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An introduction to Junction Transistors

An introduction to Junction Transistors

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An introduction to Junction Transistors

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  1. An introduction toJunction Transistors

  2. BITX20 bidirectional SSB transceiver

  3. A BITX20 single stage

  4. A simplified single stage

  5. A potential divider

  6. An NPN Transistor Collector Base Emitter

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

  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 (+) Electrons Base Electrons are negative  (Original patent used point contact)

  19. An alloy NPN Transistor (powered up) Depletion Emitter (-) Collector (+) Base 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 Collector Base 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 • 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 Collector Base Emitter

  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 Collector Base Emitter

  37. A Planar PNP Transistor on an N substrate Collector Base Emitter