Transistor Circuit Notes

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Aero2 Signals & Systems (Part 2) Notes on BJT and transistor circuits Bipolar Junction Transistors ã Physical Structure & Symbols ã NPN n-type Emitter region p-type Base region n-type Collector region C Collector (C) B E (b) Emitter (E) Emitter-base junction (EBJ) Base (B) (a) Collector-base junction (CBJ) ã PNP - similar, but: ã N- and P-type regions interchanged ã Arrow on symbol reversed ã Operating Modes Operating mode Cut-off Active Saturation Reverse-active EBJ Reverse Forward Forw
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  Aero2 Signals & Systems (Part 2)Notes on BJT and transistor circuits (Based on Dr Holmes’ notes for EE1/ISE1 course) 1 Bipolar Junction Transistors   ã Physical Structure & Symbols   ã NPN  (b)(a)BCE n-typeCollectorregionp-typeBaseregionn-typeEmitterregion Emitter(E)Collector(C)Base(B) Emitter-baseunction (EBJ)Collector-baseunction(CBJ)   ã PNP - similar, but:ã N- and P-type regions interchangedã Arrow on symbol reversed   ã Operating Modes   Cut-off ActiveSaturationReverse-active ReverseForwardForwardReverseReverseReverseForwardForwardOperating modeEBJCBJ  ã Active Mode   - voltage polarities for NPN  BCEV CB > 0V BE > 0I B I C I E    Aero2 Signals & Systems (Part 2)Notes on BJT and transistor circuits (Based on Dr Holmes’ notes for EE1/ISE1 course) 2 BJT - Operation in Active Mode I En electronsnpnI Epholes E { CBI B I E I Crecombination   ã I En , I Ep both proportional to exp(V BE /V T )ã I C   ≈ I En   ⇒ I C   ≈ I S exp(V BE /V T ) (1.1)ã I B   ≈ I Ep << I En   ⇒ can write I C = β I B where β large (1.2)ã I S = SATURATION CURRENT (typ 10 -15 to 10 -12 A)ã V T = THERMAL VOLTAGE = kT/e ≈ 25 mV at 25 ° Cã β = COMMON-EMITTER CURRENT GAIN (typ 50 to 250)ã Active Mode Circuit Model   BECI B β I B I C I E = I B + I C    Aero2 Signals & Systems (Part 2)Notes on BJT and transistor circuits (Based on Dr Holmes’ notes for EE1/ISE1 course) 3 BJT Operating Curves - 1 ã INPUT-OUTPUT I C vs V BE (for I S = 10 -13 A) BCEV CB > 0V BE I C 204060801000.80.60.40.20.0 V BE (V)I C (mA) ACTIVECUT-OFF   ã ACTIVE REGION:ã I C   ≈ 0 for V BE < ≈ 0.5 V   ã I C rises very steeply for V BE > ≈ 0.5 V   ã V BE   ≈ 0.7 V over most of useful I C rangeã I B vs V BE similar, but current reduced by factor β  ã CUT-OFF REGION:ã I C   ≈ 0ã Also I B , I E   ≈ 0  Aero2 Signals & Systems (Part 2)Notes on BJT and transistor circuits (Based on Dr Holmes’ notes for EE1/ISE1 course) 4 BJT Operating Curves - 2 ã OUTPUT I C vs V CE (for β = 50)  BCEV CE I C I B 024681012210I C (mA)V CE (V) I B = 200 µAI B = 160 µAI B = 120 µAI B = 80 µAI B = 40 µASATACTIVE   ã ACTIVE REGION (V CE > V BE ):ã I C = β I B , regardless of V CE  i.e. CONTROLLED CURRENT SOURCEã SATURATION REGION (V CE < V BE ):ã I C falls off as V CE   → 0ã V CEsat   ≈ 0.2 V on steep part of each curveã In both cases:ã V BE   ≈ 0.7 V if I B non-negligible
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