Fixed-bias circuit Emitter-stabilized bias circuit Collector-emitter loop Voltage divider bias circuit DC bias with voltage feedback

Transcription

1 Chapter 4 Fixed-bias circuit Emitter-stabilized bias circuit Collector-emitter loop Voltage divider bias circuit DC bias with voltage feedback ECET 257 Consumer Power Electronics, PNC 2 1

2 All analysis of circuits in Ch. 4 will be in DC (f=0 Hz) So All capacitors replaced with And all inductors are replaced with ECET 257 Consumer Power Electronics, PNC 3 All circuits shown in the slides and in the book for Chapter 4 are npn transistors. The analysis for pnp transistor biasing circuits is the same as that for npn transistor circuits. The only difference is that the currents are flowing in the opposite direction. ECET 257 Consumer Power Electronics, PNC 4 2

3 ECET 257 Consumer Power Electronics, PNC 5 Applying DC voltages to a transistor in order to turn it on so that it can amplify AC signals. V 0.7 V I = β + 1 I I I = βi ECET 257 Consumer Power Electronics, PNC 6 3

4 DC input establishes an operating or quiescent (Q) point Defines output characteristics based on input (I b ) ECET 257 Consumer Power Electronics, PNC 7 Active or Linear Region Operation Base Emitter junction is forward biased Base Collector junction is reverse biased Cutoff Region Operation Base Emitter junction is reverse biased Saturation Region Operation Base Emitter junction is forward biased Base Collector junction is forward biased ECET 257 Consumer Power Electronics, PNC 8 4

5 Simplest DC Bias configuration R determines the base current for Q-point ECET 257 Consumer Power Electronics, PNC 9 From Kirchhoff s voltage law: +V CC I B R B V BE = 0 Solving for base current: ECET 257 Consumer Power Electronics, PNC 10 5

6 Collector current: From Kirchhoff s voltage law: R affects V, not I ECET 257 Consumer Power Electronics, PNC 11 Current through the transistor is at its maximum possible value. Will cause distortion in output AC signal ECET 257 Consumer Power Electronics, PNC 12 6

7 Find I and I V V and V V I ECET 257 Consumer Power Electronics, PNC 13 ECET 257 Consumer Power Electronics, PNC 14 7

8 The load line end points are: I Csat I C = V CC / R C V CE = 0 V V CEcutoff V CE = V CC I C = 0 ma The Q-point is the operating point where the value of R B sets the value of I B that controls the values of V CE and I C. ECET 257 Consumer Power Electronics, PNC 15 ECET 257 Consumer Power Electronics, PNC 16 8

9 ECET 257 Consumer Power Electronics, PNC 17 ECET 257 Consumer Power Electronics, PNC 18 9

10 Given Fig 4.16 Q-point at 25 µa V CC = 20 V Find V CE I C R C R B ECET 257 Consumer Power Electronics, PNC 19 R E stabilizes the bias circuit. Counters changes in temp and input noise ECET 257 Consumer Power Electronics, PNC 20 10

11 The endpoints can be determined from the load line. V CEcutoff : I Csat : ECET 257 Consumer Power Electronics, PNC 21 Base Current Input Resistance (Fig 4.21) R ECET 257 Consumer Power Electronics, PNC 22 11

12 Since I E I C : Also: ECET 257 Consumer Power Electronics, PNC 23 Find I I V V V V V ECET 257 Consumer Power Electronics, PNC 24 12

13 ECET 257 Consumer Power Electronics, PNC 25 This is a very stable bias circuit. The currents and voltages are nearly independent of any variations in based on temperature. ECET 257 Consumer Power Electronics, PNC 26 13

14 Allows for more exact analysis on any circuit R = R R E = V = I = ( ) V = V I (R + R ) ECET 257 Consumer Power Electronics, PNC 27 Where I B << I C and I C I E : Where R E > 10R 2 : From Kirchhoff s voltage law: ECET 257 Consumer Power Electronics, PNC 28 14

15 Transistor Saturation Level Load Line Analysis Cutoff: Saturation: ECET 257 Consumer Power Electronics, PNC 29 Using General Analysis, find V I Using Approx. Analysis, find V I Conditions dictate which to use ECET 257 Consumer Power Electronics, PNC 30 15

16 Feedback loop improves stability β changes have less impact Q-point less dependent on β ECET 257 Consumer Power Electronics, PNC 31 ECET 257 Consumer Power Electronics, PNC 32 16

17 V CE = V CC I C (R C + R E ) ECET 257 Consumer Power Electronics, PNC 33 Transistor Saturation Level Cutoff Load Line Analysis Saturation ECET 257 Consumer Power Electronics, PNC 34 17

18 Find I V I ECET 257 Consumer Power Electronics, PNC 35 Many other bias configurations are possible, based on the application and the designers preference. Review pgs for these combinations as needed. ECET 257 Consumer Power Electronics, PNC 36 18

19 Pure DC source creates inverting switch circuit Used in logic controls Operates at or above saturation Shorts Collector to ground ECET 257 Consumer Power Electronics, PNC 37 Saturation current: To ensure saturation: Emitter-collector resistance at saturation and cutoff: ECET 257 Consumer Power Electronics, PNC 38 19

20 Given I = 8 ma Find standard values for R R Vi = 10 V Vcc=12 ECET 257 Consumer Power Electronics, PNC 39 Transistor switching times: ECET 257 Consumer Power Electronics, PNC 40 20

21 Approximate voltages V BE.7 V for silicon transistors V CE 25% to 75% of V CC Test for opens and shorts with an ohmmeter. Test the solder joints. Test the transistor with a transistor tester or a curve tracer. Note that the load or the next stage affects the transistor operation. ECET 257 Consumer Power Electronics, PNC 41 21