1. DMT 121/3 : ELECTRONIC I Mohd Khairuddin B Md Arshad 1.…Electronic I.…..DMT 121/3.. ChapTer ThRee BIPOLAR JUNCTION TRANSISTORS (BJTs)

2. DMT 121/3 : ELECTRONIC I Mohd Khairuddin B Md Arshad 2 TRANSISTOR STRUCTURE BJT (bipolar junction transistor) constructed with three layer semiconductor device consisting either TWO n- and ONE p-type layer (npn transistor) OR TWO-p and ONE n-type layer of material (pnp transistor) Joining between Base and Emitter region (called base-emitter junction) Joining between Base and Collector region (called base-collector junction)

3. DMT 121/3 : ELECTRONIC I Mohd Khairuddin B Md Arshad 3 TRANSISTOR STRUCTURE npn transistorpnp transistor Pointing in Not pointing in

4. DMT 121/3 : ELECTRONIC I Mohd Khairuddin B Md Arshad 4 BASIC TRANSISTOR OPERATION Fig. 3.3 Forward-biased junction of a pnp transistor. Fig. 3.4 Reverse-biased junction of a pnp transistor.

5. DMT 121/3 : ELECTRONIC I Mohd Khairuddin B Md Arshad 5 BASIC TRANSISTOR OPERATION Fig. 3.5 Majority and minority carrier flow of a pnp transistor. I E = I C + I B I C = I C majority + I CO (minority) I CO (minority) is called leakage current

6. DMT 121/3 : ELECTRONIC I Mohd Khairuddin B Md Arshad 6 npn transistor operation: oThe forward biase from base to emitter narrow the BE depletion region, and the reverse bias from base to collector widens the BC depletion region. oThe heavily doped n-type emitter region is teeming with conduction-band (free ) electrons that easily diffuse through BE juntion into the p-type base region where they become minority carriers. oThe base region is lightly doped and very thin so that it has a very limited number of holes. oThus only a small percentage of all the electrons flowing through the BE junction can combine with the available holes in the base. oA few recombined electrons flow out of the base lead as valence electrons, forming the small base electron current. oMost of electrons from the emitter diffuse into the BC depletion region. oOnce in this region they are pulled through the reverse-biased BC junction by the electric field set up by the force of attraction between the positive and negative ions. oThe electron now move through the collector region out through the collector lead into the positive terminal of the collector voltage source. oThe operation of pnp transistor is the same as for the npn except that the roles of electrons and holes, the bias voltage polarities and the current directions are all reversed. BASIC TRANSISTOR OPERATION

7. DMT 121/3 : ELECTRONIC I Mohd Khairuddin B Md Arshad 7 BASIC TRANSISTOR OPERATION Illustration of BJT action: B E C

8. DMT 121/3 : ELECTRONIC I Mohd Khairuddin B Md Arshad 8 BASIC TRANSISTOR OPERATION Look at this one circuit as two separate circuits, the base- emitter(left side) circuit and the 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. BJTs transistor are known as current controlled

9. DMT 121/3 : ELECTRONIC I Mohd Khairuddin B Md Arshad 9 TRANSISTOR CURRENTS

10. DMT 121/3 : ELECTRONIC I Mohd Khairuddin B Md Arshad 10 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 is dc current gain of transistor  DC is usually equivalent hybrid (h) parameters h FE on transistor datasheets. h FE =  DC  DC is ratio of collector current (I C ) to the emitter current (I E ). Less used parameter than beta in transistor circuits.  DC = I C /I E

11. DMT 121/3 : ELECTRONIC I Mohd Khairuddin B Md Arshad 11 Relationship between amplification factors  and  Beta (  ) Relationship Between Currents

12. DMT 121/3 : ELECTRONIC I Mohd Khairuddin B Md Arshad 12 There are three key dc voltages and three key dc currents to be considered. Note that these measurements are important for troubleshooting. 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 TRANSISTOR CHARACTERISTICS AND PARAMETERS

13. DMT 121/3 : ELECTRONIC I Mohd Khairuddin B Md Arshad 13 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. 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

14. DMT 121/3 : ELECTRONIC I Mohd Khairuddin B Md Arshad 14 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. V RB / RB = I B The collector current is determined by multiplying the base current by beta.  DC = I C /I B 0.7 V BE will be used in most analysis examples. TRANSISTOR CHARACTERISTICS AND PARAMETERS

15. DMT 121/3 : ELECTRONIC I Mohd Khairuddin B Md Arshad 15 Base-Emitter (Forward Bias) 0.7 V BE will be used in most analysis examples. TRANSISTOR CHARACTERISTICS AND PARAMETERS Collector - Emitter Collector – Base (Reverse Bias)

16. DMT 121/3 : ELECTRONIC I Mohd Khairuddin B Md Arshad 16 TRANSISTOR CHARACTERISTICS AND PARAMETERS Previously explained for npn transistor, what about pnp ???

17. DMT 121/3 : ELECTRONIC I Mohd Khairuddin B Md Arshad 17 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 ). TRANSISTOR CHARACTERISTICS AND PARAMETERS

18. DMT 121/3 : ELECTRONIC I Mohd Khairuddin B Md Arshad 18 TRANSISTOR CHARACTERISTICS AND PARAMETERS Common-base Common- EmitterCommon- Collector

19. DMT 121/3 : ELECTRONIC I Mohd Khairuddin B Md Arshad 19 TRANSISTOR CHARACTERISTICS AND PARAMETERS Fig. 3.8 Output or collector characteristics for a common-base transistor amplifier.

20. DMT 121/3 : ELECTRONIC I Mohd Khairuddin B Md Arshad 20 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 0.7V to the breakdown voltage. TRANSISTOR CHARACTERISTICS AND PARAMETERS Collector characteristics for Common emitter = common collector Fig. 3.14 Characteristics of a silicon transistor in the common-emitter configuration: (a) collector characteristics;

21. DMT 121/3 : ELECTRONIC I Mohd Khairuddin B Md Arshad 21 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 ) V CC  V CE TRANSISTOR CHARACTERISTICS AND PARAMETERS

22. DMT 121/3 : ELECTRONIC I Mohd Khairuddin B Md Arshad 22 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. Continue….at next page. TRANSISTOR CHARACTERISTICS AND PARAMETERS

23. DMT 121/3 : ELECTRONIC I Mohd Khairuddin B Md Arshad 23 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 (V CE = 0V). TRANSISTOR CHARACTERISTICS AND PARAMETERS

24. DMT 121/3 : ELECTRONIC I Mohd Khairuddin B Md Arshad 24 The dc load line graphically illustrates I C(sat) and Cutoff for a transistor. TRANSISTOR CHARACTERISTICS AND PARAMETERS

25. DMT 121/3 : ELECTRONIC I Mohd Khairuddin B Md Arshad 25 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.

26. DMT 121/3 : ELECTRONIC I Mohd Khairuddin B Md Arshad 26 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.

27. DMT 121/3 : ELECTRONIC I Mohd Khairuddin B Md Arshad 27 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.

28. DMT 121/3 : ELECTRONIC I Mohd Khairuddin B Md Arshad 28 Transistor Specification Sheet

29. DMT 121/3 : ELECTRONIC I Mohd Khairuddin B Md Arshad 29 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 (important). With live circuits it is most practical to troubleshoot with voltage measurements.

30. DMT 121/3 : ELECTRONIC I Mohd Khairuddin B Md Arshad 30 TROUBLESHOOTING 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. Opens in the external resistors or connections of the base or the collector circuit would cause current to cease (to stop) in the collector and the voltage measurements would indicate this.

31. DMT 121/3 : ELECTRONIC I Mohd Khairuddin B Md Arshad 31 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.

32. DMT 121/3 : ELECTRONIC I Mohd Khairuddin B Md Arshad 32 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.

33. DMT 121/3 : ELECTRONIC I Mohd Khairuddin B Md Arshad 33 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.

34. DMT 121/3 : ELECTRONIC I Mohd Khairuddin B Md Arshad 34 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 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 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.

35. DMT 121/3 : ELECTRONIC I Mohd Khairuddin B Md Arshad 35 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.