55:041 Electronic Circuits

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1 55:041 Electonic icuits BJT eview Sections in hapte 5 & 6 in Textbook A. Kuge BJT eview, Page- 1

2 npn Output Family of uves A. Kuge BJT eview, Page- 2

3 npn BJT dc Equivalent evese biased Fowad biased i B 1 v BE T S e Diode elationship BE ~ fo Si tansistos at oom tempeatue A. Kuge BJT eview, Page- 3

4 alculating Bias uents & oltages What is, E, E, etc., fo the cicuit below? D model fo npn BJT Key to solving cicuits such as this, is that if the tansisto is in the fowad active mode, then BE ~ 0.7 (fo Si) A. Kuge BJT eview, Page- 4

5 alculating Bias uents A. Kuge BJT eview, Page- 5

6 oltage Tansfe haacteistic fo npn icuit A. Kuge BJT eview, Page- 6

7 oltage Tansfe haacteistic fo pnp icuit A. Kuge BJT eview, Page- 7

8 Digital Logic nvete NO gate A. Kuge BJT eview, Page- 8

9 Bipola nvete as Amplifie Detemine the Q-point A. Kuge BJT eview, Page- 9

10 mpope Biasing Output voltage is clipped nput voltage A. Kuge BJT eview, Page- 10

11 Single, Base-esisto Biasing Geneally a bad idea eason 1: β vaies significantly fom device to device eason 2: Themal unaway BE deceases by about 2 m/ o. This inceases, which heats up the tansisto, which futhe deceases BE,, which inceases even moe, A. Kuge BJT eview, Page- 11

12 Stabilizing Q-Points Povides a bias voltage BAD DEA Stabilizes Q-point against vaiations in β, tempeatue, A. Kuge BJT eview, Page- 12

13 npn BJT dc Equivalent Find B,, etc. BE ~0.7 B not zeo, so )] B [ 2 /( 1 2 eplace base netwok with Thevenin equivalent )] TH [ 2 /( 1 2 TH 1 2 B 0 TH B TH 0.7 (1 ) B E 0 B TH B TH A. Kuge BJT eview, Page- 13

14 oltage Divide Biasing and Emitte esisto BE ~0.7 i b 0 )] TH [ 2 /( 1 2 A. Kuge BJT eview, Page- 14

15 Example Poblem 5.48 (a) Find Q-point voltages and cuents. Fo the Si tansisto, = 50 BE ~0.7 A. Kuge BJT eview, Page- 15

16 Example Poblem 5.44 (a) Find Q-point voltages and cuents. Fo the Si tansisto, = 75 BE ~0.7 A. Kuge BJT eview, Page- 16

17 ntegated icuit Biasing onstant uent Souce Q Question: what is 1? uent mio A. Kuge BJT eview, Page- 17

18 ascade icuit npn tansisto pnp tansisto A. Kuge BJT eview, Page- 18

19 Moving On to hapte 6 Mateial A. Kuge BJT eview, Page- 19

20 E with Time-aying nput Sect i S e v BE T Slope = -1/ v s changes v BE, which moves the Q-point along the load line A. Kuge BJT eview, Page- 20

21 B esus BE haacteistic i S e v BE T What is the incemental esistance? O, if v BE changes by a small amount, by how much will i B change? i S e v BE T i B S e v BE T v be o v π 1 i v B BE Q pt v BE S e v BE T Q pt 1 T 1 T S e BQ v BE T Q pt T BQ T Q The symbol v π is going to be shothand fo Δv BE A. Kuge BJT eview, Page- 21

22 esus BE haacteistic i S e v BE T i S e v BE T What is the incemental incease in as a function of an incemental change in BE? Q i S e v BE T g m i vbe T Se vbe Q pt BE Q v pt v be o v π 1 T vbe T Se Q pt 1 T Q g m Q T A. Kuge BJT eview, Page- 22

23 A Equivalent icuit fo E A. Kuge BJT eview, Page- 23

24 Oiginal Small Signal Equivalent icuits Small Signal Model Sect A Equivalent Small Signal Model A. Kuge BJT eview, Page- 24

25 Small-Signal Hybid- Model fo npn BJT v π Q gm 40 g m T T Q Q T =? T ~ 26 m at oom temp., => g m ~ 40 Q at oom temp. A. Kuge BJT eview, Page- 25

26 Why? A. Kuge BJT eview, Page- 26

27 Small Signal Equivalent icuits Small Signal Model Sect Small Signal Model A Equivalent A. Kuge BJT eview, Page- 27

28 Hybid- Model fo npn with Ealy Effect Sect How do we account fo the slope? o A Q A. Kuge BJT eview, Page- 28

29 Hybid- Model fo pnp with Ealy Effect A. Kuge BJT eview, Page- 29

30 A. Kuge BJT eview, Page- 30 Small-Signal Equivalent icuit fo npn E B m v g A ) ( ) ( g m o? A v S O v A B S B S m o g O + g m π = 0 KL at Output

31 E with E Sect B B E E B E A. Kuge BJT eview, Page- 31

32 A. Kuge BJT eview, Page- 32 E with E S i i E v A ) 1 ( S i E S i i E v A if b in ib b E b b b ib ib E ) 1 ( i ib 2 1 S O v A S b 0 E b b b in KL: E b b b in S ib in 1

33 E with E A v if i S E A v E A. Kuge BJT eview, Page- 33

34 E with E A v? A v E A. Kuge BJT eview, Page- 34

35 o Emitte-Followe Amplifie Sect. 6.6 ecall MOSFET Souce Followe A v < 1 i is high (fo BJT) O is low A. Kuge BJT eview, Page- 35

36 ommon-base Amplifie E Sect. 6.7 B B E Note sign of π and diection of o A. Kuge BJT eview, Page- 36

37 A. Kuge BJT eview, Page- 37 B Small Signal Model L m o g 0 S s m E g KL fo node E: sum of cuents flowing away fom node is 0 E g m 1 S E S S 1 S E S L m v g A 1 0 as S L m v g A Sect

38 A. Kuge BJT eview, Page- 38 nput esistance: B Sect g g m m b i 1 1 i ie

39 B Output esistance o? Sect Pevious Exam Question Tun off independent souces and add test souce x o x x x x g m KL at collecto (): 0 x x g m g m? g m constant Why? x cannot change the cuent flowing though the cuent souce so g m π is fixed and we can emove it fom the cicuit just like any nomal cuent souce. Thus x x O x x n othe wods, looking into the cuent souce fom, one sees and infinite esistance. A. Kuge BJT eview, Page- 39

B Output esistance f the pevious esult seems counteintuitive, below is an altenative appoach. Stat with a BJT model that includes the output esistance o.

40 B Output esistance f the pevious esult seems counteintuitive, below is an altenative appoach. Stat with a BJT model that includes the output esistance o. Then tun off independent souces, add a test souce, etc., to detemine the output esistance. KL equations at the collecto and emitte nodes, using the convention that cuents flow away fom the node (caefully note the sign of π ) give x + g m π + x π o = 0 g m π π S E π + π x o = 0 ombining gives x X = O = o 1 + g m S E π + S E π Letting o esults in O Same esult as befoe: looking into the cuent souce, once sees an infinite esistance See section in 4 th Edition of Neaman s text A. Kuge BJT eview, Page- 40

41 Dalington Pai Sect A i 1 2 i? A v? We will see how to answe the input impedance question using BJT scaling a little bit late. A. Kuge BJT eview, Page- 41

42 D and A Load Lines Sect At dc, capacitos ae open, and load line is detemined by A load line D load line At opeating fequencies capacitos ae shots, and load line is detemined by L A. Kuge BJT eview, Page- 42

43 ascode icuit ommon Base oltages ae in blue uents ae in geen ommon Emitte A. Kuge BJT eview, Page- 43

44 BTJ mpedance Scaling o esistance eflection ules Sections in hapte 6 in Textbook + Additional Mateial not n Textbook A. Kuge BJT eview, Page- 44

Emitte Followe Analysis Sect. 6.6.2 o =?

45 Emitte Followe Analysis Sect o =? Notice that o is in paallel with E i =? ib =? A. Kuge BJT eview, Page- 45

in ib o b o in b ( 1 ) b( o E ) } in ( 1 )( o

each time Notice that ib is π plus esistance

is base esistance scaled down with (1+β), in

46 Emitte Followe Analysis ib =? o =? o o o E o 1 b o 1 b o E ib =? o =? in ib o b o in b ( 1 ) b( o E ) } in ( 1 )( o E ) b 1 2 S E o 1 Somewhat involved to deive each time Notice that ib is π plus esistance at emitte, scaled up with (1+β) Notice that o is base esistance scaled down with (1+β), in paallel with othe esistance at emitte A. Kuge BJT eview, Page- 46

47 Emitte Followe Small Signal Model o π o ib =? ib ( 1 )( o E ) Example of BJT esistance scaling: looking fom the base, the emitte impedance is scaled up by (1+ A. Kuge BJT eview, Page- 47

48 BJT esistance Scaling o S E o esistance in base cicuit divided by β+1 ib =? ib ( 1 )( o E ) A. Kuge BJT eview, Page- 48

49 esistance Scaling Example The tansisto in the cicuit below has β =150. Neglect o, and estimate the input and output esistances shown. The collecto cuent is = 0.8 ma, A = g m ma/ g m 4.7K π ib o ( 1 ) ib E 4.7K 306K 302K o S E 4.7K 0.49K 2K A. Kuge BJT eview, Page- 49

50 A. Kuge BJT eview, Page- 50 BJT esistance Scaling ) 1 ( g m o o E m o E o g 1 ) ( E m o g 1 The following sub-cicuit often appeas in small signal cicuits Question: can you deive this?

51 Dalington Pai Sect A i 1 2 i? A v? A. Kuge BJT eview, Page- 51

52 omposite Tansistos Sect i i p i p BP i ) n i (1 i 2 n Bn 2 p Bn (1 ) i Bp p i n Bp i 2 p i n Bp A. Kuge BJT eview, Page- 52

53 Example Dalington Woked example on boad A. Kuge BJT eview, Page- 53

54 A. Kuge BJT eview, Page- 54

Bipolar transistors. Two port representation of the bipolar transistor Models of bipolar transistors

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