CHAPTER 3 THE BIPOLAR JUNCTION TRANSISTOR (BJT)

Transcription

1 HAPT 3 TH IPOLA JUNTION TANSISTO (JT) 1 In this chapter, we will: JT Discuss the physical structure and operation of the bipolar junction transistor. Understand the dc analysis of bipolar transistor circuits. xamine three basic applications of bipolar transistor circuits. Investigate various dc biasing schemes of bipolar transistor circuits, including integrated circuit biasing. Look at biasing stability 2 1

2 JT asic device in electrical system Used as an amplifier or a switch Has 3 terminals ase, ollector, mitter, ipolar since conduction current is due to both the majority and minority carriers 3 JT 2 types npn and pnp n p n npn - structure and symbol p n p pnp - structure and symbol 4 2

3 JT n p n ollector () ase () mitter () p n n Symbol - arrow points from p n - ase is very thin with very small amount of majority carrier -mitter is wider with the most number of majority carrier. -ollector is wider than emitter but with little majority carrier. Structure - mitter : emitter of charge - ollector : collects charge - ase : a gate that controls flow of current 5 JT - Arrow marks emitter p n p Structure ollector () ase () mitter () n Symbol p p - points from p n -ase layer is thin with little majority carrier. -mitter is thicker with the most majority carrier - ollector is thickest with little majority carrier. 6 3

4 JT DIOD ANALOGY Pnp transistor Npn transistor 7 JT ross Section of Integrated ircuit npn Transistor 8 4

5 JT 4 Possible Modes of Operation Forward-Active - amplifier - junction is forward biased - junction is reverse biased Saturation switch (on) - and - junctions are forward biased ut-off switch (off) - and - junctions are reverse biased Inverse-Active (or everse-active) (off) - junction is reverse biased - junction is forward biased 9 JT Transistor currents in JT in Forward- Active (npn ) Forward biased everse biased emitter base collector 10 5

6 - junction forward biased JT - lectrons (majority) from emitter injected into base through junction. - ecome minority in base - junction reversed biased - ase very thin, get swept across - junction to become collector current - ery little electrons left in base to become base current 11 JT urrent flow in npn JT 12 6

7 JT urrents in a Transistor mitter current is the sum of the collector and base currents: I I + I The collector current is comprised of two currents: I I + I majority Ominority 13 ommon ase JT Transistor configuration The base is the common terminal between input and output ommon mitter The emitter is the common terminal between input and output ommon ollector The collector is the common terminal between input and output 14 7

8 JT ommon-ase onfiguration The base is common to both input (emitter base) and output (collector base) of the transistor. 15 JT Input haracteristics This curve shows the relationship between of input current (I ) to input voltage ( ) for various levels of output voltage ( ). 16 8

9 This graph demonstrates the relationship between the output current (I ) to an output voltage ( ) for various levels of input current (I ). JT Output haracteristics 17 JT Operating egions Active Operating range of the amplifier. utoff The amplifier is basically off. There is voltage, but little current. Saturation The amplifier is full on. There is current, but little voltage. Approximations mitter and collector currents: I I ase-emitter voltage:

10 Ideally: α 1 JT Alpha (α)( Alpha (α) relates the D currents I and I : α dc I I In reality: α is between 0.9 and Alpha (α) in the A mode: α ac 19 JT ommon mitter onfiguration The emitter is common to both input (base-emitter) and output (collectoremitter). The input is on the base and the output is on the collector

11 JT - ase/input haracteristics 21 JT ommon-mitter Output haracteristics ollector haracteristics 22 11

12 JT ommon-mitter Amplifier urrents Ideal urrents I I + I I α I Actual urrents I α I + I O where I O minority collector current This is usually so small that it can be ignored, except in high power transistors and in high temperature environments. When I 0 µa the transistor is in cutoff, but there is some minority current flowing called I O. IO I O I 0µA 1 α 23 JT eta (β)( β represents the amplification factor of a transistor. (β is sometimes referred to as h fe, a term used in transistor modeling calculations) In D mode: β dc I I In A mode: β ac cons tan t 24 12

13 JT eta (β)( Determining β from a graph β A (3.2mA 2.2mA) (30µA 20µA) 1mA 10µA ma 25 µ A β D Note: β A β D 25 JT eta (β)( elationship between amplification factors β and α α β β + 1 β α 1 α elationship etween urrents I βi I + 1)I I αi (β 26 13

14 JT ommon ollector ollector onfiguration The input is on the base and the output is on the emitter. 27 JT ommon ollector ollector onfiguration The characteristics are similar to those of the common-emitter configuration, except the vertical axis is I. I (ma) 28 14

15 JT Limitations of Operation for ach onfiguration is at maximum and I is at minimum (I max I O ) in the cutoff region. I is at maximum and is at minimum ( max sat O ) in the saturation region. The transistor operates in the active region between saturation and cutoff. 29 JT Power Dissipation ommon-base: P max I ommon-emitter: P max I ommon-collector: P max I 30 15

16 JT Transistor Specification Sheet 31 JT Transistor Specification Sheet 32 16

17 JT Transistor Testing urve Tracer Provides a graph of the characteristic curves. DMM Some DMMs measure b D or h fe. Ohmmeter Simulate 33 JT iasing iasing refers to the D voltages applied to a transistor in order to turn it on so that it can amplify the A signal

18 JT Operating Point The D input establishes an operating or quiescent point called the Q-point. 35 JT D iasing ircuits Fixed-bias circuit mitter-stabilized bias circuit ollector-emitter loop oltage divider bias circuit D bias with voltage feedback 36 18

19 JT Fixed ias ircuit SIMULAT 37 JT ase-mitter Loop From Kirchhoff s voltage law: + I 0 Solving for the base current: I 38 19

20 JT ollector-mitter Loop The collector current is given by: I βi From Kirchhoff s voltage law: I 39 JT Transistor Saturation Level When the transistor is operating in the saturation region it is conducting at maximum current flow through the transistor. I sat

21 JT Load Line Analysis The end points of the load line are: I sat I / 0 cutoff I 0 ma The Q-point is the particular operating point: where the value of sets the value of I where I and the load line intersect that sets the values of and I 41 ircuit alues Affect the Q-PointQ more 42 21

22 JT mitter-stabilized ias ircuit Adding a resistor ( ) to the emitter circuit stabilizes the bias circuit. 43 JT ase-mitter Loop From Kirchhoff s voltage law : + - I - - I 0 Since I (β + 1)I : - I - ( β + 1)I 0 Solving for I : I - + ( β + 1) 44 22

23 From Kirchhoff s voltage law : JT ollector-mitter Loop + I + + I 0 Since I I : (very true if β is HIGH) I( + ) Also: I + - I I + 45 JT Improved iased Stability Adding to the emitter improves the stability of a transistor. Stability refers to a bias circuit in which the currents and voltages will remain fairly constant for a wide range of temperatures and transistor eta (β) values

24 JT Saturation Level The endpoints can be determined from the load line. cutoff : I sat : I 0 ma 0 I + 47 JT oltage Divider ias This is a very stable bias circuit. The currents and voltages are almost independent of variations in β

25 JT Approximate Analysis Where I << I 1 and I 2 and I 1 I 2 : Where β > 10 2 : I ; oltage divider From Kirchhoff s voltage law: - I - I I I -I( + ) 49 JT oltage Divider ias Analysis Transistor Saturation Level I sat I max + Load Line Analysis utoff: I 0mA I Saturation:

26 JT D ias with oltage Feedback Another way to improve the stability of a bias circuit is to add a feedback path from collector to base. In this bias circuit the Q-point is only slightly dependent on the transistor beta, β. 51 From Kirchhoff s voltage law: JT ase-mitter Loop I I I 0 Where I << I : I I + I I Knowing I βi and I I, the loop equation becomes: βi I βi 0 Solving for I : I + β( + ) 52 26

27 JT ollector-mitter Loop Applying Kirchoff s voltage law: I + + I 0 Since I I and I βi : I ( + ) + 0 Solving for : I ( + ) 53 JT ase-mitter ias Analysis Transistor Saturation Level Isat Imax + Load Line Analysis I utoff 0mA I Saturation

28 JT PNP Transistors The analysis for pnp transistor biasing circuits is the same as that for npn transistor circuits. The only difference is that the currents are flowing in the opposite direction. 55 JT System stability depends on sensitivity to change in parameter values. I is sensitive to: ias Stability β changes with temperature (T increases, β increases) - decreases at a rate of 7.5m/ o with increase in temperature. I O (reverse saturated current) - doubles with every 10 o increase in temperature. All these can cause Q point to drift from original position 56 28

29 JT ias Stability Stability factors - S(I O ), S( ), S(β) Definition: S S(I S' S( S" S O ( β) ) ) β O A stable network has a LOW stability factor S or S(I O ) / O I is given by Due to heat JT ias Stability I βi + (β+1)i O (1) I O -> I O + O ; I -> I + ; I -> I + quation (1) becomes I + β(i + ) + (β+1)(i O + O ) (2) Subtract (1) from (2) β( ) + (β+1)( O ) earrange; O 1 β (β + 1) so that (β + 1) S

30 When applied to a fixed bias circuit, When temp increases, I increases by, but and remains the same, i.e. 0 and 0 So, I 0 0 so JT ias Stability 0 β + 1 and S β This is the worse case scenario for any stability factor due to reverse saturated current, I O. 59 JT When applied to a D ias with oltage Feedback I( + ) + I + ( I + I )( + ) + I + I ( + ) + I ( + + ) + With change in temperature, ias Stability I I + ; I I + ; 0; 0 So, 0 Therefore, S S(I ( + ) + ( + + ) + 0 ( + + ) ( + ) or + ( + + ) O β + 1 ) β + + If there is no β + 1 S β( ) S is smaller than 1+β ircuit is more stable 60 30

31 When applied to a voltage divider bias I + + I or I I I I 0 ( ( ) + ; I I + (I + I ) + ) I With change in temperature Stability factor is 1 + β S S(I O ) 1 + β + can be rewritten as S S(I O >> + ; JT ias Stability 0 ; (1 + β) 1 + ) (1 + β) + so can be ignored. So S ct has TH ST STAILITY 61 JT ias Stability 2. S or S( ) / A. When applied to a Fixed iased ircuit I ;So, I βi β Due to heat -> + ; I -> I + ; 0 So, β( ) [β( )]/( ) Therefore the stability factor S' β 62 31

32 JT. When applied to a D ias with oltage Feedback I + (β + 1)( + ) β( ) I βi + (β + 1)( + ) With change in temperature, + ; I I + ; 0; ias Stability So. S' β(0 ) + (β + 1)( + ) Therefore the stability factor : β + (β + 1)( + ) If there is no -β S' + ( β + 1) S is smaller than -β/ ircuit is more stable 63 I + (β + 1) JT β( ) and I βi + (β + 1) With change in temperature, So. + ; I I + ; 0; β(0 ) + (β + 1) S' β + (β + 1) ias Stability. When applied to a voltage divider bias, can be rewritten as β S' S() + ( β + 1) For a well designed circuit, -β S' ; and β >> 1 ; So, (1 + β) ( β + 1) >> S' - 1 S depends on A large gives good stability

33 3. S or S(β) JT ias Stability A. When applied to a Fixed iased ircuit S / β I 1 / β 1 where: I 1 the value of I at temperature T 1 β 1 the value of β at temperature T 1 65 JT ias Stability. When applied to a D ias with oltage Feedback ( 0) S S(β) β I1( + ) β ( + (1 + β 1 2 ) where: I 1 the value of I at temperature T 1 β 1 the value of I at temperature T 1 β 2 the value of β at temperature T

34 S' ' JT. When applied to a voltage divider bias, where: I + 1(1 ) I S( β) β β + β I 1 the value of I at temperature T 1 β 1 the value of β at temperature T 1 β 2 the value of β at temperature T To find the stability factor due to al three factors, add up the changes due to all 3 factors I (total) S O + S' + S' ' β 67 XAMPL For the circuit shown: At 25 o ; I 2mA, β 100; I O 0.1nA; 0.65 At 100 o ; I O 20n,; 0.48 alculate I at 100 o JT 240kΩ 68 34

35 A. hange due to I O O 20n 0.1 n 19.9 na For a fixed biased circuit; S β JT And by definition, S x O 101 x 19.9nA 2µA A. hange due to For a fixed biased circuit; S -β/ -100/240k x 10-3 And by definition, S x x10-3 x µ Therefore, total change (tot) A + 2µ µ 72.9µA alue of I at 100 O I 2m µ 2.07 ma 69 JT Amplifier oncept 70 35

36 Objectives Understand the concept of amplifiers Understand and analyze commonemitter, common-base, and commoncollector amplifiers 71 Introduction One of the primary uses of a transistor is to amplify ac signals. This could be an audio signal or perhaps some high frequency radio signal. It has to be able to do this without distorting the original input

37 Amplifier Operation ecall from the previous chapter that the purpose of dc biasing was to establish the Q-point for operation. The collector curves and load lines help us to relate the Q-point and its proximity to cutoff and saturation. The Q-point is best established where the signal variations do not cause the transistor to go into saturation or cutoff. What we are most interested in is the ac signal itself. Since the dc part of the overall signal is filtered out in most cases, we can view a transistor circuit in terms of just its ac component. 73 Amplifier Operation For the analysis of transistor circuits from both dc and ac perspectives, the for D values, letters and subscripts are upper case. Instantaneous values use both italicized lower case letters and subscripts

38 Amplifier Operation The boundary between cutoff and saturation is called the linear region. A transistor which operates in the linear region is called a linear amplifier. 75 Transistor quivalent ircuits The two graphs best illustrate the difference between β D and β ac. The two only differ slightly

39 The ommon-mitter Amplifier The common-emitter amplifier exhibits high voltage and current gain. The output signal is 180º out of phase with the input. 77 The ommon-mitter Amplifier The emitter bypass capacitor helps increase the gain by allowing the ac signal to pass more easily. The X (bypass) should be about ten times less than

40 The ommon-mitter Amplifier The bypass capacitor makes the gain unstable since transistor amplifier becomes more dependent on I. This effect can be swamped or somewhat alleviated by adding another emitter resistor( 1 ). 79 The ommon-ollector Amplifier The common-collector amplifier is usually referred to as the emitter follower because there is no phase inversion or voltage gain. The output is taken from the emitter. The common-collector amplifier s main advantages are its high current gain and high input resistance

41 The ommon-ase Amplifier The common-base amplifier has high voltage gain with a current gain no higher than 1. It has a low input resistance making it ideal for low impedance input sources. The ac signal is applied to the emitter and the output is taken from the collector. 81 Summary Most transistors amplifiers are designed to operate in the linear region. Transistor circuits can be view in terms of its ac equivalent for better understanding. The common-emitter amplifier has high voltage and current gain. The common-collector has a high current gain and voltage gain of 1. It has a high input impedance and low output impedance

42 Summary The common-base has a high voltage gain and a current gain of 1. It has a low input impedance and high output impedance Multistage amplifiers are amplifier circuits cascaded to increased gain. We can express gain in decibels (d). 83 JT ND OF HAPT