OLEMISS PHYS 321 - Vacuum Tube (16 pages)

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Vacuum Tube



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Vacuum Tube

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Lecture Notes


Pages:
16
School:
The University of Mississippi
Course:
Phys 321 - Electronics

Unformatted text preview:

Vacuum Tube cathode anode Vacuum diode A Evacuated tubes used in PhotoElectric Effect and early X ray work circa 1890 Vacuum tube invented circa 1903 for radio Marconi group Diode action Triode allowed modulation of anode current by small change in grid voltage 1 Solid State Transistors Lilienfeld Heil Germany Shockley and Pearson Bell Labs Patent circa 1930 Current modulation and gain ICE iBE 100 collector emitter n p n base bipolar n p gate field effect 2 BiPolar Transistor Operation base IB signal VBE E emitter IE N P E IE IC IB IB 0 small IC IE 1 IC IB 1 hFE transistor gain E VCC E N collector IC Quiescent State Open diode action No current flow Conducting State Closed Forward Bias Internal Electric Fields E are cancelled Holes injected into base lower barrier potential VCE 2V to allow electrons to flow across barrier They can be captured by injected holes 3 Cross Section of an NPN Bipolar Device C E B Metal contact n p n p substrate Figure 3 13 4 Transistor Curves IC the current through the collector is almost constant when the transistor is operated in its linear range IC IB http www st andrews ac uk jcgl Scots Guide info comp active BiPolar bpcur html 5 Transistor Curves The base current IB will begin to flow freely when VBE 0 7V We say the transistor has turned on at this point http www st andrews ac uk jcgl Scots Guide info comp active BiPolar bpcur html 6 Amplification and Zin Amplification 1 I E I b I C I E I C 2 I C I b I E I b I b 0 few A 2 VE Vb 0 7V Vb VE I E RE 2 VCC VC VE 0 7V VE VC I C RC V I C RC I R R AV C C C C Vb I E RE I C RE RE Vb Vcc RC V C IC Ib 0 7V AV RC RE IE Input Impedance Im pedance looking in to base RE VE Z IN Vb I b VE I b I E RE I b I E I b RE RE Z IN RE 7 Temperature Dependence The base emitter voltage of the silicon pn junction in a transistor is written as a function of temperature T reference temp T0 bandgap gap voltage Vg collector current IC and current IC0 at T0 is a device constant as IC IC 0 e qV qVg 1 T T0 BE 0 T T0 kT



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