1. Chapter 4 DC Biasing–BJTs

2. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Introduction the transistor is a magical device that can raise the level of the applied ac input without the assistance of an external energy source. In actuality, the improved output ac power level is the result of a transfer of energy from the applied dc supplies. The analysis or design of any electronic amplifier therefore has two components: the dc portion and the ac portion.

3. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Biasing Biasing: T Biasing: The DC voltages applied to a transistor in order to turn it on so that it can amplify the AC signal. The term biasing is an all-inclusive term for the application of dc voltages to establish a fixed level of current and voltage. For transistor amplifiers the resulting dc current and voltage establish an operating point, Since the operating point is a fixed point on the characteristics, it is also called the quiescent point (abbreviated Q-point). DC Biasing – used to set the initial values of I B, I C, and V CE for ac operation of the transistor

4. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Operating Point The DC input establishes an operating or quiescent point Q-point called the Q-point. The Q-Point the combination of I C and V CE on the dc load line where a transistor has no input signal (Quiescent – at rest)

5. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky If no bias were used, the device would initially be completely off, resulting in a Q-point at A. Since it is necessary to bias a device so that it can respond to the entire range of an input signal, point A would not be suitable. For point B, if a signal is applied to the circuit, the device will vary in current and voltage from operating point, but not enough to drive the device into cutoff or saturation. Point C would allow some positive and negative variation of the output signal, but the peak-to-peak value would be limited by the proximity of VCE=0 Point D sets the device operating point near the maximum voltage and power level. The output voltage swing in the positive direction is thus limited if the maximum voltage is not to be exceeded. Point B therefore seems the best operating point in terms of linear gain and largest possible voltage and current swing.

6. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky The Three States of Operation 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 Saturation Region Operation Base–Emitter junction is forward biased Base–Collector junction is forward biased

7. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky DC Biasing Circuits Fixed-bias circuit Emitter-stabilized bias circuit Collector-emitter loop Voltage divider bias circuit DC bias with voltage feedback

8. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Fixed Bias For the dc analysis the network can be isolated from the indicated ac levels, by replacing the capacitors with an open circuit equivalent. In addition, the dc supply VCC can be separated into two supplies (for analysis purposes only)

9. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky

10. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky The Base-Emitter Loop From Kirchhoff’s voltage law: Solving for base current : +V CC – I B R B – V BE = 0

11. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Collector-Emitter Loop Collector current: From Kirchhoff’s voltage law:

12. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Saturation When the transistor is operating in saturation, current through the transistor is at its maximum possible value.

13. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky

14. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Load Line Analysis I Csat CCCC I C = V CC / R C CE V CE = 0 V V CEcutoff CECC V CE = V CC C I C = 0 mA This line called the load line, since it is defined by the load resistance Rc BBwhere the value of R B sets the value of I B CECthat sets the values of V CE and I C The Q-point is the operating point: The end points of the load line are:

15. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Circuit Values Affect the Q-Point more … Effect of lower values of V CC on the load line and Q-point.

16. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Circuit Values Affect the Q-Point more … Effect of increasing levels of R C on the load line and Q-point.

17. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Circuit Values Affect the Q-Point If the level of I B is changed by varying the value of R B the Q- point moves up or down the load line

18. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky

19. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Emitter-Stabilized Bias Circuit Adding a resistor (R E ) to the emitter circuit stabilizes the bias circuit.

20. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Base-Emitter Loop From Kirchhoff’s voltage law: Since I E = (  + 1)I B : Solving for I B :

21. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Collector-Emitter Loop From Kirchhoff’s voltage law: Since I E  I C : Also:

22. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Improved 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 (  ) values. Adding R E to the emitter improves the stability of a transistor. The addition of the emitter resistor to the dc bias of the BJT provides improved stability, that is, the dc bias currents and voltages remain closer to where they were set by the circuit when outside conditions, such as temperature, and transistor beta change.

23. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Saturation Level : V CEcutoff :I Csat : The endpoints can be determined from the load line.

24. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky

25. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky

26. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Voltage Divider Bias This is a very stable bias circuit. any The currents and voltages are nearly independent of any variations in , since it is temperature sensitive especially for silicon transistors

27. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Exact Analysis

28. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky

29. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Approximate Analysis Where I B << I 1 and I 1  I 2 : Where  R E > 10R 2 : From Kirchhoff’s voltage law:

30. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Voltage Divider Bias Analysis Transistor Saturation Level Load Line Analysis Cutoff:Saturation:

31. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Determine the exact and the approximate dc bias voltage V CE and the current I C for the following voltage-divider configuration: EXAMPLE

32. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky

33. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky DC Bias with Voltage Feedback Another way to improve the stability of a bias circuit is to add a feedback path from collector to base. In this bias circuit the Q-point is only slightly dependent on the transistor beta, .

34. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Base-Emitter Loop From Kirchhoff’s voltage law: Where I B << I C : Knowing I C =  I B and I E  I C, the loop equation becomes: Solving for I B :

35. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Collector-Emitter Loop Applying Kirchoff’s voltage law: I E R E + V CE + I ’ C R C – V CC = 0 Since I C  I C and I C =  I B : I C (R C + R E ) + V CE – V CC =0 Solving for V CE : V CE = V CC – I C (R C + R E )

36. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Base-Emitter Bias Analysis Transistor Saturation Level Load Line Analysis Cutoff:Saturation:

37. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Determine the quiescent levels of I CQ and V CEQ for the network EXAMPLE

38. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Transistor Switching Networks Transistors with only the DC source applied can be used as electronic switches.

39. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Switching Circuit Calculations Saturation current: To ensure saturation: Emitter-collector resistance at saturation and cutoff:

40. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Switching Time Transistor switching times:

41. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky 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.

42. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Determine R B and R C for the transistor inverter if I Csat 10 mA. EXAMPLE

43. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky