CHAPTER 3: BIPOLAR JUNCION TRANSISTOR DR. PHẠM NGUYỄN THANH LOAN

Transcription

1 CHAPTER 3: BIPOLAR JUNCION TRANSISTOR DR. PHẠM NGUYỄN THANH LOAN Hanoi, 9/24/2012

2 Contents 2 Structure and operation of BJT Different configurations of BJT Characteristic curves DC biasing method and analysis Base bias Collector-feedback bias Voltage divider bias AC signal analysis Impact of other parameters (temperature, leakage currents)

3 Structure and operation of BJT 3 BJT structure BJT :Bipolar Junction Transistor 2 kinds of BJT: NPN & PNP 3 terminals: E, B và C E: Emitter; B: Base, C: Collector Base located in the middle: thinner than E & C; and lower dope

4 Structure and operation of BJT 4 Bias condition for 2 junctions: J BE & J BC Junction BE in forward bias: electrons (e) move from E region to B region to create the current I E (diffusion current; flow of majority carriers) Junction BC in reverse bias: e that moved from E to B then move from B to C to create the current I C (drift current, flow of minority carriers) The combination of some electrons with holes in B region creates the current I B So: I E = I C + I B

5 Structure and operation of BJT 5 BJT symbol I B I C I E 3 terminals: B, E và C Arrow instructs the current direction between B & E Conventional current is the flow of positive charges (holes) NPN: B E PNP: E B

6 Technical parameters 6 I E = I C + I B I C = αi E + I CBO I C = βi B β = (may be higher) I C αi E (neglect leakage I CBO ) α = β is DC current gain α is DC current transfer coefficient + 1 = β β α

7 Technical parameters 7 I E = I C + I B I C = αi E + I CBO I C = β*i B β = (may be higher) I C αi E (neglect leakage I CBO ) α = β is DC current gain α is DC current transfer coefficient β α = β + 1

8 8 BJT as an amplifier Different amplifier configurations 3 configurations Common emitter (CB) Common base (CB) Common collector (CC) Look at the input and output to distinguish these configurations Configuration BC EC CC Input E B B Output C C E

9 9 CE configuration E is used in common for in and out Input: r e is considered as AC resistor of diode BE r e =26mV/I E Output: I c = βi b 9

10 Characteristic curves: CE 10 Input and output characteristic curves of CE configuration 10

11 Characteristic curves: CE 11 0<V CE <0.7V: Junction BE starts moving to forward bias I C increases gradually V CE >0.7V: Junction BE is in FB and Junction BC in reverse I C = β*i B

12 CB configuration 12 B is used in common for in and out Input: r e is considered as AC resistor of diode BE r e =26mV/I E Isolation between in and out Output: I c =αi e 12

13 CB configuration 13 1) Z i = r e (nω-50 Ω) 2) Z o = r o (nmω) 3) A v = αr L /r e R L /r e quite big, U o & U i in phase 4) A i = -α 1 13

14 Characteristic curves: CB 14 Input and output characteristic curves of CB configuration

15 15 CC configuration Similar to CE configuration Refer to Electronic Devices Thomas Floyd 15

16 Limits of operation 16 Two limits: cut-off region Saturation region

17 Cutoff and saturation 17 Cutoff state Saturation state

18 18 DC bias: DC operating point & DC load line

19 DC bias 19 A transistor must be properly biased in order to operate as an amplifier DC bias can be considered as supply power properly to BJT so that NPN: V E < V B < V C (J E : in Forward; J C : in Reverse bias) PNP: V E > V B > V C DC bias is characterized by Q-point (DC operating point) and DC load line

20 DC bias 20 NOTES: REMEMBER some equations: V BE 0,6 0,7V (Si) ; 0,2 0,3(Ge) I E = I C + I B I C = βi B There 3 types of bias circuits Base bias Collector-feedback bias Voltage divider bias I C αi E Question: How many amplifier circuits can be designed?

21 21 3 types of baising Base bias Voltage divider bias Collector feedback bias

22 Example of DC bias 22 Questions: 1. What are the amplifier configuration of these circuits? 2. What kind of DC bias? And then draw DC equivalent circuit. (a) (b) (c) Question: How many amplifier circuits can be designed?

23 Base bias 23 Consider the analysis for only EC configuration (similar analysis can be obtained for BC and CC)

24 Base bias 24 BE loop: Vcc I B R B U BE = 0 I B = (Vcc - U BE )/R B I C =β*i B CE loop: U CE = Vcc - I C R C

25 Voltage divider bias 25 Method 1: Thevenin equivalent circuit: * Group R1, R2 and Vcc can be considered as follows: R BB =R 1 //R 2 V BB = V cc * R 2 /(R 1 +R 2 ) Now it is similar to base-bias analysis Current and voltage do not depend on β Method 2: Approximative analysis If β*r 2 10R 2 -> I 2 I 1 V BB =V cc *R 2 /(R 1 +R 2 ) V E =V BB -U BE I c I e =V e /R e U CE =V cc - I C (R C +R E )

26 Collector-feedback bias 26 BE loop: (1) V cc - I c R C I B R B U BE I E R E =0 (2) I C = β *I B ; I E I C (3) Kirchoof cho dòng tại C: I C = I B + I c I c = I C - I B = (β-1)i B (1)+(2)+(3) I B = (Vcc - U BE )/[R B + β(rc+re)] CE loop: U CE = Vcc I C (R C +R E ) Quite stable

27 Example 27 Analyze the following circuit and determine Q-point and its DC loadline

28 Example 28 Analyze the following circuit and determine Q-point and its DC loadline

29 29 Analysis by method 1

30 Example 30 Analyze the following circuit and determine Q-point and its DC loadline

31 Example 31 Analyze the following circuit and determine Q-point and its DC loadline

32 Example 32 Analyze the following circuit and determine Q-point and its DC loadline

33 33 AC analysis (Small signal analysis)

34 Small signal analysis 34 Small signal analysis: Small signal refers to AC signal with small amplitude that take up a relatively small percentage of an amplifier s operation range (compared to DC power supply) The operation region on amplifier should be in linear BJT model for small signal analysis Represent the BJT by an equivalent circuit that allows to visualize and analyze the operation of BJT as an amplifier

35 35 Example of CE configuration

36 Gain and impedances 36

37 AC equivalent circuit 37

38 EC configuration 38 EC circuit AC eq. circuit (remove R L ) AC eq. circuit (w. T model)

39 EC analysis: results 39 AC eq. circuit (w. T model)

40 Overall gain 40 Voltage gain base to collector and overall gain

41 CC configuration 41 CC circuit AC eq. circuit

42 BC configuration 42 BC circuit AC equivalent circuit

43 Summary 43 EC configuration: power amplifier A u ; A i >1 A P >1 CC configuration: used as load buffer A u ~1; A i >1 BC configuration (emitter follower) A u >1; A i ~1