIRCUIT SET UP Above shows the most common configuration for a stable operating amplifier V CC is the power supply voltage, i.e. the circuit needs power in order to perform its function (amplification) Such circuits have to be biased in order to give the required value of I C In order to help achieve this, a voltage V BB, is set up using a voltage divider, which has an effect on the size of I C 9

10 IMPORTANT AMPLIFIER CIRCUIT RELATIONSHIPS V BB = V BE + V RE In practice V RE > V BE. For silicon transistors, V RE 2V R 1 and R 2 chosen so that base-emitter junction is forward biased resistors form a potential divider For good stability, R 2 < 10R E Typical values for current gain h FE are around 100 As an approximation V BB = (V CC R 2 )/(R 1 + R 2 ) V RE = I E R E Since I B << I C and I C I E, then I C V RE /R E Since V RE = V BB - V BE Then V BE is approximately 0.7V R C is chosen so that V CE is V CC /2 to ensure that the amplifier output does not exceed the boundary limits of -V CC /2 to V CC /2 V CE = V CC I C R C - I C R E I C 1 VCC R2 = VBE RE R1 + R2 10

11 WORKED AMPLIFIER EXAMPLE Given that h FE = 150, V BE = 0.7V and V CC = 15V, find R 1, R 2, R C and R E (i.e. bias the amplifier circuit) so that I C is 1mA. h FE = I C / I B I B = /150 = A I E I C A For silicon transistors V RE 2V V BB = V BE + V RE = 2.7V V RE = I E R E R E = 2kΩ R 2 < 10R E R 2 = 20kΩ V BB = (V CC R 2 )/(R 1 + R 2 ) R 1 = 91.1kΩ V CE = V CC /2 = 7.5V V CE = V CC I C R C I C R E R C = 9.5kΩ 11

12 APPLYING AN AC INPUT SIGNAL TO THE AMPLIFIER Coupling capacitors C 1 and C 2 are added. C 1 protects the base from being short circuited by the source. C 2 blocks any DC component at the output. The AC input signal, V S, is superimposed on the base bias voltage, so that the new base emitter voltage becomes v be = V BE + V S The output voltage V out = V CC /2 + AV S A is the overall gain of the amplifier 12

13 TRANSISTOR AS A SWITCH So far we have looked at transistors as amplifying devices. However another important application is their use as a switch. Advantages Very high speed of operation present day transistor can operate at up to 10 9 times a second Very high reliability Electronically controlled operation Low cost Small size Disadvantages The switch is not a true open circuit in the OFF condition a small but finite current still flows The switch is not a true short circuit in the ON condition there is a small but finite voltage drop across it of about 0.1V 13

14 PRACTICALITY OF TRANSISTOR SWITCHING OPERATION In many practical cases, the disadvantages are of little significance In many applications the use if transistor switches provides great improvement in operation over alternative methods such as relays and other mechanical switches One of the most important applications is in the control of logic levels in digital circuits Modern digital computers rely almost exclusively for their operation on the use of transistor switches The operation of a transistor as a switch is the basics of switching (digital) circuits In digital circuits, the outputs and inputs involve only two levels of voltage HIGH or LOW i.e. two state circuits HIGH logic level is referred to as logic 1 level, which in voltage terms is 5V or near the supply voltage value LOW is referred to as logic 0 s are cheap, reliable, there are no moving parts with almost indefinite life and can switch millions of times a second A perfect switch would have zero resistance when ON and infinite resistance when OFF with no power consumption 14

15 TRANSISTOR SWITCHING CIRCUIT When transistor is switched on I C = V CC /R C We already know that I C = h FE * I B Substituting: I B = V CC /h FE R C This value of I B is the minimum for satisfactory switching Also, I B = (V BB V BE )/R B However, in practice, V BB >> V BE So I B V BB /R B 15