BIPOLAR TRANSISTORS Celso Jos Faria de Arajo Dr

BIPOLAR TRANSISTORS Celso José Faria de Araújo, Dr.

THE BIPOLAR JUNCTION TRANSISTORS - BJT Objectives: ü Understand the basic principles of BJT operation ü Interpret the transport model ü Identify operating regions of the BJT and use simplified models ü Interpret the graphical representation of BJT characteristics ü Analyze and design bias circuits ü Analyze single-stage amplifiers ü Use BJT small-signal model to analyze amplifiers ü Understand the transfer characteristic of a BJT logic inverter ü Analyze and determine experimentally the characteristics of some typical BJT circuits 1 Bipolar Transistors Electronics – Celso José Faria de Araújo, M. Sc.

Physical Structure of the BJT Cross-section of an integrated npn bipolar junction transistor 2 Bipolar Transistors Electronics – Celso José Faria de Araújo, M. Sc.

A simplified structure of the pnp transistor. 3 Bipolar Transistors Electronics – Celso José Faria de Araújo, M. Sc.

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. ) 4 Bipolar Transistors Electronics – Celso José Faria de Araújo, M. Sc.

Operating Regions of the BJT and Simplified Models Regions of Operation of the Bipolar Transistors Base-Emitter Junction EBJ Forward Bias Reverse Bias Base-Collector Junction CBJ Forward Bias Reverse Bias Saturation Region (Closed Switch) Forward-Active Region (Normal-Active Region) (Good Amplifier) Reverse-Active Region (Inverse-Active Region) (Poor Amplifier) Cutoff Region (Open Switch) 5 Bipolar Transistors Electronics – Celso José Faria de Araújo, M. Sc.

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. 6 Bipolar Transistors Electronics – Celso José Faria de Araújo, M. Sc.

Large-signal equivalent-circuit models of the npn BJT operating in the active mode. 7 Bipolar Transistors Electronics – Celso José Faria de Araújo, M. Sc.

Current flow in an pnp transistor biased to operate in the active mode. 8 Bipolar Transistors Electronics – Celso José Faria de Araújo, M. Sc.

Two large-signal models for the pnp transistor operating in the active mode. 9 Bipolar Transistors Electronics – Celso José Faria de Araújo, M. Sc.

CIRCUITS SYMBOLS AND CONVENTIONS CUTOFF ACTIVE SATURATION NPN v. BE < 0. 7 V i. C = i. B = i. E = 0 V v. BE = 0. 7 V v. C < v. B PNP v. EB < 0. 7 V i. C = i. B = i. E = 0 V v. EB = 0. 7 V v. B v. C v. EB = 0. 7 V v. B < v. C 10 Bipolar Transistors Electronics – Celso José Faria de Araújo, M. Sc.

The i. C-v. CB characteristics for an npn transistor in the active mode. 11 Bipolar Transistors Electronics – Celso José Faria de Araújo, M. Sc.

(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. 12 Bipolar Transistors Electronics – Celso José Faria de Araújo, M. Sc.

Transistor output characteristics identifying the Early voltage VA 13 Bipolar Transistors Electronics – Celso José Faria de Araújo, M. Sc.

Bias Circuits Assuming BJT in the active mode A simple bias circuit IC (and VCE) is very sensitive to 14 Bipolar Transistors Electronics – Celso José Faria de Araújo, M. Sc.

A Simple Example of Bias Circuit = 100 Assume VBE = 0. 7 V Calculate IC and VCE if: = 50 ? = 200 ? 15 Bipolar Transistors Electronics – Celso José Faria de Araújo, M. Sc.

A Bias Cicuit Example The four resistor bias network ( Assume F = 75 for analysis ) ( sensitivity to is low if RE >> REQ ) 16 Bipolar Transistors Electronics – Celso José Faria de Araújo, M. Sc.

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 vbe eliminated for dc (bias) analysis. 17 Bipolar Transistors Electronics – Celso José Faria de Araújo, M. Sc.

The Small-Signal Model Parameters of the BJT The Collector Current and the Transconductance The Base Current and the Input Resistence at the Base The Emitter Current and the Input Resistance at the Emitter 18 Bipolar Transistors Electronics – Celso José Faria de Araújo, M. Sc.

Linear operation of the transistor under the small-signal condition: A small signal vbe with a triangular waveform is superimpose din the dc voltage VBE. It gives rise to a collector signal current ic, also of triangular waveform, superimposed on the dc current IC. Ic = gm vbe, where gm is the slope of the ic - v. BE curve at the bias point Q. 19 Bipolar Transistors Electronics – Celso José Faria de Araújo, M. Sc.

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). Bipolar Transistors Electronics – Celso José Faria de Araújo, M. Sc. 20

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 re rather than the base resistance r featured in the hybrid- model. 21 Bipolar Transistors Electronics – Celso José Faria de Araújo, M. Sc.

Example to Show Wave Forms ( =100) 22 Bipolar Transistors Electronics – Celso José Faria de Araújo, M. Sc.

Signal waveforms in the circuit of former Example 23 Bipolar Transistors Electronics – Celso José Faria de Araújo, M. Sc.

(a) circuit; (b) dc analysis; (c) small-signal model; (d) small-signal analysis performed directly on the circuit. Bipolar Transistors Electronics – Celso José Faria de Araújo, M. Sc. 24

Distortion due to Cutoff or Nonlinearity 25 Bipolar Transistors Electronics – Celso José Faria de Araújo, M. Sc.

Augmenting the hybrid- model to account for the Early effect for the small-signal operation of the BJT. 26 Bipolar Transistors Electronics – Celso José Faria de Araújo, M. Sc.

Augmenting the T-Model to Account for the Early effect for the small-signal operation of the BJT. 27 Bipolar Transistors Electronics – Celso José Faria de Araújo, M. Sc.

Relationships Between the Small. Signal Model Parameters of the BJT Model Parameters in Terms of DC Bias Currents: In terms of gm In terms of re Relationships between and : 28 Bipolar Transistors Electronics – Celso José Faria de Araújo, M. Sc.

Graphical construction for the determination of the dc base current in the shown circuit. 29 Bipolar Transistors Electronics – Celso José Faria de Araújo, M. Sc.

Graphical construction for determining the dc collector current IC and the collector-to-emmiter voltage VCE in the shown circuit. 30 Bipolar Transistors Electronics – Celso José Faria de Araújo, M. Sc.

Graphical determination of the signal components vbe, ib, ic, and vce when a signal component vi is superimposed on the dc voltage VBB 31 Bipolar Transistors Electronics – Celso José Faria de Araújo, M. Sc.

Effect of bias-point location on allowable signal swing: 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. 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. 32 Bipolar Transistors Electronics – Celso José Faria de Araújo, M. Sc.

= 75 Note: Early effect has been neglacted ( VA ) Load line for the four resistor bias circuit Exercice: What’s the Q-point if a) = 500 b) c) = 75 and RC = 56 k 33 Bipolar Transistors Electronics – Celso José Faria de Araújo, M. Sc.

180 phase shift between input and output signals Load Line Q-Point and signals for the BJT amplifier Bipolar Transistors Electronics – Celso José Faria de Araújo, M. Sc. 34

To make IE insensitive to temperature and variation, we design the circuit to satisfy the following two constraints: As is the case in any design problem, we have a set of conflicting requirements, and the solution must be a compromise. As a rule of thumb, one designs Bias Arrangement Using a Single Power Supply 35 Bipolar Transistors Electronics – Celso José Faria de Araújo, M. Sc.

Resistor RB is need only if the signal is to be couple to the base. Otherwise, the base can be connected directly to ground, resulting in almost total independence of the bias current with regard to the value of . Bias Arrangement Using Two Power Supply 36 Bipolar Transistors Electronics – Celso José Faria de Araújo, M. Sc.

To obtain a value of IE that is insensitive to variation of , we select Note, however, that the value of RB determines the allowable signal swing at the collector since An alternative Biasing Arrangement 37 Bipolar Transistors Electronics – Celso José Faria de Araújo, M. Sc.

Biasing Using a Current Source 38 Bipolar Transistors Electronics – Celso José Faria de Araújo, M. Sc.

Analysis Circuits Step-by-Step DC Analysis 1. ; 2. Find the Q-point using large-signal model AC Analysis 3. ; 4. BJT small-signal model 5. Analyze the circuit 6. Combine DC AC results 39 Bipolar Transistors Electronics – Celso José Faria de Araújo, M. Sc.

The common-emitter amplifier. (a) Circuit. (b) Equivalent circuit with the BJT replaced with its model. 40 Bipolar Transistors Electronics – Celso José Faria de Araújo, M. Sc.

The common-emitter amplifier with a resistance Re in the emitter. (a) Circuit. (b) Equivalent circuit with the BJT replaced with its T model (c) The circuit in (b) with ro eliminated. Bipolar Transistors Electronics – Celso José Faria de Araújo, M. Sc. 41

R 2 = 100 K ; R 1 = 20 K ; RC = 6. 8 K ; RE 1 = 2 K RE 2 = 150 ; RL = 100 K ; CI = 10 F; CC = 15 F CE = 47 F; = 200; Rs = 2 K e Vcc = 30 V DC ANALYSIS VBB = 25 V RBB = 50/3 K IE = 1. 926 m. A; IB = 9. 58 A; IC = 1. 916 m. A VE = 25, 86 V; VB = 25. 16 V; VC = 13. 029 V VB > VC (Active Operation) AC ANALYSIS (SMALL – SIGNAL) DC + AC ANALYSIS OK The common-emitter amplifier with a resistance RE in the emitter (Example) Bipolar Transistors Electronics – Celso José Faria de Araújo, M. Sc. Suggestion: Do again without CE 42

The common-base amplifier. (a) Circuit. (b) Equivalent circuit obtained by replacing the BJT with its T model. 43 Bipolar Transistors Electronics – Celso José Faria de Araújo, M. Sc.

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 ro is in parallel with RL. (d) Circuit for determining Ro. Bipolar Transistors Electronics – Celso José Faria de Araújo, M. Sc. 44

The transistor as a switch-cutoff and saturation To analyse verify if BJT is in saturation region. If yes, don’t use any type of . Just remember that: to PNP use VECsat = 0. 2 V ; to NPN use VCEsat = 0. 2 V (if not specified). Also remember IE = IC+IB Exemple: = 30 ; VECsat = 0. 2 V To design use f = min/OF OF = Overdrive Factor min from Data Sheet f =Forced ; 2 OF 10 45 Bipolar Transistors Electronics – Celso José Faria de Araújo, M. Sc.

RC = 1 k RB = 10 k VCC=5 V = 50 VCEsat = 0. 2 V Simplified Analysis: 1. At v. I =VIH v. O = VOL = VCEsat= 0. 2 V; 2. At v. I =VIL v. O = VOH = VCC = 5 V 3. At v. I = VIL , the BJT begins to turn on, thus VIL 0. 7 V 4. For VIL < v. I < VIH , the BJT is in the active region 5. At v. I = VIH , the BJT enters the saturation region Basic BJT digital logic inverter. Bipolar Transistors Electronics – Celso José Faria de Araújo, M. Sc. 46

Figure 5. 67 The high-frequency hybrid- model. 47 Bipolar Transistors Electronics – Celso José Faria de Araújo, M. Sc.

Figure 5. 68 Circuit for deriving an expression for hfe(s) ; Ic/Ib. 48 Bipolar Transistors Electronics – Celso José Faria de Araújo, M. Sc.

49 Bipolar Transistors Electronics – Celso José Faria de Araújo, M. Sc.

Figure 5. 71 (a) Capacitively coupled common-emitter amplifier. (b) Sketch of the magnitude of the gain of the CE amplifier versus frequency. The graph delineates the three frequency bands relevant to frequency-response determination. Bipolar Transistors Electronics – Celso José Faria de Araújo, M. Sc. 50

Figure 5. 72 Determining the high-frequency response of the CE amplifier: (a) equivalent circuit; (b) the circuit of (a) simplified at both the input side and the output side; (c) equivalent circuit with C replaced at the input side with the equivalent capacitance Ceq; (d) sketch of the frequencyresponse plot, which is that of a low-pass STC circuit. Bipolar Transistors Electronics – Celso José Faria de Araújo, M. Sc. 51

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Figure 5. 73 Analysis of the low-frequency response of the CE amplifier: (a) amplifier circuit with dc sources removed; (b) the effect of CC 1 is determined with CE and CC 2 assumed to be acting as perfect short circuits; Bipolar Transistors Electronics – Celso José Faria de Araújo, M. Sc. 55

Figure 5. 73 (Continued) (c) the effect of CE is determined with CC 1 and CC 2 assumed to be acting as perfect short circuits; (d) the effect of CC 2 is determined with CC 1 and CE assumed to be acting as perfect short circuits; Bipolar Transistors Electronics – Celso José Faria de Araújo, M. Sc. 56

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Implementation of na RTL ( resistor-transistor logic ) NOR-gate 59 Bipolar Transistors Electronics – Celso José Faria de Araújo, M. Sc.

The High-Frequency Model of the BJT Inclusion of capacitances in the hybrid-pi model of BJT 60 Bipolar Transistors Electronics – Celso José Faria de Araújo, M. Sc.

The unity-gain frequency ( f. T ) : the frequency at which drops to one + v be C + i r b v be - C ic i 0 gm v be ro Common-emitter current gain versus frequency for the BJT Finding the short-circuit current gain of the BJT 61 Bipolar Transistors Electronics – Celso José Faria de Araújo, M. Sc.

Dependence of the unity-gain frequency on collector current high-level injection low currents: ( IC << ICM ) f. T IC moderate currents ICM f. T independent of current high-level injection: IC > ICM f. T decreases with IC Current dependence of f. T 62 Bipolar Transistors Electronics – Celso José Faria de Araújo, M. Sc.

R 2 = 100 K ; R 1 = 20 K ; RC = 6. 8 K ; RE 1 = 2 K RE 2 = 150 ; RL = 100 K ; CI = 10 F; CC = 15 F CE = 47 F; = 200; Rs = 2 K e Vcc = 30 V DC ANALYSIS VBB = 25 V RBB = 50/3 K IE = 1. 926 m. A; IB = 9. 58 A; IC = 1. 916 m. A VE = 25, 86 V; VB = 25. 16 V; VC = 13. 029 V VB > VC (Active Operation) DC + AC ANALYSIS (SMALL – SIGNAL) Suggestion: Do again without CE The common-emitter amplifier with a resistance RE in the emitter (Example) Bipolar Transistors Electronics – Celso José Faria de Araújo, M. Sc. 63