note6 - Handout 6 ECE 315, Cornell University 1 Basic BJT...

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Unformatted text preview: Handout 6 ECE 315, Cornell University 1 Basic BJT Analog Circuits What you will learn: The basic difference between MOSFET and BJT as a circuit component The BJT DC operations and biasing circuits The BJT single-stage amplifiers The BJT cascode circuits and Wilson mirror The BJT diff pair Textbook reading: 5.2, 5.3, 5.5 5.7, 6.2, 6.11, 7.3 Handout 6 Handout 6 ECE 315, Cornell University 2 6.1 BJT Large-/Small-Signal Models Four modes of large-signal operations in the Ebers-Moll model. The input, transfer and output characteristics Small-signal and T models Critical comparison between MOSFET and BJT Textbook reading: 5.1 5.4 Handout 6 ECE 315, Cornell University 3 Two Choices: NPN and PNP Fig. 5.2 Handout 6 ECE 315, Cornell University 4 Ebers-Moll BJT Model of PNP (Large Signal) ( ) ( ) ( ) ( ) 1 1 1 1 / / / / = + = = + = kT qV R kT qV F F Cp Cn C kT qV R R kT qV F Ep En E CB EB CB EB e I e I I I I e I e I I I I E B C I F I R R I R F I F Main current path in active mode I B = I E I C Two components in I B : back injection from base to emitter ( R I R ), and recombination in base ( I F F I F ). Handout 6 ECE 315, Cornell University 5 Forward Active npn: Large Signal This is the similar mode to the MOSFET saturation , where a large A vo (and current gain for BJT) can be obtained: the through current ( I C ) is mostly a function of the V BE , less for V CE . The criteria for npn in the active mode is: forward- bias V BE V ON (around 0.7V) and reverse-bias V BC ( V BC , or V CE V ON ) v CE +- v BE +- v BC +- i B i C i E E C B C C B E i i i i i i i = = + = ( ) + + = = = + = A CE T BE S C B T B E V v V v I i mV q T k V i i 1 exp 1 1 26 1 Handout 6 ECE 315, Cornell University 6 Large Signal Example =100 V BE =0.7V Assume active, then check Fig. 5.34 Given , V CC , R C , R E and V B , Find I E , I B , I C , V E , and V C In forward active mode, V BE is often quite close to 0.7V. It will not be much smaller, as the circuits will be rather slow, and not much bigger, since the exponential function will give too large current!!! Handout 6 ECE 315, Cornell University 7 Large Signal Example Solution Fig. 5.34 Handout 6 ECE 315, Cornell University 8 Large Signals (DC Biasing) and Small Signals (Signals to Be Amplified) in BJT Large Signal DC Biasing Fig. 5.48 Handout 6 ECE 315, Cornell University 9 Large and Small Signals in BJT v CE +- v BE +- v BC +- i B i C i E ( ) T be T be C V v C V v V V S V v V S V v S C V v if V v I e I e e I e I e I i T be T be T BE T be BE T BE << + = = = = + 1 / / / / / Since i B and i C both follow the exponential function of v BE , is the same for large and small signals!!...
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note6 - Handout 6 ECE 315, Cornell University 1 Basic BJT...

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