Lecture_18_first_posting

Lecture_18_first_posting - Lecture 18-19 Basic BJT Analog...

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Basic BJT Analog Circuits Difference between MOSFET and BJT as a circuit component The BJT DC operations and biasing circuits The BJT single-stage amplifiers Textbook reading: Sedra Chap 5 Lecture 18-19
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Two Choices: NPN and PNP Fig. 5.2
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Physical Mechanisms Inside NPN in the Active Mode Fig. 5.3
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Minority Carrier Concentration Inside NPN in the Active Mode p n not important here due to reverse bias N E >> N B : gain N B > N C : base width modulation W B << L B : gain base p emitter n n p << base p emitter n n p 0 0 << sustaining large output voltage depletion region into base: base width modulation Fig. 5.4
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Physical realization of the diode. Note that V BE junction area and V CB junction areas are not equal. This is taken into account in the Ebers-Moll model
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Four Modes in BJT Mode Base junction Collector junction V BE V BC V CE Forward Active Forward bias Reverse bias npn: small + large + large + pnp: small - large - large - Inverted Active Reverse bias Forward bias npn: large + small + large - pnp: large - small - large + Saturation Forward bias Forward bias npn: small + small + small + pnp: small - small - small - Cutoff Reverse bias Reverse bias npn: large - large + unsure pnp: large + large - unsure Words of caution: in MOSFET, when both S and D junctions are forward biased: linear, S forward and D reverse: saturation!!
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i C vs v CB Minority carrier concentration in saturation region i C vs v BE Operation in saturation mode Common base characteristic
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Ebers-Moll BJT Model of NPN (Large Signal) ( 29 ( 29 ( 29 ( 29 1 1 1 1 / / / / - - - = + - = - - - = - = kT qV R S kT qV S F F R C kT qV S kT qV F S R R F E BC BE BC BE e I e I I I I e I e I I I I α α α α E B C I F I R α R I R α F I F Main current path in active mode Main current path in inverted mode I B = I E - I C I FO and I RO are the individual diode sat currents Two components in I B : back injection from base to emitter ( α R I R ), and recombination in base ( I F - α F I F ). Junction laws: α F I F0 = α R I R0 I S Table 5.3
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NPN Common-Emitter Gummel Plot V BE V CE V BE log( I B , I C ) Limited by series resistance Slope: η × 60mV/decade of current I B I C current gain β BJT can maintain exponential increase of current only limited by the series resistance. Input characteristics Input characteristics (in term of r π ) Transfer characteristics (in term of g m )
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NPN Common-Emitter IV Characteristics Output characteristics (in term of r o ) Transfer characteristics (in term of g m ) Early Voltage C A I V r ' 0 =
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NPN Common-Emitter Current Gain Increasing I B I B V CE Output characteristics (in term of r o ) Transfer characteristics (in term of β ) B CEsat forced I I = β
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Forward Active npn: Large Signal This is the similar mode to the MOSFET saturation , where a large g m or current gain can be obtained: the main output current ( I C ) is mainly a function of the input voltage V BE , less for V CE .
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