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An introduction to Junction Transistors BITX20 bidirectional SSB transceiver A BITX20 single stage A simplified single stage +12 V R1 R2 0V A potential divider +12 V +12 V R1 R1 V R2 0V R2 0V An NPN Transistor +12 V R1 Collector Base R2 0V Emitter The transfer resistor (transistor) Emitter (-) Collector (+) Electrons N P Base (Original patent used point contact) N Electrons are negative A silicon atom (Si) ? ? 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 Silicon (group 4) bonds Si Si Si Si Si Si Si Si Si Si Si Si A pure silicon crystal lattice An Arsenic atom (As) Has 5 outer electrons ? One surplus for fitting in to the lattice As ? ? Arsenic doping (group 5) – N type Si Si As Si Si Si Si Si Si Si Si As A Gallium atom (Ga) ? ? Has 3 outer electrons ? Ga ? ? One short for fitting in to the lattice Gallium doping (group 3) – P type Si Ga Si Si Si Si Si + Si + Si Si Ga Si Holes are positive A P-N Junction (N on left) N Type Si Si Depleted Si Si As Si Si P Type Si Si - Si Si Si + Si As Si Si Si Si Ga Si Ga Si Si Si Si Si As Si Si Si Si Ga Si Si Si Ga Si Si Si + Si As Si Si Si Si - Si Si Si + Si As Si Si Si Si Si Si Ga Si Si Si Si Si As Si Si As Si Ga Si Si Si Ga Si Si Si + Si As Si Si Si Si Si As Si Si Si Si Si Si + Si Si Si Si Ga Si Ga 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) Diode junction (BC107 base-emitter) Milliamps 0.5 0.4 0.3 0.2 0.1 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Volts The transfer resistor (transistor) Emitter (-) Collector (+) Electrons N P Base (Original patent used point contact) N Electrons are negative An alloy NPN Transistor (powered up) P Emitter (-) Base N Depletion N Collector (+) Most alloy transistors (e.g. OC71) were germanium PNP 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 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. NPN Transistor circuits • Common base • Emitter follower (common collector) • Common emitter Collector Base Emitter The first transistor circuit: Common base +12V R Output Collector Current Base 0V Variable R -1 V Input Emitter Current Common Emitter +12 V R Input Base Voltage (Positive) 0V Emitter Grounded Diode junction (BC107 base-emitter) Milliamps 0.5 0.4 0.3 0.2 0.1 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Volts Emitter follower Collector to supply line +12 V Input Base Voltage R 0V Output Emitter Voltage Our original circuit •Potential divider bias to linear region +12 V R1 Collector Base •Partly common Emitter Emitter R2 0V •Partly an Emitter follower •Voltage gain set by Collector / Emitter resistor ratios •More in a later talk Questions? (Summary follows) Common Emitter +12 V R Input Base Voltage (Positive) 0V Emitter Grounded 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. Emitter follower Collector to supply line +12 V Input Base Voltage R 0V Output Emitter Voltage 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. Common base +12 V R 0V Input Emitter Voltage (Negative) Output Collector Voltage 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. Appendix A Planar NPN Transistor Collector N Base P Emitter N A Planar PNP Transistor on an N substrate Collector Base P N N substrate Emitter P