Chapter 4 Bipolar Junction Transistors (BJTs)

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1 Chapter 4 Bipolar Junction Transistors (BJTs) Introduction

2 Physical Structure and Modes of Operation A simplified structure of the npn transistor.

3 Physical Structure and Modes of Operation A simplified structure of the pnp transistor.

4 Physical Structure and Modes of Operation Mode EBJ CBJ Active Forward Reverse Cutoff Reverse Reverse Saturation Forward Forward

5 Operation of The npn Transistor Active Mode Current flow in an npn transistor biased to operate in the active mode, (Reverse current components due to drift of thermally generated minority carriers are not shown.)

6 Operation of The npn Transistor Active Mode Profiles of minority-carrier concentrations in the base and in the emitter of an npn transistor operating in the active mode; v BE 0 and v CB 0.

7 Operation of The npn Transistor Active Mode The Collector Current v BE i C I S e V T The Base Current v BE i B i C I S e V T Physical Structure and Modes of Operation i E i C i B 1 i C 1 v BE V T I S e i C I E 1

8 Equivalent Circuit Models Large-signal equivalent-circuit models of the npn BJT operating in the active mode.

9 The Constant n The Collector-Base Reverse Current The Structure of Actual Transistors

10 The pnp Transistor Current flow in an pnp transistor biased to operate in the active mode.

11 The pnp Transistor Two large-signal models for the pnp transistor operating in the active mode.

12 Circuit Symbols and Conventions C C B B E E

13 Circuit Symbols and Conventions

14 Example 4.1 VCC 15 IC VBE 0.7 Design circuit such that VC 5 IC VEE 15 VT B C E VCC VC RC RC IC2 Since VBE=0.7V at IC=1mA, the value of VBE at IC=2mA is VBE 0.7 VT ln 2 VBE VE VBE VE IC2 IE IE i C I S e v BE V T VE ( VEE) RE RE IE IC2 IB IB

15 Example 4.1

16 Example 4.1 IC2 IB IB

17 Summary of the BJT I-V Relationships in the Active Mode v BE v BE v BE i C i VT C I S e i B I S i e VT C i E I S e VT Note : for pnp transitor, replace vbe for veb i C i C i E i B 1 i E i B i E 1i B i E 1 i E 1 VT 25mV

18 Exercise 4.8

19 Exercise 4.9

20 The Graphical Representation of the Transistor Characteristics

21 The Graphical Representation of the Transistor Characteristics Temperature Effect (10 to 120 C)

22 Dependence of ic on the Collector Voltage The i C -v CB characteristics for an npn transistor in the active mode.

23 Dependence of ic on the Collector Voltage

24 Dependence of ic on the Collector Voltage Early Effect VA 50 to 100V (a) Conceptual circuit for measuring the i C -v CE characteristics of the BJT. (b) The i C -v CE characteristics of a practical BJT. I C v BE VT I S e 1 v CE V A

25 Dependence of ic on the Collector Voltage Early Effect

26 Nested DC Sweeps

27 Example

28 Example

29 Example

30 Monte Carlo Analysis Using PSpice

31 Monte Carlo Analysis Using PSpice

32 Monte Carlo Analysis Using PSpice

33 Monte Carlo Analysis Using PSpice Probe Output Ic(Q), Ib(Q), Vce

34 The Transistor As An Amplifier (a) Conceptual circuit to illustrate the operation of the transistor of an amplifier. (b) The circuit of (a) with the signal source v be eliminated for dc (bias) analysis. The Collector Current and The Transconductance The Base Current and the Input Resistance at the Base The Emitter Current and the Input Resistance at the Emitter

35 The Transistor As An Amplifier Linear operation of the transistor under the small-signal condition: A small signal v be with a triangular waveform is superimpose din the dc voltage V BE. It gives rise to a collector signal current i c, also of triangular waveform, superimposed on the dc current I C. I c = g m v be, where g m is the slope of the i c - v BE curve at the bias point Q.

Small-Signal Equivalent Circuit Models Two slightly different versions of the simplified hybrid- model for the small-signal operation of the BJT.

36 Small-Signal Equivalent Circuit Models Two slightly different versions of the simplified hybrid- model for the small-signal operation of the BJT. The equivalent circuit in (a) represents the BJT as a voltage-controlled current source ( a transconductance amplifier) and that in (b) represents the BJT as a current-controlled current source (a current amplifier).

The circuit in (a) is a voltage-controlled current source representation and that in (b) is a

37 Small-Signal Equivalent Circuit Models Two slightly different versions of what is known as the T model of the BJT. The circuit in (a) is a voltage-controlled current source representation and that in (b) is a current-controlled current source representation. These models explicitly show the emitter resistance r e rather than the base resistance r featured in the hybrid- model.

38 Signal waveforms in the circuit of Fig

39 Fig Example 4.11: (a) circuit; (b) dc analysis; (c) small-signal model; (d) small-signal analysis performed directly on the circuit.

40 Fig Circuit whose operation is to be analyzed graphically.

41 Fig Graphical construction for the determination of the dc base current in the circuit of Fig

42 Fig Graphical construction for determining the dc collector current I C and the collector-to-emmiter voltage V CE in the circuit of Fig

43 Fig Graphical determination of the signal components v be, i b, i c, and v ce when a signal component v i is superimposed on the dc voltage V BB (see Fig. 4.34).

44 Fig Effect of bias-point location on allowable signal swing: Load-line A results in bias point Q A with a corresponding V CE which is too close to V CC and thus limits the positive swing of v CE. At the other extreme, load-line B results in an operating point too close to the saturation region, thus limiting the negative swing of v CE.

Fig. 4.44 The common-emitter amplifier with a resistance R e in the emitter. (a) Circuit.

45 Fig The common-emitter amplifier with a resistance R e in the emitter. (a) Circuit. (b) Equivalent circuit with the BJT replaced with its T model (c) The circuit in (b) with r o eliminated.

46 Fig The common-base amplifier. (a) Circuit. (b) Equivalent circuit obtained by replacing the BJT with its T model.

Fig. 4.46 The common-collector or emitter-follower amplifier. (a) Circuit.

47 Fig The common-collector or emitter-follower amplifier. (a) Circuit. (b) Equivalent circuit obtained by replacing the BJT with its T model. (c) The circuit in (b) redrawn to show that r o is in parallel with R L. (d) Circuit for determining R o.

48 A General Large-Signal Model For The BJT: The Ebers-Moll Model i DE I SE e v BE V T 1 i DC I SC e v BC V T 1 ISC > ISE (2-50) An npn resistor and its Ebers-Moll (EM) model. ISC and ISE are the scale or saturation currents of diodes D E (EBJ) and D C (CBJ). More General Describe Transistor in any mode of operation. Base for the Spice model. Low frequency only

49 A General Large-Signal Model For The BJT: The Ebers-Moll Model I DE I SE e v BE V T 1 I DC I SE e v BC V T 1 F forwarded of the transistor source (close to 1) R reverse of the transistor source (

50 A General Large-Signal Model For The BJT: The Ebers-Moll Model Terminal Currents F I SE R I SC I S i E i DE R i DC i C i DC R i DE i B 1 F i DE 1 R i DC i E v BE I S V T e 1 I S e F v BC V T 1 F F 1 F i C I S F e v BE V T 1 I S R e v BC V T 1 R R 1 R i B I S F e v BE V T 1 I S R e v BC V T 1

51 A General Large-Signal Model For The BJT: The Ebers-Moll Model Forward Active Mode i E v BE I S V T e I S 1 F 1 F Since vbc is negative and its magnitude Is usually much greater than VT the Previous equations can be approximated as v BE i C V T 1 I S e I S 1 R v BE i B I S V T 1 1 e I S F F R

52 A General Large-Signal Model For The BJT: The Ebers-Moll Model Normal Saturation Collector current will be forced IB forced F In saturation both junctions are forwarded biased. T hus VBE and VB are positive and their values greater than VT. Making these approximations and substituting i B I B and i C forced IB results in two equations that can be solved to obtain VBE and VBC. The saturatuion voltage can be obtained as the difference between th VCEsat V T ln 1 1 forced 1 R forced F

53 A General Large-Signal Model For The BJT: The Ebers-Moll Model Reverse Mode IB I1 I2 Note that the currents indicated have positive values. Thus, since ic = -I2 and ie = -I1, both ic and IE will be negative. Since the roles of the emitter and collector are interchanged, the transistor in the circuit will operate in the active mode (called the reverse active mode) when the emitter-base junction is reverse-biased. In such a case I1 = beta_r. IB This circuit will saturate (reverse saturation mode) when the emitter-base junction becomes forward-biased. I1/IB < beta_r

54 A General Large-Signal Model For The BJT: The Ebers-Moll Model Reverse Saturation We can use the EM equations to find the expression of VECSat VECsat V T ln 1 1 F 1 I1 IB I1 IB 1 R 1 F From this expression, it can be seen that the minimum VECSat is obtained when I1 = 0. This minimum is very close to zero. The disadvantage of the reverse saturation mode is a relatively long turnoff time.

55 A General Large-Signal Model For The BJT: The Ebers-Moll Model Example For the circuit below, let RB 1000 VI 5 VCC 5 VB C 0.6 R 0.1 F 50 Calculate approximate values ofe VE for th e following cases: RC = 1K, 10K, 100K From VBC = 0.6 VB 0.6 IB VI VB RB IB a) for RC = 1 K, assume that the transitor is in the reverse active mode. thusrc 1000 I1 R IB I VE VCC I1RC VE 4.56

56 A General Large-Signal Model For The BJT: The Ebers-Moll Model Example b) For RC = 10K, assume reverse acti ve mode RC I1 R IB I VE VCC I1RC VE 0.6 Since VE = VB, the BJT is still in the reverse active mode. b) For RC = 100K, assume reverse saturation mode RC Since VECsat is liekly to be very sma ll, we can assume VE = 0, and I1 VCC 0 RC I a better estimate for VE is to use the equation below (4.115) V T 25 VECsat V T ln 1 1 F 1 I1 IB I1 IB 1 R 1 F VEC sat 3.5 mv Since I1 R IB the BJT is saurated

57 A General Large-Signal Model For The BJT: The Ebers-Moll Model Transport Model npn BJT The transport model of the npn BJT. This model is exactly equivalent to the Ebers-Moll model. Note that the saturation currents of the diodes are given in parentheses and i T is defined by Eq. (4.117).

58 Basic BJT Digital Logic Inverter. Basic BJT digital logic inverter. vi high (close to power supply) - vo low vi low vo high

59 Basic BJT Digital Logic Inverter. Sketch of the voltage transfer characteristic of the inverter circuit of Fig for the case R B = 10 k, R C = 1 k, = 50, and V CC = 5V. For the calculation of the coordinates of X and Y refer to the text.

60 The Voltage Transfer Characteristics (a) The minority-carrier concentration in the base of a saturated transistor is represented by line (c). (b) The minority-carrier charge stored in the base can de divided into two components: That in blue produces the gradient that gives rise to the diffusion current across the base, and that in gray results in driving the transistor deeper into saturation.

61 Complete Static Characteristics, Internal Impedances, and Second-Order Effects Common Base Avalanche Saturation Slope The i c -v cb or common-base characteristics of an npn transistor. Note that in the active region there is a slight dependence of i C on the value of v CB. The result is a finite output resistance that decreases as the current level in the device is increased.

62 Complete Static Characteristics, Internal Impedances, and Second-Order Effects Common Base The hybrid- model, including the resistance r, which models the effect of v c on i b.

63 Complete Static Characteristics, Internal Impedances, and Second-Order Effects Common-Emitter Common-emitter characteristics. Note that the horizontal scale is expanded around the origin to show the saturation region in some detail.

64 Complete Static Characteristics, Internal Impedances, and Second-Order Effects Common-Emitter An expanded view of the common-emitter characteristics in the saturation region.

65 The Transistor Beta

66 Transistor Breakdown

67 Internal Capacitances of a BJT C de I C F V T Base charging or Diffusion capacitance C je C je0 V BE 1 V 0e m Base Emitter Junction capacitance m grading coefficient C 1 C 0 V CB V 0c m Collector Base Juntion Capacitance C C de C je r x

68 The Cut-Off Frequency

69 The Spice BJT Model and Simulation Examples

70 The Spice BJT Model and Simulation Examples

71 The Spice BJT Model and Simulation Examples

72 The Spice BJT Model and Simulation Examples.model Q2N2222-X NPN( Is=14.34f Xti=3 Eg=1.11 Vaf=74.03 Bf=200 Ne=1.307 Ise=14.34f Ikf=.2847 Xtb=1.5 Br=6.092 Nc=2 Isc=0 Ikr=0 Rc=1 Cjc=7.306p Mjc=.3416 Vjc=.75 Fc=.5 Cje=22.01p Mje=.377 Vje=.75 Tr=46.91n Tf=411.1p Itf=.6 Vtf=1.7 Xtf=3 Rb=10) *National pid=19 case=to bam creation

73 The Spice BJT Model and Simulation Examples

74 BJT Modeling - Idealized Cross Section of NPN BJT

77 + Sunday, March 08, 1998 The Spice BJT Model and Simulation Examples RX_I N C18 120pF R K/1% BANDSPREAD MAIN TUNE L4 2.0uH R28 C21 R24 5KPOT 100KPOT R K/1% 180pF C19 C pF L10 1mH R31 6.8pF L5 2.0uH L4, L5 26t AWG32 ON AMIDON T37-6 L? 100uH C? 0.01uF PTT KEY 1.00M/1% C uF TX_ON RX_ON 12VREG C23 8VREG 180pF D5 42pF C uF T1 BIFILAR XFMR 2 x 10t AWG32 ON AMIDON FT37-61 E3 E4 J2 C? 0.01uF C7 0.01uF RF PREAMP C20 D5: 18-36pF (6-1.5V) MHz C50 56pF LSB O/S USB O/S CENTER = ZERO O/S D10 C pF S2 R4 3.2K R pF C uF C pF R10 1K VF0 / BFO C pF R41 3.2K C uF L18 C uF L11 5.6uH T1 BIFXFMR Q4 2N2222A R C4 0.1uF T3 TRI XFMR 100uH R15 75 C pF D8 1N4148 R K/1% C uF 13 VDC (BATT) RX MIXER C uF L3 1mH C pF RX_ON J3 R K/1% 12VREG F2 D1 1N4148 D2 1N4148 Q8 2N2222A L15 100uH C uF C49 L6 0.01uF R K/1% C uF R5 1K C16 100uH R37 C5 10uFNP T3 TRIFILAR XFMR 3 x 10t AWG32 ON AMIDON FT ASB R R /2W 10K DET_AUD uF RX_BFO Q13 2N2222A 2N2222A Q14 D9 C uF C uF C uF 8VREG R40 33K R43 10K 1N4002 R26 47 R32 1K Q10 2N2222A L17 C uF L1 C uF Q9 2N2222A 100uH C53 12VREG 0.022uF R CONTROL CKT C uF 82mH C11 0.1uF R42 15K C uF TX_ON LP L2 L12 1mH D6 1N4148 D7 1N4148 L16 1mH C uF 82mH C uF E1 E2 S5 RCVR FILTER TO LO-Z MIC TX_VFO 12VREG C uF C36 10uFNP BP F-LP = 2.5KHz / F-BP = 800Hz C39 0.1uF RX_ON TX_ON C uF 12VREG DSB C uF RX GAIN C40 L uF T6: PRI: 36t AWG 32 SEC: 4t AWG 32 ON AMIDON T50-6 TX_ON R8 1KPOT 47mH C uF C2 100uF C17 10uFNP + 12VREG C15 10uF 2.75 KHz LOW PASS FILTER C uF L13 100uH C57 0.1uF Q11 2N2222A R R55 R46 2.2K C uF 5uH L9 R :1 R mH C43 0.1uF C55 0.1uF T6 C63 82pF 0.01uF C69 C3 0.1uF R14 10K 12VREG R6 R2 10K 10K Q3 2N2222A C pF R11 27K C44 + C uF 10uF C27 100uF T5 TX_ON R1 1K R3 10K Q2 2N2222A RX AUDIO AMP 600/3K T5 PRI: 360t AWG40 SEC: 800t AWG40 ON AMIDON PC POT CORE RF DRIVERS C uF R51 TBD AS REQD TO ADJ GAIN L14 100uH C70 R12 C25 10uF + 51K C37 10uFNP C58 0.1uF C33 10uF R47 1K R K C28 0.1uF T2 2K/SPKR Q1 2N2222A R34 10K R9 100 DRV_COLL Q12 2N2222A R53 39 T2 PRI: 650t AWG40 SEC: 50t AWG32 ON AMIDON PC POT CORE RX_ON R22 10K 12 OHM R18 10K Q7 2N2222A R29 27K + C29 10uF TX_ON J1 PHJACK R17 1K R20 10K Q5 2N2222A Q6 2N2222A TX AUDIO AMP + R30 51K C46 10uF 1mH L19 HEADPHONES (LO-Z) BAL MODULATOR TX VFO T4 TRIFILAR XFMR 3 x 12t AWG32 ON AMIDON FT37-61 R19 2K R T4 TRI XFMR 10uF C30 DSB R36 1K C uF R S1 0dB CW R33 1K C uF C38 D3 1N4148 CARRIER BALANCE R16 100POT 0. 01uF D4 1N4148 L7 1mH TX_ON RX_I N RCVR ATTEN S3 20dB DSB 16 VDC UNREG F3 1ASB J4 C74 220uF PWR ON/OFF S7 + C75 0.1uF R /2W C91 0.1uF Q18 2N2222A 1N4002 Q16 2N2222A Q22 2N2222A D19 6.2V/1W R65 R67 Q19 2N2222A 12V REGULATOR 10-1/2W 10-1/2W 10-1/2W R59 R64 R66 I-LIM = 0.42A C82 0.1uF R72 357/1% R77 475/1% + C76 47uF 12VREG 1N5822 D11 1K R68 C89 0.1uF D12 1N4002 Q17 2N2222A Q20 2N2222A D16 6.2V/1W 8V REGULATOR R /1% R73 475/1% C83 0.1uF + C77 47uF TX_ON 8VREG DRV_COLL T7: PRI: 36t AWG 32 SEC: 2 x 9t AWG 32 ON AMIDON T50-2 R D18 8.2V/1W R58 20 C72 0.1uF C86 82pF C90 R76 5uH 2KPOT T7 3:1:1 C pF 0.1uF BIAS (SET FOR Ic=1.5mA QUIESCENT) D17 1N4148 2K R62 20 C uF R70 20 R74 2K C uF C78 0.1uF Q15 2N2222A Q21 2N2222A 0.01uF C87 (THERMAL COUPLING) PUSH-PULL POWER AMP 1.5W PEP 0.1uF T8 BIFCHOKE C88 0.1uF L22 C uF + C93 47uF T9 22uH C71 120pF C94 0.1uF 15K 1.0uH L20 C79 470pF 12VREG R56 D14 1.0uH L21 C pF 1N4148 T9: LOW-PASS PRI: 2 x 8t AWG 26 RF FILTER SEC: 7t AWG 26 ON AMIDON T68-6 T8: BIFILAR CHOKE 2 x 8t AWG26 ON AMIDON FT50-61 Tit le C81 470pF J5 R57 15K BNC ANTENNA 50 OHMS N5FC 2N2222 DSB/ CW TRANSCEIVER DESIGNED BY Size Document Number Rev C {Doc} -- M. NORTHRUP N5FC Date: Sheet of 1 2 R60 36 R63 20 R61 36

Electronic Circuits EE359A

Electronic Circuits EE359A Bruce McNair B206 bmcnair@stevens.edu 201-216-5549 Lecture 4 0 Bipolar Junction Transistors (BJT) Small Signal Analysis Graphical Analysis / Biasing Amplifier, Switch and Logic