CHAPTER 2: Characteristics of Bipolar Junction Transistor Val de Loire Program p.31 CHAPTER 2: CHARACTERISTICS OF BIPOLAR JUNCTION TRANSISTOR Table of Contents 2.1. BJT CONSTRUCTION AND SYMBOLS 32 2.2. COMMON-EMITTER TERMINAL CHARACTERISTICS 33 2.3. CURRENT RELATIONSHIPS 34 2.4. BIAS AND DC LOAD LINES 35 2.5. CAPACITORS AND AC LOAD LINES 38 Table of Figures Fig. 2-1 Constructions and Symbols of BJT 32 Fig. 2-2 Common-emitter characteristics (npn, Si device) 34 Fig. 2-3 CE amplifier bias circuit 36 Fig. 2-4 DC load line and Q point 38 Fig. 2-5 Capacitors in CE amplifier 38 CHAPTER 2: Characteristics of Bipolar Junction Transistor Val de Loire Program p.32 CHAPTER 2: CHARACTERISTICS OF BIPOLAR JUNCTION TRANSISTOR 2.1. BJT CONSTRUCTION AND SYMBOLS The bipolar junction transistor (BJT) is a three-element (emitter, base, and collector) device, made up of alternating layers of n- and p-type semiconductor materials. The transistor can be of pnp type (principal conduction by positive holes) or of npn type (principal conduction by negative electrons). Fig. 2-1 Constructions and symbols of BJT CHAPTER 2: Characteristics of Bipolar Junction Transistor Val de Loire Program p.33 Table 2-1 Notation for voltages and currents 2.2. COMMON-EMITTER TERMINAL CHARACTERISTICS The common-emitter (CE) connection is a two-port transistor arrangement (widely used because of its high current amplification) in which the emitter shares a common point with the input and output terminals. The independent port input variables are base current B i and emitter-to-base voltage BE v , and the independent port output variables are collector current C i and emitter-to-collector voltage CE v . CE analysis is based on: 1. Input or transfer characteristics that relate the port input variables B i and BE v , with CE v held constant. 2. Output or collector characteristics that show the functional relationship between port output variables C i and CE v for constant B i . CHAPTER 2: Characteristics of Bipolar Junction Transistor Val de Loire Program p.34 Fig. 2-2 Common-emitter characteristics (npn, Si device) 2.3. CURRENT RELATIONSHIPS The two pn junctions of the BJT can be independently biased, to result in four possible transistor operating modes as summarized in Table 2-2. A junction is forward-biased if the n material is at a lower potential than the p material, and reverse-biased if the n material is at a higher potential than the p material. CHAPTER 2: Characteristics of Bipolar Junction Transistor Val de Loire Program p.35 Table 2-2 Operating modes Saturation denotes operation (with  0.2 CE v V and  0.5 CB v V for Si devices) such that maximum collector current flows and the transistor acts much like a closed switch from collector to emitter terminals. Cutoff denotes operation near the voltage axis of the collector characteristics, where the transistor acts much like an open switch. Only leakage current (similar to o I of the diode) flows in this mode of operation; thus,   0 C CBO i I for CE connection. The inverse mode is a little-used, inefficient active mode with the emitter and collector interchanged. The active or linear mode describes transistor operation in the region to the right of saturation and above cutoff. In this mode, the base current is increased or amplified  times to become the collector current:   C B i i ,   E B C i i i and    ( 1) E B i i . 2.4. BIAS AND DC LOAD LINES Supply voltages and resistors bias a transistor; that is, they establish a specific set of dc terminal voltages and currents, thus determining a CHAPTER 2: Characteristics of Bipolar Junction Transistor Val de Loire Program p.36 point of active-mode operation (called the quiescent point or Q point). Usually, quiescent values are unchanged by the application of an ac signal to the circuit. Fig. 2-3 CE amplifier bias circuit Use of the Thevenin equivalent of the circuit to the left of a, b leads to the circuit of Fig. 2-3(b), where   1 2 1 2 B R R R R R ,   1 1 2 BB CC R V V R R      1 EQ BQ I I and assume the emitter-to-base voltage BEQ V is constant (  0.7 V and  0.3 V for Si and Ge, respectively), then KVL around the emitter loop of Fig. 2-3(b) yields:      1 EQ BB B BEQ EQ E I V R V I R CHAPTER 2: Characteristics of Bipolar Junction Transistor Val de Loire Program p.37      1 EQ CQ CQ I I I         / 1 BB BEQ CQ EQ B E V V I I R R If component values and the worst-case  value are such that:      1 B B E R R R Then EQ I (and thus CQ I ) is nearly constant, regardless of changes in  ; the circuit then has  - independent bias. Considering   1 :                    CC CEQ CEQ CC CQ C E C E C E CE CC C C E C E dc C E CE CC C dc C E V V V V I R R R R R R v V i R R R R R R R v V i R R R   dc C E R R R     CE CC C dc C E v V i R R R : dc load line with slope  1 dc R . CHAPTER 2: Characteristics of Bipolar Junction Transistor Val de Loire Program p.38 Fig. 2-4 DC load line and Q point 2.5. CAPACITORS AND AC LOAD LINES Fig. 2-5 Capacitors in CE amplifier CHAPTER 2: Characteristics of Bipolar Junction Transistor Val de Loire Program p.39 1. Coupling capacitors   C C confine dc quantities to the transistor and its bias circuitry. 2. Bypass capacitors   E C effectively remove the gain-reducing emitter resistor E R insofar as ac signals are concerned, while allowing E R to play its role in establishing  - independent bias. 1 0 C Z C    when C is large enough. With ac signal:   ce c ac v i R    // C L ac C L C L R R R R R R R     CEQ CE C CQ ac ac V v i I R R : ac load line with slope 1 ac R  and intersects the dc load line at the Q point. . CHAPTER 2: Characteristics of Bipolar Junction Transistor Val de Loire Program p. 32 CHAPTER 2: CHARACTERISTICS OF BIPOLAR JUNCTION TRANSISTOR 2. 1. BJT CONSTRUCTION AND SYMBOLS The bipolar. CHAPTER 2: Characteristics of Bipolar Junction Transistor Val de Loire Program p.31 CHAPTER 2: CHARACTERISTICS OF BIPOLAR JUNCTION TRANSISTOR Table of Contents 2. 1. BJT CONSTRUCTION. CE v for constant B i . CHAPTER 2: Characteristics of Bipolar Junction Transistor Val de Loire Program p.34 Fig. 2- 2 Common-emitter characteristics (npn, Si device) 2. 3. CURRENT RELATIONSHIPS