Bipolar Junction Transistors (BJT)

1. Bipolar Junction Transistors (BJT) NPN PNP

2. BJT Cross-Sections Emitter Collector NPN PNP

3. Common-Emitter NPN Transistor Reverse bias the CBJ Forward bias the BEJ

4. Input Characteristics • Plot IB as f(VBE, VCE) • As VCE increases, more VBE required to turn the BE on so that IB>0. • Looks like a pn junction volt-ampere characteristic.

5. Output Characteristics • Plot IC as f(VCE, IB) • Cutoff region (off) • both BE and BC reverse biased • Active region • BE Forward biased • BC Reverse biased • Saturation region (on) • both BE and BC forward biased

6. Transfer Characteristics

7. Large-Signal Model of a BJT KCL >> IE = IC + IB βF = hFE = IC/IB IC = βFIB + ICEO IE = IB(1 + βF) + ICEO IE = IB(1 + βF) IE = IC(1 + 1/βF) IE = IC(βF + 1)/βF

9. Transistor Operating Point

10. DC Load Line VCC/RC VCC

11. BJT Transistor Switch

12. BJT Transistor Switch (continued)

13. BJT in Saturation

14. Model with Current Gain

15. Miller Effect iout vbe vce

16. Miller Effect (continued)

17. Miller Effect (continued) • Miller Capacitance, CMiller = Ccb(1 – A) • since A is usually negative (phase inversion), the Miller capacitance can be much greater than the capacitance Ccb • This capacitance must charge up to the base-emitter forward bias voltage, causing a delay time before any collector current flows.

18. Saturating a BJT • Normally apply more base current than needed to saturate the transistor • This results in charges being stored in the base region • To calculate the extra charge (saturating charge), determine the emitter current

19. The Saturating Charge • The saturating charge, Qs storage time constant of the transistor

20. Transistor Switching Times

21. Switching Times – turn on • Input voltage rises from 0 to V1 • Base current rises to IB1 • Collector current begins to rise after the delay time, td • Collector current rises to steady-state value ICS • This “rise time”, tr allows the Miller capacitance to charge to V1 • turn on time, ton = td + tr

22. Switching Times – turn off • Input voltage changes from V1 to –V2 • Base current changes to –IB2 • Base current remains at –IB2 until the Miller capacitance discharges to zero, storage time, ts • Base current falls to zero as Miller capacitance charges to –V2, fall time, tf • turn off time, toff = ts + tf

23. Charge Storage in Saturated BJTs Charge storage in the Base Charge Profile during turn-off

24. Example 4.2

25. Waveforms for the Transistor Switch VCC = 250 V VBE(sat) = 3 V IB = 8 A VCS(sat) = 2 V ICS = 100 A td = 0.5 µs tr = 1 µs ts = 5 µs tf = 3 µs fs = 10 kHz duty cycle k = 50 % ICEO = 3 mA

27. Power Loss due to IC for ton = td + tr • During the delay time, 0 ≤t ≤td • Instantaneous Power Loss • Average Power Loss

28. During the rise time, 0 ≤t ≤tr

30. Average Power during rise time

31. Total Power Loss during turn-on

33. Power Loss during the Conduction Period

35. Power Loss during turn offStorage time

37. Power Loss during Fall time

38. Power Loss during Fall time (continued)

40. Power Loss during the off time

41. The total average power losses

42. Instantaneous Power for Example 4.2

43. BJT Switch with an Inductive Load

44. Load Lines