1. Transistor Circuit DC Bias Part 1 ENGI 242

2. February 2003ENGI 2422 DC Biasing Circuits Fixed-Bias Circuit Emitter-Stabilized Bias Circuit Collector-Emitter Loop Voltage Divider Bias Circuit DC Bias with Voltage Feedback Miscellaneous Bias Circuits

3. February 2003ENGI 2423 Maximum Power Curve

4. February 2003ENGI 2424 Fixed-Bias Circuit

5. February 2003ENGI 2425 DC Equivalent circuit

6. February 2003ENGI 2426 Base-Emitter (Input) Loop Using Kirchoff’s voltage law: – V CC + I B R B + V BE = 0 Solving for I B :

7. February 2003ENGI 2427 Collector-Emitter (Output) Loop Since: I C =  I B Using Kirchoff’s voltage law: V CE – V CC – I C R C Because: V CE = V C - V E Since V E = 0V, then: V C = V CE Also: V BE = V B - V E with V E = 0V, then: V B = V BE

8. February 2003ENGI 2428 BJT Saturation Regions When the transistor is operating in the Saturation Region, the transistor is conducting at maximum collector current (based on the resistances in the output circuit, not the spec sheet value) such that:

9. February 2003ENGI 2429 Determining I csat

10. February 2003ENGI 24210 Determining Icsat for the fixed-bias configuration

11. February 2003ENGI 24211 Load Line Analysis

12. February 2003ENGI 24212 Load Line Analysis The end points of the line are : I Csat and V CE cutoff For load line analysis, use V CE = 0 for I CSAT, and I C = 0 for V CEcutoff I Csat : V CEcutoff : Where I B intersects with the load line we have the Q point Q-point is the particular operating point: Value of R B Sets the value of I B Where I B and Load Line intersect Sets the values of V CE and I C.

13. February 2003ENGI 24213 Circuit values effect Q-point

14. February 2003ENGI 24214 Circuit values effect Q-point (continued)

15. February 2003ENGI 24215 Circuit values effect Q-point (continued)

16. February 2003ENGI 24216 DC Fixed Bias Circuit Example

17. February 2003ENGI 24217 Load-line analysis

18. February 2003ENGI 24218 Fixed-bias load line

19. February 2003ENGI 24219 Movement of Q-point with increasing levels of I B

20. February 2003ENGI 24220 Effect of R C on the load line and Q-point

21. February 2003ENGI 24221 Effect of V CC on the load line and Q-point

22. February 2003ENGI 24222 Example

23. Emitter Stabilized Bias

24. February 2003ENGI 24224 Emitter-Stabilized Bias Circuit Adding a resistor to the emitter circuit (between the emitter lead and ground) stabilizes the bias circuit

25. February 2003ENGI 24225 Improved Bias Stability The addition of R E to the Emitter improves the stability of a transistor Stability refers to a bias circuit in which the currents and voltages will remain fairly constant for a wide range of temperatures and transistor forward current gain (  ) The temperature surrounding the transistor circuit is not always constant Therefore, the transistor  is not a constant value

26. February 2003ENGI 24226 Base-Emitter Loop

27. February 2003ENGI 24227 Equivalent Network

28. February 2003ENGI 24228 Reflected Input impedance of R E

29. February 2003ENGI 24229 Base-Emitter Loop Applying Kirchoffs voltage law:- V CC + I B R B + V BE +I E R E = 0 Since: I E = (  + 1) I B We can write: - V CC + I B R B + V BE + (  + 1) I B R E = 0 Grouping terms and solving for I B : Or we could solve for I E with:

30. February 2003ENGI 24230 Collector-Emitter Loop

31. February 2003ENGI 24231 Collector-Emitter Loop Applying Kirchoff’s voltage law: - V CC + I C R C + V CE + I E R E = 0 Assuming that I E  I C and solving for V CE : I C = V CC – V CE – (R E + R C ) Solve for V E : V E = I E R E Solve for V C : V C = V CC - I C R C or V C = V CE + I E R E Solve for V B : V B = V CC - I B R B or V B = V BE + I E R E

32. February 2003ENGI 24232 Transistor Saturation At saturation, V CE is at a minimum We will find the value V CEsat = 0.2V For load line analysis, we use V CE = 0 To solve for I CSAT, use the output KVL equation:

33. February 2003ENGI 24233 Load Line Analysis The load line end points can be calculated: At cutoff: At saturation:

34. February 2003ENGI 24234 Emitter Stabilized Bias Circuit Example

35. February 2003ENGI 24235 Load Line For The Emitter-bias Configuration.