Chapter 4 DC Biasing BJTs. BJTs

Transcription

1 hapter 4 D Biasing BJTs BJTs

2 Biasing Biasing: The D voltages applied to a transistor in order to turn it on so that it can amplify the A signal.

3 Operating Point The D input establishes an operating or quiescent point called the Q-point.

4 The Three States of Operation Active or Linear egion Operation Base mitter junction is forward biased Base ollector junction is reverse biased utoff egion Operation Base mitter junction is reverse biased Saturation egion Operation Saturation egion Operation Base mitter junction is forward biased Base ollector junction is forward biased

5 D Biasing ircuits Fixed-bias circuit mitter-stabilized bias circuit ollector-emitter loop oltage divider bias circuit D bias with voltage feedback

6 Fixed Bias

7 The Base-mitter Loop From Kirchhoff s voltage law: + B B B 0 Solving for base current: B B B

8 ollector-mitter Loop ollector current: β B From Kirchhoff s voltage law:

9 Saturation When the transistor is operating in saturation, current through the transistor is at its maximum possible value. sat 0

10 Load Line Analysis The end points of the load line are: sat / 0 cutoff 0 ma The Q-point is the operating point: where the value of B sets the value of B that sets the values of and

11 ircuit alues Affect the Q-Point more

12 ircuit alues Affect the Q-Point more

13 ircuit alues Affect the Q-Point

14 mitter-stabilized Bias ircuit Adding a resistor ( ) to the emitter circuit stabilizes the bias circuit.

15 Base-mitter Loop From Kirchhoff s voltage law: B - 0 Since (β + 1) B : Solving for B : - B B - ( β+ 1) B 0 B B - B + ( β+ 1)

16 ollector-mitter Loop From Kirchhoff s voltage law: Since : ( + ) Also: B + B - B +

17 mproved Biased Stability Stability refers to a circuit condition in which the currents and voltages will remain fairly constant over a wide range of temperatures and transistor Beta (β) values. Adding to the emitter improves the stability of a transistor.

18 Saturation Level The endpoints can be determined from the load line. cutoff : sat : 0 ma 0 +

19 oltage Divider Bias This is a very stable bias circuit. The currents and voltages are nearly independent of any variations in β.

20 Where B << 1 and 1 2 : Approximate Analysis B Where β > 10 2 : B B From Kirchhoff s voltage law: ( + )

21 oltage Divider Bias Analysis Transistor Saturation Level sat max + Load Line Analysis utoff: 0mA Saturation: + 0

22 D Bias with oltage Feedback Another way to improve the stability of a bias circuit is to add a feedback path from collector to base. n this bias circuit the Q-point is only slightly dependent on the transistor beta, β.

23 From Kirchhoff s voltage law: Base-mitter Loop BB B 0 Where B << : ' + B Knowing β B and, the loop equation becomes: βb BB B βb 0 Solving for B : B B +β( B + )

24 ollector-mitter Loop Applying Kirchoff s voltage law: Since and β B : Solving for : ( + ) + 0 ( + )

25 Base-mitter Bias Analysis Transistor Saturation Level sat max + Load Line Analysis utoff: 0 ma Saturation: 0 +

26 Transistor Switching Networks Transistors with only the D source applied can be used as electronic switches.

27 Switching ircuit alculations Saturation current: sat To ensure saturation: B > sat β dc mitter-collector resistance at saturation and cutoff: sat sat sat cutoff O

28 Switching Time Transistor switching times: t t + on r t t + off s t t d f

29 Troubleshooting Hints Approximate voltages B.7 for silicon transistors 25% to 75% of 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.

30 PNP 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.