Chapter 3 Bipolar Junction Transistor BJT SJTU Zhou

Chapter 3 Bipolar Junction Transistor (BJT) SJTU Zhou Lingling 1

Outline • • Introduction Operation in the Active Mode Analysis of Transistor Circuits at DC The transistor as an Amplifier Graphical Analysis Biasing the BJT for Discrete-Circuit Design Configuration for Basic Single Stage BJT Amplifier High frequency Model SJTU Zhou Lingling 2

Introduction • • Physical Structure Circuit Symbols for BJTs Modes of Operation Basic Characteristic SJTU Zhou Lingling 3

Physical Structure A simplified structure of the npn transistor. SJTU Zhou Lingling 4

Physical Structure A dual of the npn is called pnp type. This is the simplified structure of the pnp transistor. SJTU Zhou Lingling 5

Circuit Symbols for BJTs The emitter is distinguished by the arrowhead. SJTU Zhou Lingling 6

Modes of Operation Modes EBJ Cutoff Reverse Saturation Forward Active Forward Reverse Amplifier Reverse active Reverse Forward Performance degradation SJTU Zhou Lingling Application Switching application in digital circuits 7

Basic Characteristics • Far more useful than two terminal devices (such as diodes) • The voltage between two terminals can control the current flowing in the third terminal. We can say that the collector current can be controlled by the voltage across EB junction. • Much popular application is to be an amplifier SJTU Zhou Lingling 8

Operation in the Active Mode • Current flow • Current equation • Graphical representation of transistor’s characteristics SJTU Zhou Lingling 9

Current Flow Current flow in an npn transistor biased to operate in the active mode. SJTU Zhou Lingling 10

Collector Current • Collector current is the drift current. • Carriers are successful excess minority carriers. • The magnitude of collector current is almost independent of voltage across CB junction. • This current can be calculated by the gradient of the profile of electron concentration in base region. SJTU Zhou Lingling 11

Base Current • Base current consists of two components. ØDiffusion current ØRecombination current • Recombination current is dominant. • The value of base current is very small. SJTU Zhou Lingling 12

Emitter Current • • Emitter current consists of two components. Both of them are diffusion currents. Heavily doped in emitter region. Diffusion current produced by the majority in emitter region is dominant. SJTU Zhou Lingling 13

Profiles of Minority-Carrier Concentrations SJTU Zhou Lingling 14

Current Equation • Collector current • Base current • Emitter current SJTU Zhou Lingling 15

Explanation for Saturation Current • Saturation current is also called current scale. • Expression for saturation current: • Has strong function with temperature due to intrinsic carrier concentration. • Its value is usually in the range of 10 -12 A to 10 -18 A. SJTU Zhou Lingling 16

Explanation for Common-Emitter Current Gain • Expression for common –emitter current gain: • Its value is highly influenced by two factors. • Its value is in the range 50 to 200 for general transistor. SJTU Zhou Lingling 17

Explanation for Common-Base Current Gain • Expression for common –base current gain: • Its value is less than but very close to unity. • Small changes in α correspond to very large changes in β. SJTU Zhou Lingling 18

Recapitulation • Collector current has the exponential relationship with forward-biased voltage as long as the CB junction remains reversebiased. • To behave as an ideal constant current source. • Emitter current is approximately equal to collector current. SJTU Zhou Lingling 19

Graphical Representation of Transistor’s Characteristics • Characteristic curve relates to a certain configuration. • Input curve is much similar to that of the diode, only output curves are shown here. • Three regions are shown in output curves. • Early Effect is shown in output curve of CE configuration. SJTU Zhou Lingling 20

Output Curves for CB Configuration SJTU Zhou Lingling 21

Output Curves for CB Configuration • Active region § EBJ is forward-biased, CBJ is reverse-biased; § Equal distance between neighbouring output curves; § Almost horizontal, but slightly positive slope. • Saturation region § EBJ and CBJ are not only forward-biased but also turned on; § Collector current is diffusion current not drift current. § Turn on voltage for CBJ is smaller than that of EBJ. • Breakdown region § EBJ forward-biased, CBJ reverse-biased; § Great voltage value give rise to CBJ breakdown; § Collector current increases dramatically. SJTU Zhou Lingling 22

Output Curves for CE Configuration (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. SJTU Zhou Lingling 23

The Early Effect • Curves in active region are more sloped than those in CB configuration. • Early voltage. • Effective base width and base width modulation. SJTU Zhou Lingling 24

The Early Effect(cont’d) • Assuming current scale remains constant, collector current is modified by this term: • Narrow base width, small value of Early voltage, strong effect of base width modulation, strong linear dependence of on. SJTU Zhou Lingling 25

Analysis of Transistor Circuit at DC • Equivalent Circuit Models • Analysis Steps • Examples SJTU Zhou Lingling 26

Equivalent Circuit Models Large-signal equivalent-circuit models of the npn BJT operating in the forward active mode. In practical DC analysis, constant voltage drop model is popular used. SJTU Zhou Lingling 27

DC Analysis Steps a. Using simple constant-voltage drop model, assuming irrespective of the exact value of currents. b. Assuming the device operates at the active region, we can apply the relationship between IB, IC, and IE, to determine the voltage VCE or VCB. c. Check the value of VCE or VCB, if , i. VC>VB (or VCE>0. 2 V), the assumption is correct. ii. VC<VB (or VCE<0. 2 V), the assumption is incorrect. It means the BJT is operating in saturation region. Thus we shall assume VCE=VCE(sat) to obtain IC. Here the common emitter current gain is defined as forced=IC/IB, we will find forced< . SJTU Zhou Lingling 28

Examples • Example 5. 4 shows the order of the analysis steps indicated by the circled numbers. • Example 5. 5 shows the analysis of BJT operating saturation mode. • Example 5. 6 shows the transistor operating in cutoff mode. SJTU Zhou Lingling 29

Examples(cont’d) • Example 5. 7 shows the analysis for pnp type circuit. It indicates the current is affected by illspecified parameter β. As a rule, one should strive to design the circuit such that its performance is as insensitive to the value of β as possible. • Example 5. 8 is the bad design due to the currents critically depending on the value of β. • Example 5. 9 is similar to the example 5. 5 except the transistor is pnp type. SJTU Zhou Lingling 30

Examples(cont’d) • Example 5. 10 shows the application of Thévenin’s theorem in calculating emitter current and so on. This circuit is the good design for the emitter is almost independent of β and temperature. • Example 5. 11 shows the DC analysis for two stage amplifier. • Example 5. 12 shows the analysis of the power amplifier composed of the complimentary transistors. SJTU Zhou Lingling 31

The Transistor as an Amplifier • Conceptual Circuits • Small-signal equivalent circuit models • Application of the small-signal equivalent circuit models • Augmenting the hybrid π model. SJTU Zhou Lingling 32

Conceptual Circuit (a) Conceptual circuit to illustrate the operation of the transistor as an amplifier. (b) The circuit of (a) with the signal source vbe eliminated for dc (bias) analysis. SJTU Zhou Lingling 33

Conceptual Circuit(cont’d) With the dc sources (VBE and VCC) eliminated (short circuited), thus only the signal components are present. Note that this is a representation of the signal operation of the BJT and not an actual amplifier circuit. SJTU Zhou Lingling 34

Small-Signal Circuit Models • Transconductance • Input resistance at base • Input resistance at emitter • Hybrid π and T model SJTU Zhou Lingling 35

Transconductance • Expression • Physical meaning gm is the slope of the i. C–v. BE curve at the bias point Q. • At room temperature, SJTU Zhou Lingling 36

Input Resistance at Base and Emitter • Input resistance at base • Input resistance at emitter • Relationship between these two resistances SJTU Zhou Lingling 37

The Hybrid- Model • The equivalent circuit in (a) represents the BJT as a voltage-controlled current source (a transconductance amplifier), • The equivalent circuit in (b) represents the BJT as a current-controlled current source (a current amplifier). SJTU Zhou Lingling 38

The T Model • These models explicitly show the emitter resistance re rather than the base resistance r featured in the hybrid- model. SJTU Zhou Lingling 39

Augmenting the Hybrid- Model ØExpression for the output resistance. ØOutput resistance represents the Early Effect(or base width modulation) SJTU Zhou Lingling 40

Models for pnp Type • Models derived from npn type transistor apply equally well to pnp transistor with no changes of polarities. Because the small signal can not change the bias conditions, small signal models are independent of polarities. • No matter what the configuration is, model is unique. Which one to be selected is only determined by the simplest analysis. SJTU Zhou Lingling 41

Graphical Analysis a. Graphical construction for the determination of the dc base current in the circuit. b. Load line intersects with the input characteristic curve. SJTU Zhou Lingling 42

Graphical Analysis(cont’d) Graphical construction for determining the dc collector current IC and the collector-to-emitter voltage VCE in the circuit. SJTU Zhou Lingling 43

Small Signal Analysis Graphical determination of the signal components vbe, ib, ic, and vce when a signal component vi is superimposed on the dc voltage VBB SJTU Zhou Lingling 44

Effect of Bias-Point Location on Allowable Signal Swing SJTU a. Load-line A results in bias point QA with a corresponding VCE which is too close to VCC and thus limits the positive swing of v. CE. b. 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. Zhou Lingling 45

Biasing in BJT Amplifier Circuit • Biasing with voltage ØClassical discrete circuit bias arrangement § Single power supply § Two-power-supply ØWith feedback resistor • Biasing with current source SJTU Zhou Lingling 46

Classical Discrete Circuit Bias Arrangement by fixing VBE by fixing IB. SJTU Zhou Lingling 47

Classical Discrete Circuit Bias Arrangement • Both result in wide variations in IC and hence in VCE and therefore are considered to be “bad. ” • Neither scheme is recommended. SJTU Zhou Lingling 48

Classical Biasing for BJTs Using a Single Power Supply ØCircuit with the voltage divider supplying the base replaced with its Thévenin equivalent. ØStabilizing the DC emitter current is obtained by considering the negative feedback action provided by RE SJTU Zhou Lingling 49

Classical Biasing for BJTs Using a Single Power Supply • Two constraints • Rules of thumb SJTU Zhou Lingling 50

Two-Power-Supply Version • Resistor RB can be eliminated in common base configuration. • Resistor RB is needed only if the signal is to be capacitively coupled to the base. • Two constraints should apply. SJTU Zhou Lingling 51

Biasing with Feedback Resistor ØResistor RB provides negative feedback. ØIE is insensitive to β provided that ØThe value of RB determines the allowable signal swing at the collector. SJTU Zhou Lingling 52

Biasing Using Current Source (a) Q 1 and Q 2 are required to be identical and have high β. (b) Short circuit between Q 1’s base and collector terminals. (c) Current source isn’t ideal due to finite output resistor of Q 2 SJTU Zhou Lingling 53

Application of the Small-Signal Models a. Determine the DC operating point of BJT and in particular the DC collector current IC(ICQ). b. Calculate the values of the small-signal model parameters, such as gm=IC/VT, r = /gm=VT/IB, re= /gm=VT/IE. c. Draw ac circuit path. d. Replace the BJT with one of its small-signal models. The model selected may be more convenient than the others in circuits analysis. e. Determine the required quantities. SJTU Zhou Lingling 54

Basic Single-Stage BJT Amplifier • Characteristic parameters • Basic structure • Configuration Ø Common-Emitter amplifier § Emitter directly connects to ground § Emitter connects to ground by resistor RE Ø Common-base amplifier Ø Common-collector amplifier(emitter follower) SJTU Zhou Lingling 55

Characteristic Parameters of Amplifier ØThis is the two-port network of amplifier. ØVoltage signal source. ØOutput signal is obtained from the load resistor. SJTU Zhou Lingling 56

Definitions • Input resistance with no load • Input resistance • Open-circuit voltage gain • Voltage gain SJTU Zhou Lingling 57

Definitions(cont’d) • Short-circuit current gain • Current gain • Short-circuit transconductance SJTU Zhou Lingling 58

Definitions(cont’d) • Open-circuit overall voltage gain • Overall voltage gain SJTU Zhou Lingling 59

Definitions(cont’d) Output resistance of amplifier proper SJTU Output resistance Zhou Lingling 60

Definitions(cont’d) Voltage amplifier Transconductance amplifier SJTU Zhou Lingling 61

Relationships • Voltage divided coefficient SJTU Zhou Lingling 62

Basic Structure Basic structure of the circuit used to realize single-stage, discrete-circuit BJT amplifier configurations. SJTU Zhou Lingling 63

Common-Emitter Amplifier SJTU Zhou Lingling 64

Common-Emitter Amplifier Equivalent circuit obtained by replacing the transistor with its hybrid- model. SJTU Zhou Lingling 65

Characteristics of CE Amplifier • Input resistance • Voltage gain • Overall voltage gain • Output resistance • Short-circuit current gain SJTU Zhou Lingling 66

Summary of CE amplifier • • • Large voltage gain Inverting amplifier Large current gain Input resistance is relatively low. Output resistance is relatively high. Frequency response is rather poor. SJTU Zhou Lingling 67

The Common-Emitter Amplifier with a Resistance in the Emitter SJTU Zhou Lingling 68

The Common-Emitter Amplifier with a Resistance in the Emitter SJTU Zhou Lingling 69

Characteristics of the CE Amplifier with a Resistance in the Emitter • Input resistance • Voltage gain • Overall voltage gain • Output resistance • Short-circuit current gain SJTU Zhou Lingling 70

Summary of CE Amplifier with RE • The input resistance Rin is increased by the factor (1+gm. Re) • The voltage gain from base to collector is reduced by the factor (1+gm. Re). • For the same nonlinear distortion, the input signal vi can be increased by the factor (1+gm. Re). • The overall voltage gain is less dependent on the value of β. SJTU Zhou Lingling 71

Summary of CE Amplifier with RE • The reduction in gain is the price for obtaining the other performance improvements. • Resistor RE introduces the negative feedback into the amplifier. • The high frequency response is significant improved. SJTU Zhou Lingling 72

Common-Base Amplifier SJTU Zhou Lingling 73

Common-Base Amplifier SJTU Zhou Lingling 74

Characteristics of CB Amplifier • Input resistance • Voltage gain • Overall voltage gain • Output resistance • Short-circuit current gain SJTU Zhou Lingling 75

Summary of the CB Amplifier • • Very low input resistance High output resistance Short-circuit current gain is nearly unity High voltage gain Noninverting amplifier Current buffer Excellent high-frequency performance SJTU Zhou Lingling 76

The Common-Collector Amplifier or Emitter-Follower SJTU Zhou Lingling 77

The Common-Collector Amplifier or Emitter-Follower SJTU Zhou Lingling 78

The Common-Collector Amplifier or Emitter-Follower SJTU Zhou Lingling 79

Characteristics of CC Amplifier • Input resistance • Voltage gain • Overall voltage gain • Output resistance • Short-circuit current gain SJTU Zhou Lingling 80

Summary for CC Amplifier or Emitter-Follower • High input resistance • Low output resistance • Voltage gain is smaller than but very close to unity • Large current gain • The last or output stage of cascade amplifier • Frequency response is excellent well SJTU Zhou Lingling 81

Summary and Comparisons • The CE configuration is the best suited for realizing the amplifier gain. • Including RE provides performance improvements at the expense of gain reduction. • The CB configuration only has the typical application in amplifier. Much superior high-frequency response. • The emitter follower can be used as a voltage buffer and exists in output stage of a multistage amplifier. SJTU Zhou Lingling 82

Internal Capacitances of the BJT and High Frequency Model • Internal capacitance ØThe base-charging or diffusion capacitance ØJunction capacitances § The base-emitter junction capacitance § The collector-base junction capacitance • High frequency small signal model • Cutoff frequency and unity-gain frequency SJTU Zhou Lingling 83

The Base-Charging or Diffusion Capacitance • • • Diffusion capacitance almost entirely exists in forward-biased pn junction Expression of the small-signal diffusion capacitance Proportional to the biased current SJTU Zhou Lingling 84

Junction Capacitances • The Base-Emitter Junction Capacitance • The collector-base junction capacitance SJTU Zhou Lingling 85

The High-Frequency Hybrid- Model • Two capacitances Cπ and Cμ , where • One resistance rx. Accurate value is obtained form high frequency measurement. SJTU Zhou Lingling 86

The Cutoff and Unity-Gain Frequency • Circuit for deriving an expression for • According to the definition, output port is short circuit SJTU Zhou Lingling 87

The Cutoff and Unity-Gain Frequency(cont’d) • Expression of the short-circuit current transfer function • Characteristic is similar to the one of firstorder low-pass filter SJTU Zhou Lingling 88

The Cutoff and Unity-Gain Frequency (cont’d) SJTU Zhou Lingling 89