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

Transcription

1 D iasing JT

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

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

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

5 D iasing ircuits Fixed-bias circuit mitter-stabilized bias circuit oltage divider bias circuit D bias with voltage feedback mitter Follower configuration ommon base configuration

6 Fixed ias

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

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

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

10 The end points of the load line are: sat = / Load Line Analysis = 0 cutoff = = 0 ma The Q-point is the operating point: where the value of sets the value of 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 ias ircuit Adding a resistor ( ) to the emitter circuit stabilizes the bias circuit.

15 ase-mitter Loop From Kirchhoff s voltage law: Since = ( + 1) : ( 1) 0 Solving for : - ( 1)

16 ollector-mitter Loop From Kirchhoff s voltage law: 0 Since : ( ) Also: -

17 mproved iased 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 eta () 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 ias This is a very stable bias circuit. The currents and voltages are nearly independent of any variations in.

20 xact Analysis

21 xact Analysis (cont.)

22 Approximate Analysis Where << 1 and 1 2 : Where > 10 2 : From Kirchhoff s voltage law: ) (

23 oltage Divider ias Analysis Transistor Saturation Level sat max Load Line Analysis utoff: 0mA Saturation: 0

24 D ias with oltage Feedback/ ollector Feedback onfiguration 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,.

25 From Kirchhoff s voltage law: ase-mitter Loop 0 Where << : ' Knowing = and, the loop equation becomes: 0 Solving for : ( )

26 ollector-mitter Loop Applying Kirchoff s voltage law: + + = 0 Since and = : ( + ) + =0 Solving for : = ( + )

27 Saturation Analysis Transistor Saturation Level sat max Load Line Analysis utoff: 0 ma Saturation: 0

28 mitter follower onfiguration oltage is taken off from collector Saturation:

29 mitter follower onfiguration (cont.)

30 ommon ase onfiguration

31 ommon ase onfiguration (cont.)

32 Design Problem xample

33 Design Problem: Solution

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

35 i = 5 Saturation current: sat To ensure saturation: Switching ircuit alculations sat dc

36 i= 0

37 mitter-collector resistance at saturation and cutoff: sat sat sat cutoff O

38 Switching Time Transistor switching times: t on t r t d t off t s t f t r td ise time 10% to 90% Delay time t s t f Storage time Fall time 90% to 10%

39 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.

40 PNP Transistors (cont.) ase-emitter loop ollector-emitter loop

41 Troubleshooting Hints Approximate voltages.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.