ch1_BJT_01-14 - ELEC202 Electronic Circuits II BJT Bipolar...

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ELEC202 – Electronic Circuits II BJT Last updated: 8-Sep-03 1 Bipolar Transistor (BJT) Modes of Operation Note: Amplifier - BJT operates at Active Forward Region Switch - BJT operates between Cutoff and Saturation Regions BJT in Active Mode Operations: n n p Collector C Emitter E B Base E B C Ideal Structure (npn) Symbol (npn) Actual Structure in IC (npn) N Emitter > N Base > N collector Reverse E/B reverse C/B forward Saturation Both forward Active Forward E/B forward C/B reverse Cutoff Both reverse V BC(on) V BE(on) 0 V BC V BE V CE =V CB +V BE V CE(sat) = V BC(on) + V BE(on) 0.5V + 0.7V 0.2V Typical values V BE(on) = 0.6V - 0.7V V CE(sat) = 0.2V- 0.3V C E V CB > 0 V BE >V BE(on) C/B junction Reverse bias E/B junction Forward bias Good BJT transistor requires: (1) doping concentration in Emitter (N D ) >> in Base (N A ) (2) very narrow base width W (3) large Emitter-Base junction A E n+ n p C B E I C I E I B Collected electrons diffusing electrons Injected holes B n p n+ W
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ELEC202 – Electronic Circuits II BJT Last updated: 8-Sep-03 2 Equations: Recall = T D S D nV V I I exp Collector Current: = T BE S C V V I I exp , n = 1 W N n qD A I A i n E S 2 = W N A A E Base Current: I B is a factor of I C . β C B I I = Emitter Current: B C E I I I + = C C C I I I β β β 1 + = + = or E C i i α = where α α β β β α = + = 1 1 or α – common-base current gain for large β , say 100, 1 99 . 0 101 100 = α β = 20, 95 . 0 21 20 = α + V D I D I C is a function of V BE and independent of collector voltage as long as C/B junction is reverse biased. A E = E/B junction area, D n = diffusion constant N A = Base doping concentration, W = base width n i = intrinsic carrier concentration = 1.5×10 10 carrier/cm 3 where β - current gain (common-emitter current gain) - function of W and N D / N A ratio β with W and/or N D / N A typical values of β is between 50 - 200
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ELEC202 – Electronic Circuits II BJT Last updated: 8-Sep-03 3 Large Signal Model Common Emitter Configuration ( π -model ) Common Base Configuration ( T-model ) Voltage-Controlled Current Source Current-Controlled Current Source pnp transistor I C I B Size of this diode = (I S / β ) B C E + V BE T BE S V V I exp B (I S / α ) C E T BE S V V I exp β = common-emitter current gain α = common-base current gain B (I S / α ) C E α I E I E p p n Collector E Emitter C B Base Ideal Structure (pnp) Symbol (pnp) Actual Structure in IC (pnp) In general, β pnp < β npn I C I B I E B C E E B C n p-substrate p+ p+ E β I B I C I B (I S / β ) B C
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ELEC202 – Electronic Circuits II BJT Last updated: 8-Sep-03 4 Voltage Polarities and Current Flow = T BE S C V V I I exp = T EB S C V V I I exp B C I I β = B C I I β = E C I I α = E C I I α = DC Analysis: Example 1: Find V C , V B , V E , I C , I B and I E . Assume β = 100, V BE = 0.7V at I C = 1mA.
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