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Chapter 4 Bipolar Junction Transistors Objectives Describe the basic structure of the bipolar junction transistor (BJT) Explain and analyze basic transistor bias and operation Discuss the parameters and characteristics of a transistor and how they apply to transistor circuits Discuss how a transistor can be used as an amplifier or a switch Troubleshoot various failures typical of transistor circuits 4.1 Introduction A transistor is a three-terminal device whose output current, voltage, and/or power controlled by its input. The three terminals are called collector, base, and emitter. The arrow on the symbol is important for two reasons: 1. It identifies the component terminals. The arrow is always drawn on the emitter terminal. Terminal opposite the emitter is the collector, and the center terminal is the base. 2. The arrow always points toward the n-type materials. If the arrow points toward the emitter, the transistor is a npn type. If it points toward the base, the transistor is a pnp type. (a) n-p-n (b) p-n-p Fig.4-1: Standard transistor symbols 4.2 Bipolar Junction Transistor Structure Different with diodes having one p-n junction, bipolar junction transistors (BJT) have three layers and two p-n junctions. Fig.4-2: Basic BJT construction. 4.3 Transistor Currents The values of the collector and emitter currents are determined by the value of the base current. An increase or decrease in base current (IB) causes a similar change in collector current (IC) and emitter current (IE). Fig.4-3: Transistor terminal currents. According to Kirchhoff’s current law, the current leaving a component must equal the current entering the component. By formula, I E IC I B 4-1 Since IB is normally much less than IC, IC and IE are approximately equal, expressed as follows: IC I E 4-2 The value of IC is normally some multiple of the value of IB. The factor by which current increases from base to collector is referred to as dc current gain (βDC) of a transistor. I E I B ( DC 1) 4-3 If we combine this Eq.4-3 with Eq.4-1, we get I C DC I B 4-4 βDC is usually designated as an equivalent hybrid parameter, hFE. hFE DC 4-5 The ratio of the dc collector current (IC) to the dc emitter current (IE) is a dc alpha (αDC). DC IC IE 4-6 Using the relationships of Eq.4-1 and 4-6, we can calculate base current as I B I E (1 ) 4-7 The relationship between alpha and beta: DC DC DC 1 4-8 4.4 BJT Characteristics and Parameters Several voltages are normally involved in the transistor operation, described in Table 4-1. VCC and VBB are dc voltage sources that are used to bias the transistor. Fig.4-4: Transistor currents and voltages. TABLE 4-1: Transistor voltages Voltage Abbreviation Definition VCC Collector-bias voltage source. This is a dc voltage source applied to the base-collector junction. Base-base voltage source. This is a dc voltage source applied to the base-emitter junction. DC voltage across base-emitter junction. DC voltage across collector-base junction. DC voltage from collector to emitter. VBB VBE VCB VCE For proper operation, the base-emitter junction is forward-biased by VBB and conducts just like a diode and has a nominal forward voltage drop of VBE 0.7 V 4-9 The collector-base junction is reverse-biased by VCC and blocks current flow through it’s junction just like a diode. Since the emitter is at ground (0 V), by Kirchhoff’s voltage law, the voltage across RB is VRB VBB VBE 4-10 Also, by Ohm’s law, VRB I B RB Substituting for VRB yields I B RB VBB VBE Solving for IB, VBB VBE IB RB 4-11 The voltage at the collector with respect to the grounded emitter is VCE VCC VRC Since the drop across RC is VRC I C RC the voltage at the collector with respect to the emitter can be written as VCE VCC I C RC 4-12 The voltage across the reverse-biased collector-base junction is VCB VCE VBE 4-13 4.4.1 Collector characteristic curves These curves give a graphical illustration of the relationship between collector current (IC) and VCE with specified amounts of base current. With greater increases of VCC , VCE continues to increase until it reaches breakdown. When VCE exceeds 0.7 V, the base-collector junction becomes reversebiased and the transistor goes into the active or linear region. In this region, the current remains from 0.7 V to the breakdown voltage. Fig.4-5: Collector characteristic curves. 4.4.2 Cutoff Cutoff is a non-conducting state of a transistor. This occurs when the base lead opens and the base current is zero. There is only a very small amount of collector leakage current , ICEO, caused by thermally produced carriers. However, it will usually be neglected so that VCE = VCC. In the cutoff, neither the base-emitter nor the basecollector junctions are forward-biased. The subscript CEO represents collector-toemitter with the base open. Fig.4-6 4.4.3 Saturation Saturation is the state of a BJT in which the collector current has reached a maximum and is independent of the base current. Note that saturation value of IC can be determined by application of Ohm’s law. When VCE reaches its saturation value (VCE(sat) = 0, we obtain, I C sa t VCC RC 4-14 Fig.4-7 4.4.4 DC Load Line DC load line graphically illustrates IC(sat) and cutoff for a transistor. The bottom of the load line is at ideal cutoff where IC = 0 and VCE = VCC. The top of the load line is at saturation where IC = IC(sat) and VCE = VCE(sat). Along the load line is the active region of the transistor’s operation. Fig.4-8 4.5 The BJT As An Amplifier Amplification is the process of increasing the amplitude of an electrical signal. The circuits used to provide the amplification are referred to as amplifiers. Fig.4-9: Basic transistor amplifier circuit. ' If the internal ac emitter resistance designated re is in series with RB, the ac base voltage can be written as Vb I e re' 4-15 The ac collector voltage, Vc, equals the ac voltage drop across RC. Vc I c RC Since Ic 4-16 I c I e , the ac collector voltage is Vc I e RC 4-17 The ratio of Vc to Vb is the ac voltage gain, Av, of the transistor. Vc I e RC RC Av ' ' Vb I e re re Because RC is always larger than input voltage. 4-18 re' , the output voltage is greater than 4.6 The BJT As A Switch A BJT can be used as a switching device. If the base-emitter junction is not forward-biased, the BJT is in the cutoff (switched off) and there is an open circuit between collector and emitter, as shown in Fig. 4-10(a). If the base-emitter junction and base-collector junction are forwardbiased, the BJT is in the saturation (switched on). There is an short circuit between collector and emitter, as shown in Fig. 4-10(b). Fig.4-10: Switching action of an ideal transistor. Conditions in Cutoff All of the currents are zero, and VCE is equal to VCC. VCE ( cut ) VCC 4-19 Conditions in Saturation The formula for collector saturation current is I C ( sat) VCC VCE ( sat) RC 4-20 The minimum value of base current needed to produce saturation is I B (min) I C ( sat) DC 4-21 4.7 Troubleshooting Several faults that can occur in a transistor bias circuit are: 1. Open bias resistors. 2. Open or resistive connections. 3. Shorted connections. 4. Opens or shorts internal to transistor itself. These faults will cause the current to cease in the collector. 4.7.1 Troubleshooting a Biased Transistor This figure shows the correct voltage measurements at base and collector of a basic transistor bias circuit. Erroneous voltage measurements are a result of point that is not “solidly connected”. This called a floating point. This is typically indicative of an open. A simple check at the top of the collector resistor and at the collector itself will quickly ascertain if VCC is present and the transistor is conducting normally or is in cutoff (VC = VCC) or saturation (VC ≈ 0). Fig.4-11: A basic transistor bias circuit with all voltages referenced to ground. 4.7.2 Testing a Transistor with a DMM Several faults that can occur in the circuit and the accompanying symptoms are illustrated in Fig. 4-12. Symptoms are shown in terms of measured voltages that are incorrect. Fig. 4–12: Typical faults and symptoms in the basic transistor bias circuit. 4.7.2 Testing a Transistor with a DMM A fast and simple way to check a transistor for open or shorted junction is by using a digital multimeter. Testing of a transistor can be viewed as testing of two diode junctions. The testing shows that forward bias has low resistance and reverse bias has infinite resistance. Fig.4-13: A transistor viewed as two diodes. The diode test function of a multimeter is more reliable than using an ohmmeter. However, make sure to note whether it is an npn or pnp and polarize the test leads. Fig.4-14: Typical DMM test of a properly functioning npn transistor. 4.7.3 Transistor Testers In addition to the traditional DMMs, there are also transistor testers. Some of these have the ability to test other parameters of the transistor, such as leakage and gain. Fig. 4-15: Transistor tester. Summary The bipolar junction transistor (BJT) is constructed of three regions: base, collector, and emitter. The BJT has two pn junctions, the base-emitter junction and the base-collector junction. The two types of transistors are pnp and npn. For the BJT to operate as an amplifier, the base-emitter junction is forward-biased and the collector-base junction is reverse-biased. Of the three currents IB is very small in comparison to IE and IC. Beta is the current gain of a transistor. This the ratio of IC/IB. Summary A transistor can be operated as an electronics switch. When the transistor is off it is in cutoff condition (no current). When the transistor is on, it is in saturation condition (maximum current). Beta can vary with temperature and also varies from transistor to transistor.