1. An introduction to Junction Transistors

2. BITX20 bidirectional SSB transceiver

3. A BITX20 single stage

4. A simplified single stage

5. A potential divider

6. An NPN Transistor Base Collector Emitter

7. The transfer resistor (transistor) Emitter (-) Collector (+) Base (Original patent used point contact) Electrons Electrons are negative 

8. A silicon atom (Si) Has 4 outer electrons The outer electron shell needs 8 to be “full” (standing wave pattern) Silicon will try to lend or borrow 4

9. Silicon (group 4) bonds

10. A pure silicon crystal lattice

11. An Arsenic atom (As) Has 5 outer electrons One surplus for fitting in to the lattice

12. Arsenic doping (group 5) – N type

13. A Gallium atom (Ga) Has 3 outer electrons One short for fitting in to the lattice

14. Gallium doping (group 3) – P type Holes are positive

15. A P-N Junction (N on left)

16. What causes the depletion? Electrons move from left to right to fill the + holes Where electrons and holes combine the area is “depleted” of current carriers This leaves the left (N Type) positive so eventually this prevents the depletion spreading any more. Applying negative to N type replaces the depleted carriers and the current resumes (Forward biased diode) Applying positive to the N type removes more electrons and increases the depletion. Almost no current flows. (Reverse biased diode)

17. Diode junction (BC107 base-emitter)

18. The transfer resistor (transistor) Emitter (-) Collector (+) Base (Original patent used point contact) Electrons Electrons are negative 

19. An alloy NPN Transistor (powered up) Emitter (-) Collector (+) Base Depletion Most alloy transistors (e.g. OC71) were germanium PNP

20. Characteristics of transistors Geometry Carrier movement Collector “collection” efficiency (Alpha) Asymmetry: Efficiency / Breakdown voltages NPN transistors are normally better than PNP since electron mobility is better than hole mobility

21. Current gain of transistors For the original “common base” circuit the ratio of collected current to emitted current was measured. This is called Alpha. Values have improved to well over 0.99 (always less than 1). However normally we quote the current gain, called Beta. Beta = Collector current / Base current Beta values of over 200 are common.

22. NPN Transistor circuits Common base Emitter follower (common collector) Common emitter Base Collector Emitter

23. The first transistor circuit: Common base Base

24. Common Emitter

25. Diode junction (BC107 base-emitter)

26. Emitter follower

27. Our original circuit Base Collector Emitter Potential divider bias to linear region Partly an Emitter follower Partly common Emitter Voltage gain set by Collector / Emitter resistor ratios More in a later talk

28. Questions? (Summary follows)

29. Common Emitter

30. Features of Common Emitter High voltage gain High current gain Medium input impedance due to high current gain High output impedance. For HF capacitive loading will need to be resonated reducing bandwidth. Bad HF & bandwidth as falling beta with frequency reduces gain.

31. Emitter follower

32. Features of Emitter followers Voltage gain of almost exactly 1 High current gain High input impedance (due to high current gain) Low output impedance (Good for unknown loads) Good HF & bandwidth as falling beta with frequency matters less.

33. Common base

34. Features of Common Base Current gain of approximately 1 (alpha) Low input impedance (due to low current gain) High output impedance (Base screens collector) High voltage gain (if input impedance matched) Works with a low gain transistor (beta) Good HF & bandwidth as falling beta with frequency matters less.

35. Appendix

36. A Planar NPN Transistor CollectorBaseEmitter

37. A Planar PNP Transistor on an N substrate CollectorBaseEmitter