Chapter 6. Bipolar Junction Transistors (BJTs)

1. Chapter 6. Bipolar Junction Transistors (BJTs)

2. Bipolar Junction Transistor • Three terminal device • Voltage between two terminals to control current flow in third terminal • Invented in 1948 at Bell Telephone Laboratories • Dominant until late 1980’s • Reliable under harsh operating conditions • High frequency applications • High speed designs • High power applications

3. npn transistor • n-type emitter (E) region, p-type base (B) region, n-type collector (C) region • Two pn junctions (naming basis for bipolar junction transistor) • Modes • Active: used for amplifier design • Cutoff • Saturation: used for logic design • Reverse active: limited operation

4. pnp transistor – dual of npn transistor

5. Active Mode of npn Transistor

8. Circuit Models for Active Mode npn Transistor

9. Practical Implementation E and C are not symmetrical. pnp transistors works dual to npn transistors much in the same way PMOSFET works dual to NMOSFET. (In this class, we will concentrate on npn transistors.)

10. Circuit Symbols for npn Transistors Biasing in active mode Directions of current flow

12. iC – vBECharacteristics Temperature Dependence

13. Common Base Characteristics In active region, vCB ≥ - 0.4 V Base voltage is fixed at zero.

14. Dependence of iC on Collector Voltage

15. Circuit Models with Output Resistance ro

16. Common Emitter Configuration

18. Common Emitter Saturation Model

19. Designing Linear Amplifiers (Active Region)

20. Amplifier Gain

22. Graphical Analysis

24. To determine iB, iC and vCE, you need to use both graphs.

25. Quiescent point must be selected to give a symmetric output swing.

32. Input part 10 V RB1 RBB RB1 + VBB + + 10 V RB2 _ _ VBB VBB _ RB2

34. Biasing BJT • Determining a quiescent point for linearization • Active mode operation • Considerations • Stable with respect to manufacturing parameters (e.g., ro, β) • Desired gains • Acceptable output swing

35. Biasing with Single Power Supply • Fix VBE or IB. • Output directly depends on β • Unstable with respect to temperature variation

36. Addition of Degeneration Resistor

38. Biasing with Two Power Supplies

39. Biasing with Feedback Resistor

40. Biasing with Current Source

41. Small Signal Analysis • A quiescent point has been determined by biasing. • Active mode operation • Forward biasing for base-emitter junction by VBE • Reverse biasing for collector-base junction by RC and VCC

44. The transistor performs as a voltage controlled current source with gain gm when input varies by 10 mV or less.

48. Hybrid π Model • Short circuit voltage sources • Open circuit current sources • Short circuit capacitors

50. T Model