Introduction to transistor theory [Electronics]

Introduction to Transistors

1. BIPOL A R JU N CTIO N TRANS I STO R CONFI G U RATIO N S C o m m o n- Base ( CB ) C o nf i g u rat i on 𝑉 𝐸𝐵 ELE 307 MODULE 1 1

2. Th is c o nf i g u rat i on d er i ves its n a me f r o m t h e f a ct t h at t h e b a s e is c o m m o n t o b o th t h e i n pu t ( em i tte r ) a n d o u tp u t s i d e o f t h e c o nf i g u rat i o n . Ther e are t wo s ets o f ch a r acteri st i cs n ec e ssar y t o re p rese n t its b eh a vior . T h ese are the i n pu t ( o r e m i t ter) c h arac t eri st i c s a n d t h e o u tp u t ( o r c o l l e cto r ) ch a racter i s tics. Th e i n pu t ch a ra c teri s tics a ri s e f r o m a p l o t o f e mi t ter cu r rent, 𝐼 𝐸 ve r su s e mit t er - t o b ase v o l tag e , 𝑉 𝐸𝐵 with the c o l l ec t o r - t o -b ase v o l tage, 𝑉 𝐶𝐵 as a p ar a me t er. Th e o u tp u t ch a rac t eri st i c s ar is e fr o m a p lot o f t h e c o l l ec t o r cu r re n t, 𝐼 𝐶 versus 𝑉 𝐶𝐵 with 𝐼 𝐸 as a para m eter. Th e i n pu t ch a rac t eri st ics a r e s imi l ar to the f o rward c h aracteri s tics o f a d io d e. An i n crease in 𝑉 𝐶𝐵 will b y Ear ly Ef f ect ca u s e 𝐼 𝐸 to i n crea s e with 𝑉 𝐸𝐵 h eld c o nstant . Th is g iv e s ri s e to a s et o f cu r ves a s sho wn in Figu re below 2

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5. Ther e are three reg i o n s o f o p e r ation in t h e o u tp u t c h aracteri st i cs as i l l u str a te d . In the f i rs t reg i o n cal l ed the act i ve r eg i o n , the c o l l ec t o r jun c ti o n is r e v e rs e -b iased whi l e the e mit t er is f o rwar d -b iase d . Th is is the o n l y reg i o n e m p lo y ed f or a m p l if icat i o n o f s i g n als with mi n i m u m d i s t o rtio n . In th i s reg i o n , 𝐼 𝐸 ≥ a n d 𝑉 𝐸𝐵 ≥ 𝑉 𝛾 . C o ns i d er f i rs t that 𝐼 𝐸 = , t h en 𝐼 𝐶 is s mall a n d eq u als t h e r e ver s e s a tur a t i o n cu r rent o f the c o l l e c t o r jun cti o n , 𝐼 𝐶 𝑂 . S u pp o s e n o w that 𝐼 𝐸 is ca use d to f l o w, 𝐼 𝐶 then b e c o m e s 𝐼 𝐶 = 𝛼 𝐼 𝐸 + 𝐼 𝐶 𝑂 . (1) In th i s reg i o n 𝐼 𝐶 is es s ent i a l l y i n d epen d ent o f 𝑉 𝐶 𝐵 a n d d epen d ent o n l y o n 𝐼 𝐸 . H o w e v e r, there is actua l l y a s mall i n c rease in | 𝐼 𝐶 | with | 𝑉 𝐶 𝐵 | . Sin ce 𝛼 is clo s e t o bu t l ess than 1 , t h e va l u e o f is slightly less than , i.e the current gain is less than unity   𝐸 5

6. In the cu t - o f f r eg i o n , the c o l l ect o r a n d the e mi t ter jun c ti o n s are b o th r e ve r s e -b ia s e d ( 𝐼 𝐸 < ) . The reg i o n be l o w 𝐼 𝐸 = ch a racter i s tic is re f erred to as t h e cu t - o f f reg i o n . In the satura ti o n reg i o n , b o th the c o l l e c t o r a n d e mit t er jun cti o n s are f o rwar d -b i ased resu l ti n g in the e xp o n ent i al ch ang e in 𝐼 𝐶 with s mall ch ang e in 𝑉 𝐶 𝐵 . 𝐼 𝐶 m a y e v en b e co me p o s it i ve if t h e f o r ward b ias is suff icie n t l y lar g e. 6

7. C o m m o n- E mit t er ( C E ) C o n f i g u rat i on In th i s c o nf i g u rat i o n i l l u st r ate d , the e mit t er is c omm o n t o b o th t h e b a s e a n d t h e c o l l e c t o r. It is al s o c al l ed G r o und ed – E mit t er c o nf i g u rat i o n . Th is is the m o s t frequ ent l y enco un te r ed tra ns i s t o r c o nf i g u rat i o n . 7

8. 1 Th e i n pu t ch a rac t eri st i cs s h o wn below r es e m b l e s that o f a C B t h o ug h 𝐼 𝐵 is p l o t t ed a g ai n st 𝑉 𝐵 𝐸 with 𝑉 𝐶𝐸 as a p ar a m e ter. 8

9. Th e o u tp u t ch a r a cteri s tics sho wn in Figu re b e l o w c o n s i s t o f th r ee r eg i o ns . 9

10. 𝐼 𝐵 = 𝐼 𝐸 − 𝐼 𝐶 (2) In the act i ve r eg i o n o f o p erat i o n , t h e c o l l e c t o r is r e v e r se d -b iased whi l e t h e b ase jun ct i o n is f o r w ar d-b iase d . Th e a c ti v e reg i o n o f the c o mm o n- e m i tter c o nf i g u rat i on can b e e m p l o y e d f or v o lta g e, cu r rent a n d p o w er a m p l if icat i o ns. A p p lyi n g Kirc h h o f f’s c u rre n t law ( K C L ) t o t h e circ u it in F i g u re a b o ve, we h a v e : It has been stated earlier on for a common-base configuration that: Combining Eqns. 1 and 2, we have (3) where .   10

11. Equation 3 can also be written as (4)   where   Since in the active region, is commonly called the common-emitter forward current amplification factor. Typical values vary from 20 to 600. The cut off region for the CE configuration is not as well-defined as for the CB. At cut-off, then . . is the actual collector current with the collector junction reverse-biased and base open-circuited. It is not just enough to reduce to zero in order to cut-off the transistor. Instead, it is necessary to reverse-bias the emitter junction slightly. So, cut-off is properly defined as a condition when and .   11

12. In the saturation region, the collector and the emitter junctions are forward-biased. The collector-emitter voltage at saturation is denoted by . and no longer respond appreciably to changes in . Since is usually very small (almost zero), is nominally . The minimum base current to achieve saturation is given by .   12

13. Example 1 Determine the region of operation of the transistor in the circuit shown in the Figure below given that a silicon transistor with β=100 and I CO =2×10 -5 mA is under consideration. Hence, determine the currents in the circuit, 13

14. Solution: Since the transistor is an npn transistor, the emitter-base junction is forward-biased. This shows that the transistor is not in the cut-off region. Let us assume it is in the active region. Applying Kirchhoff’s voltage law (KVL) to the input circuit gives: For silicon transistor,   14

15. To verify our assumption, we apply KVL to the collector circuit:   Since is positive and the transistor is an npn type, the collector junction is reverse-biased and so the transistor is in the active region as assumed.   15

16. Example 2 The circuit in Example 1.1 is modified by changing the base resistor from 200kΩ to 50kΩ. If β=100, determine whether or not the transistor is in saturation. (Take ). Repeat part (a) with 2kΩ-emitter resistor added. Solution: (a)Assuming the transistor is saturation, applying KVL to the input circuit yields:   16

17. Applying KVL to the collector circuit, The minimum base current required for saturation is Since , the transistor is in saturation. (b)Assuming the transistor is in saturation, applying KVL to the input circuit yields: (1) Applying KVL to the collector circuit gives: (2)   17

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19. Solving the two equations simultaneously yields: Since , the transistor is not in the saturation region but in the active region. To analyse the circuit in the active region, we apply KVL to the input circuit giving: But ,   19

20. To confirm that it is actually in the active region, we apply KVL to the collector circuit:   Example 3 A silicon transistor with V BEsat =0.8V, β=100 and V CEsat =0.2V is used in the circuit shown in the Figure below. Find the minimum value of R C for which the transistor remains in saturation. 20

21. Solution: Applying KVL to the base circuit yields: For saturation, the minimum collector current is given by: Applying KVL to the collector circuit gives:   21

22. Common Collector (CC) Configuration The common collector configuration is used primarily for impedance matching purposes since it has high input impedance and low output impedance. 22

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24. The common collector circuit (also called emitter follower) using npn transistor is shown in the Figure above with the load resistor from emitter to ground. Note that the collector is tied to ground even though the transistor is connected in a manner similar to the common emitter configuration. For all practical purposes, the output characteristics of the CC are the same as for the CE configuration. For the CC configuration, the output characteristics are plots of versus with as a parameter. The horizontal voltage axis for the CC is obtained by changing the sign of i.e. . Since , is replaced with . The input characteristics can also be obtained from CE characteristics by performing proper mathematical manipulations.   24

25. BJT Maximum Ratings For each transistor, there is a region of operation on the characteristics that will ensure that the maximum ratings are not being exceeded and the output signal exhibits minimum distortion. The standard transistor data sheet will include at least three maximum ratings: These are: Maximum collector power dissipation, expressed in mW Maximum collector voltage, expressed in V Maximum collector current, expressed in mA. The power or dissipation rating is the product of collector voltage and current. For CE configuration, The non-linear curve (maximum power curve) determined by this equation is illustrated on the characteristics.   25

26. As seen from the characteristics, the limit of operation of the transistor is defined by:   26