ชาว ศวกรรมคอมพ วเตอร คณะว ศวกรรมศาสตร มหาว ทยาล ยเทคโนโลย ราชมงคลพระนคร

EN2042102 วงจรไฟฟ าและอ เล กทรอน กส Circuits and Electronics บทท 7 ทรานซ สเตอร Bipolar Junction Transistor สาขาว ชาว ศวกรรมคอมพ วเตอร คณะว ศวกรรมศาสตร มหาว ทยาล ยเทคโนโลย ราชมงคลพระนคร

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

BJT Construction With diodes there is one p-n junction. With bipolar junction transistors (BJT), there are three three layers and two p-n junctions. Transistors can be a either pnp or npn type. The emitter (E) and is heavily doped (n-type). The collector (C) is also doped (n-type). The base (B) is lightly doped with opposite type to the emitter and collector (i.e. p-type in the npn transistor). The base is physically very thin. The p-n junction joining the base and emitter regions is called the baseemitter (B-E) junction. (or emitter-base, it doesn t really matter) The p-n junction between the base and collector regions is called the collector-base (C-B) junction.(or base-collector)

BJT Construction

Symbols

Current Flows The forward bias between the base and emitter injects electrons from the emitter into the base and holes from the base into the emitter. As the emitter is heavily doped and the base lightly doped most of the current transport across this junction is due to the electrons flowing from emitter to base The base is lightly doped and physically very thin. Thus only a small percentage of electrons flowing across the base-emitter (BE) junction combine with the available holes in this region. I E = I C + I B

BJT Operation A transistor is a device which can be used as either an: Amplifier Switch Let s first consider it s operation in a more simple view as a current controlling device.

BJT Operation Look at this one circuit as two separate circuits: base-emitter(left side) circuit collector-emitter(right side) circuit. Note that the emitter leg serves as a conductor for both circuits. The amount of current flow in the base-emitter circuit controls the amount of current that flows in the collector circuit. Small changes in base-emitter current yields a large change in collector-current. I C is approx 100 times I B (ratio is called beta )

Modes of Operation Depending on the bias condition (forward or reverse) of each of the two junctions, there are different modes of operation of the BJT. Modes EBJ CBJ Application Cutoff Reverse Reverse Saturation Forward Forward Switching application in digital circuits Active Forward Reverse Amplifier Reverse active Reverse Forward Performance degradation

Amplification

Input Characteristics The ic-vbe characteristic for an npn transistor

Output Characteristics I C (ma) Saturation Region 30 22.5 Active Region I B = 200 ma I B = 150 ma 15 I B = 100 ma 7.5 0 Cutoff Region I B = 50 ma I B = 0 V CE (V) 0 5 10 15 20

Transistor Characteristics and Parameters As previously discussed, base-emitter current changes yields large changes in collector-emitter current. The factor of this change is called beta( ). DC = I C /I B α DC = I C / I E

Transistor Characteristics and Parameters There are three key dc voltages and three key dc currents to be considered. Note that these measurements are important for troubleshooting.

Transistor Characteristics and Parameters I B : dc base current I E : dc emitter current I C : dc collector current V BE : dc voltage across base-emitter junction V CB : dc voltage across collector-base junction V CE : dc voltage from collector to emitter

Transistors Characteristics and Parameters For proper operation the base-emitter junction is forward biased by V BB and conducts just like a diode. The collector-base junction is reverse biased by V CC and blocks current flow through it s junction just like a diode.

Transistors Characteristics and Parameters Remember current flow through the base-emitter junction will help establish the path for current flow from the collector to emitter.

Transistor Characteristics and Parameters Analysis of this transistor circuit to predict the dc voltages and currents requires use of Ohm s law, Kirchhoff s voltage law and the beta for the transistor. Application of these laws begins with the base circuit to determine the amount of base current. Using Kirchhoff's voltage law, subtract the 0.7 V BE and the remaining voltage is dropped across R B. Determining the current for the base with this information is a matter of applying of Ohm s law. I B = ( V BB V BE )/R B

Transistor Characteristics and Parameters I C = I B I E = I C + I B V BE = 0.7 (Will be used in most analysis examples) V CE = V CC V RC = V CC I C R C V CB = V CE - V BE

Transistor Characteristics and Parameters What we ultimately determine by use of Kirchhoff s voltage law for series circuits is that in the base circuit V BB is distributed across the base-emitter junction and R B in the base circuit. In the collector circuit we determine that V CC is distributed proportionally across R C and the transistor(v CE ).

Example Find out the DC Currents and Voltages in the CCT.

Transistor Characteristics and Parameters Collector characteristic curves gives a graphical illustration of the relationship of collector current and V CE with specified amounts of base current. With greater increases of V CC, V CE continues to increase until it reaches breakdown, but the current remains about the same in the linear region from.7v to the breakdown voltage.

Transistor Characteristics and Parameters With no I B the transistor is in the cutoff region and just as the name implies there is practically no current flow in the collector part of the circuit. With the transistor in a cutoff state the the full V CC can be measured across the collector and emitter(v CE )

Transistor Characteristics and Parameters Current flow in the collector part of the circuit is, as stated previously, determined by I B multiplied by. However, there is a limit to how much current can flow in the collector circuit regardless of additional increases in I B.

Transistor Characteristics and Parameters Once this maximum is reached, the transistor is said to be in saturation. Note that saturation can be determined by application of Ohm s law. I C(sat) =V CC /R C The measured voltage across this now seemingly shorted collector and emitter is 0V.

Transistor Characteristics and Parameters The dc load line graphically illustrates I C(sat) and Cutoff for a transistor.

Example Determine whether or not the transistor is in saturation region. Assume V CE(sat) =0.2V. Steps: 1. Find I C(sat) 2. Find I B 3. Find I C for I B 4. If I C > I C(sat) The BJT is in saturation region. Can I C ever exceed I C(sat) in normal operation????

Transistor Characteristics and Parameters The beta for a transistor is not always constant. Temperature and collector current both affect beta, not to mention the normal inconsistencies during the manufacture of the transistor. There are also maximum power ratings to consider. The data sheet provides information on these characteristics.

Example P D(max) =800mW, V CE(max) =15V, I C(max) =100mA Determine the maximum value of V CC that can be adjusted without exceeding a rating. Which rating will be crossed first? Steps: 1. Determine I B 2. Determine I C for I B 3. I C > I C(max)??? 4. What will be the max value of V CC for which V CE = 15V for calculated value of I C?? 5. Calculate P D 6. P D > P D(max)??? 7. What if V BB is cut out???

Transistor Amplifier Amplification of a relatively small ac voltage can be had by placing the ac signal source in the base circuit. Recall that small changes in the base current circuit causes large changes in collector current circuit. The small ac voltage causes the base current to increase and decrease accordingly and with this small change in current the collector current will mimic the input only with greater amplitude.

Transistor Amplifier I e = V b / r e = I c V c = I c R C A v = V out / V in = V c / V b = I c R C / I e r e A v = R C / r e

Transistor Switch A transistor when used as a switch is simply being biased so that it is in cutoff (switched off) or saturation (switched on). Remember that the V CE in cutoff is V CC and 0V in saturation. Cut off: I B = 0 I C = 0 V CE = V CC Saturation: I C =(V CC V CE(sat) )/R C I B(min) = I C(sat) / β DC V CE = 0 ideally or few tens of a volt. Some text assume V CE(sat) = 0.2V

Troubleshooting Troubleshooting a live transistor circuit requires us to be familiar with known good voltages, but some general rules do apply. Certainly a solid fundamental understanding of Ohm s law and Kirchhoff s voltage and current laws is imperative. With live circuits it is most practical to troubleshoot with voltage measurements.

Troubleshooting Opens in the external resistors or connections of the base or the circuit collector circuit would cause current to cease in the collector and the voltage measurements would indicate this. Internal opens within the transistor itself could also cause transistor operation to cease. Erroneous voltage measurements that are typically low are a result of point that is not solidly connected. This called a floating point. This is typically indicative of an open. More in-depth discussion of typical failures are discussed within the textbook.

Troubleshooting Testing a transistor can be viewed more simply if you view it as testing two diode junctions. Forward bias having low resistance and reverse bias having infinite resistance.

Troubleshooting The diode test function of a multimeter is more reliable than using an ohmmeter. Make sure to note whether it is an npn or pnp and polarize the test leads accordingly.

Troubleshooting 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. Curve tracers give us even more detailed information about a transistors characteristics.

Summary The bipolarjunction transistor (BJT) is constructed of three regions: base, collector, and emitter. The BJT has two pn junctions, the base-emitter junction and the basecollector 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 I B is very small in comparison to I E and I C. Beta is the current gain of a transistor. This the ratio of I C /I B.

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.

Quiz 1. If V IN = 0, V CE =?? 2. I B(min) =?? for saturating transistor. β DC = 200, V CE(sat) = 0V 3. R B(max) when V IN = 5V