Bipolar Junction Transistors

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2 Bipolar Junction Transistors Invented in 1948 at Bell Telephone laboratories Bipolar junction transistor (BJT) - one of the major three terminal devices Three terminal devices more useful than two terminal ones - can be used in signal amplification, design of digital logic and memory circuits Basic principle: Voltage between two terminals is used to control the current in third terminal

3 Bipolar Junction Transistors Constructed of doped semiconductor material Mainly used in amplifying or switching applications Named so because their operation involves both electrons and holes Two types of BJTs: o npn transistors o pnp transistors Has three regions: emitter region, base region and the collector region

4 Bipolar Junction Transistors Collector Collector C C n p Base p B Base n B n p Emitter E Emitter E npn transistor pnp transistor

5 Bipolar Junction Transistors For each type of transistor: o There are two pn junctions and a transistor is a combination of two diodes connected back to back o There are three terminals o The middle layer is very thin, which is the most important factor in the operation of a transistor Transistor is created basically by connecting two pn junctions back to back Two junctions are known as: emitter-base junction (EBJ) and collector base junction (CBJ)

6 Bipolar Junction Transistors Collector C Collector C n p Base p B Base n B n p Emitter E npn transistor Emitter E pnp transistor

7 Bipolar Junction Transistors Depending on the bias condition (forward or reverse) of each of these junctions, different modes of operation are defined o Active mode o Cut off mode o Saturation mode Active mode is used commonly when the transistor is used as an amplifier Switching applications, such as logic circuits, utilize both cut off and saturation modes

8 Bipolar Junction Transistors Mode EBJ CBJ Amplifier Active Forward Reverse Switch Cut-off Reverse Reverse Saturation Forward Forward

9 Transistor Terminals Base terminal is a thin layer of semiconductor placed between the thicker emitter and collector Emitter is on the side that supplies charge carriers Emitter is always forward biased with respect to base so that it can supply a large number of majority carriers Collector is on the other side that collects the charges Collector is always reverse biased Collector removes charges from its junction with base

10 Transistor Terminals Base emitter junction is forward biased -> allows low resistance in emitter circuit Base collector junction is reverse biased -> provides high resistance in collector circuit

11 Transistor Terminals Emitter n-type Heavy doping Base p-type Light doping Collector n-type Moderate doping

12 Transistor Terminal Currents & Voltages Collector Collector I C I C Base I B Base I B I E I E Emitter npn transistor Emitter pnp transistor

13 Transistor current gain (β) The factor by which current increases from the base of a transistor to its collector Represented using the Greek letter Beta (β) I I C B

14 Beta (β) The ratio of collector current ( I C ) to base current ( I B ) I I C B

15 Beta can be used to calculate any of the transistor terminal currents. I E I B I C I I I C E B I I I B C B ( 1) I E 1

16 Transistor Terminal Currents & Voltages Voltage Abbreviation V CC V BB V EE V C V B V E V CE V BE V CB Definition Collector supply voltage Base supply voltage Emitter supply voltage Voltage between collector and ground Voltage between base and ground Voltage between emitter and ground Voltage between collector and emitter Voltage between base and emitter Voltage between collector and base

17 Operation of npn Transistor I C R C C I C I C βi B n V BB R B V BE I B 0.7 V B E p n +5.0 V V CE V CC 10 V I E R E V BE < V CE < V CC

18 Operation of npn Transistor Active mode is the most important One junction is forward biased while the other is reverse biased Forward biased junction has a low resistance path Reverse biased junction has a high resistance path Weak signal is introduced into low resistance circuit and the output is taken from high resistance circuit

19 Operation of npn Transistor Transistor transfers a signal from low resistance to high resistance Name transistor indicates transfer property and classifies it in same family with resistors Base-emitter junction is forward biased Electrons injected at emitter diffuse across emitter-base junction to base Holes in base diffuse across to emitter, but not as many as electrons

20 Operation of npn Transistor Few electrons in base recombine with holes Because base is too thin, most diffuse across to reverse biased base-collector junction where the electrons are accelerated across the depletion region Collector is very lightly dope and relatively few electrons recombine with base holes Most of the collector electrons make it to the collector terminal

21 Operation of npn Transistor In cut off mode both junctions are reverse biased I C = 0 I C = 0 C n R C Terminal currents are approximately 0 A V BB R B B E p n V CC R E

22 Operation of npn Transistor In saturation mode both junctions are forward biased Collector current reaches its maximum value V BB I C I C VCC 0.2V VCC R R R R R I B B V BE C E C E C I C n p B n 0.8 V E I E R E R C 0.2 V V CE V CE < V BE V CC

23 Common Emitter Configuration

24 Common Emitter Configuration Base current I B is proportional to collector current I c and can be expressed as a fraction of it - common-emitter current gain and in range of The following expression can be derived: I B = I C / Emitter current I E can be expressed as: I E = I C + I B = I C + I C / = I C [( +1)/ ] I C = [ /( +1)].I E = I E Since is usually large, then 1 (typically 0.99) - common-base current gain and is always less than

25 Common Emitter Configuration: Input characteristics I B V K V BE

26 Common Emitter Configuration: Input characteristics Input characteristics - relation between input current and input voltage for different values of output voltage Input voltage V BE is across forward biased base-emitter junction Graph of input current I B vs input voltage V BE resembles forward biased diode

27 Common Emitter Configuration: Output characteristics I C 30 ma 20 ma 10 ma Saturation I B = 200 A I B = 150 A I B = 100 A I B = 50 A I B = 0 A 10 V 20 V 30 V 40 V Breakdown Cutoff V CE

28 Common Emitter Configuration: Output characteristics Output characteristics - relation between output current I C and output voltage V CE for different values of input current I B Region where curves are approximately horizontal is the active region of common-emitter configuration and is generally constant Saturation - V CE drops enough so that the collectorbase junction is forward biased V CE = V CB + V BE (for Si V BE 0.7V) When V CE < 0.2V or 0.3V collector base junction becomes well forward biased and collector current diminishes rapidly

29 Common Collector Configuration

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31 Purpose of Bias V CC V B(ac) I B(ac) R C R B Q 1 V CE(ac) I C(ac)

32 Purpose of Bias For transistor to pass complete signal onto the output side the input signal must be completely positive or negative, in single transistor amplifiers AC waveform is alternating between positive and negative Transistor cannot amplify it as emitter base junction will only be forward biased during positive cycle When biasing the transistor dc values of circuit must be set to constant values, somewhere in the middle of the total range This is to superimpose the ac waveform on it AC voltage will vary above and below bias voltage

33 Purpose of Bias Consider the signal applied is A sin ωt and the bias voltage is V B Output of the amplifier is given by: V O = V B + A sin ωt V O V IN V O V B +A V MAX V B V B -A V MIN t

34 Clipping V B +A < V MAX and V B -A > V MIN must be satisfied If not output will reach minimum or maximum before a complete AC variation has taken place Clipping Output flattens at V MIN or V MAX Clipping distorts signal In transistor amplifiers the minimum and maximum voltages are the saturation and cut off voltages

35 Clipping

36 Over-driving When amplitude A is too larger the amplifier is overdriven Clipping on both sides

37 Coupling Capacitors Coupling or blocking capacitor capacitor connected in series to prevent the flow of DC current from signal source to amplifier Must be large enough to provide negligible impedance to AC signals

38 Typical Amplifier Operation V B(ac) V CC I B(ac) R C R B Q 1 V CE(ac) I C(ac)

39 Bias Circuits Biasing does not mean the individual biasing of PN junctions Assume the emitter-base is forward biased and the collector-base is reverse biased Changing degree of bias to keep the output at the exact value desired Analysis of BJT circuits to which only dc voltages are applied Assume V BE is a constant voltage drop of 0.7V and V CE saturates at 0.2V

40 Bias Circuits Assume transistor is operating in active mode and determine the corresponding voltages and current In practical circuits bias is controlled by connecting external resistors in series with external voltage sources V CC, V EE etc Changing resistor values instead of voltage source values to control the DC input and output voltages and currents Circuit used to set the bias point : bias circuit

41 DC Load Line A line that represents every possible combination of I C and V CE for a given amplifier Load line indicates that there is a unique value of V CE for every value of I C Ends of the load line are labeled I C(sat) and V CE(off)

42 DC Load Line I C I C(sat) V CC R C I C V CC V R C CE V CE(off ) V CC V CE

43 Optimum Q-point When a transistor does not have an input signal, its output rests at specific dc values of l C and V CE These values correspond to a specific point on the dc load line - Q-point or Quiescent point A quiescent amplifier is one that has no input signal applied and therefore, has constant dc values of I c and V CE When the dc load line of an amplifier is superimposed on collector curves for the transistor, the Q-point value can easily be determined

44 Optimum Q-point Q-point is the point where the load line intersects the appropriate collector curve For linear operation of an amplifier, it is desirable to have the Q-point centered on the load line With a centered Q-point, V CE is half the value of V CC, and I c is half the value of I C(sat) A centered Q-point provides values of I c and V CE that are half their maximum possible values When a circuit is designed to have a centered Q- point, the amplifier is said to be midpoint biased

45 Optimum Q-point I C I C βi B I C(sat) I B = 50 A I B = 40 A I B I C(sat) /2 Q-Point I B = 30 A V CC /2 V CC I B = 20 A I B = 10 A I B = 0 A V CE V V I R CE CC C C

46 Base Bias (Fixed Bias) V CC I B V CC V R B BE I C R C I C βi B R B I B Output V V I R CE CC C C Input Q 1 = dc current gain = h FE +0.7 V I E V BE

47 Base Bias Characteristics Circuit recognition: A single resistor (R B ) between the base terminal and V CC. No emitter resistor. Advantage: Circuit simplicity Disadvantage: Q-point shift with temperature

48 V CC Load line equations: I C R B I B R C Output I V C(sat) CE(off ) V CC R C V Q-point equations: CC Input V BE +0.7 V Q 1 I E I B V CC C FE B V R I h I B BE V V I R CE CC C C 48

49 Fixed-bias circuit

50 Determine the following for the fixed-bias configuration shown in below. (a) I BQ and I CQ. (b) V CEQ. (c) VB and VC. (d) VBC.

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52 Emitter-stabilized bias circuit

53 Load-Line Analysis The load-line analysis of the emitter-bias network is only slightly different from that encountered for the fixed-bias configuration.

54 For the emitter bias network shown in below, determine: (a) IB. (b) IC. (c) VCE. (d) VC. (e) VE. (f) VB. (g) VBC.

55 Voltage-divider bias

56 Load Line for Voltage Divider Bias Circuit I C (ma) I C(sat) VCC 10V R R 260Ω+240Ω C E 20mA V CE(off ) V CC 10V V CE (V)

57 Determine the levels of ICQ and VCEQ

58 Voltage Divider Bias Characteristics Circuit recognition: The voltage divider in the base circuit Advantages: The circuit Q-point values are stable against changes in β Disadvantages: Requires more components than most other biasing circuits Applications: Used primarily to bias linear amplifiers

59 DC bias with voltage feedback

60 Determine the quiescent levels of ICQ and VCEQ

61 Other Transistor Biasing Circuits Emitter-bias circuits : consists of a dual polarity power supply and a grounded base resistor Feedback-bias circuits : circuit connection that feeds a portion of output voltage or current back to the input to control the circuit s operating characteristics o Collector-feedback bias : a bias circuit constructed so that collector voltage V C has a direct effect on base voltage V B o Emitter-feedback bias : a bias circuit constructed so that emitter voltage V E has a direct effect on base voltage V B 61