Lecture20-BJT Small Signal Model

Lecture20-BJT Small Signal Model - Lecture 20 Lecture 20...

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ecture 20 Lecture 20 Bipolar Junction Transistors (BJT): Part 4 Small Signal BJT Model Reading: Jaeger 13.5-13.6, Notes ECE 3040 - Dr. Alan Doolittle Georgia Tech
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Further Model Simplifications (useful for circuit analysis) (y ) Ebers-Moll Forward Active Mode Neglect Small Terms T EB T EB T CB T EB V V S C R V V F F C V V R V V F F C e I I I e I I e I e I I 0 0 0 0 1 1 1 ECE 3040 - Dr. Alan Doolittle Georgia Tech
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Modeling the “Early Effect” (non-zero slopes in IV curves) i B1 < i B2 <i B3 I C •Base width changes due to changes in the base- i B3 (theory) collector depletion width with changes in V CB . •This changes , which i B1 (theory) i B2 (theory) V V CE T changes I C , DC and B F A Major BJT Circuit Relationships T EB T EB T EB V V FO S F C B A CE FO F A CE V V S C V V S C e I i i V v V v e I i e I i 1 1 ECE 3040 - Dr. Alan Doolittle Georgia Tech
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Small Signal Model of a BJT •Just as we did with a p-n diode, we can break the BJT up into a large signal analysis and a small signal analysis and “linearize” the non-linear behavior of the Ebers-Moll model. •Small signal Models are only useful for Forward active mode and thus, are derived under this condition. (Saturation and cutoff are used for switches which involve very large voltage/current swings from the on to off states.) •Small signal models are used to determine amplifier characteristics (Example: “Gain” = Increase in the magnitude f a signal at the output of a circuit relative to it’s magnitude at of a signal at the output of a circuit relative to it s magnitude at the input of the circuit). arning: Just like when a diode voltage exceeds a certain Warning: Just like when a diode voltage exceeds a certain value, the non-linear behavior of the diode leads to distortion of the current/voltage curves (see previous lecture), if the ECE 3040 - Dr. Alan Doolittle Georgia Tech inputs/outputs exceed certain limits, the full Ebers-Moll model must be used.
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Consider the BJT as a two-port Network Two Port Network + V 1 - i 1 i 2 + V 2 - i 1 =y 11 v 1 + y 12 v 2 i b =y v be + y 12 v ce General “y-parameter” Network BJT “y-parameter” Network i 2 =y 21 v 1 + y 22 v 2 i c =y 21 v be + y 22 v ce ECE 3040 - Dr. Alan Doolittle Georgia Tech
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Consider the BJT as a two-port Network i b =y 11 v be + y 12 v ce i c =y 21 v be + y 22 v ce ECE 3040 - Dr. Alan Doolittle Georgia Tech
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Consider the BJT as a two-port Network o is most ECE 3040 - Dr. Alan Doolittle Georgia Tech often taken as a constant, F
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Alternative Representations C T C m I V I y g 21 40 Transconductance m o C T o g I V y r 11 1 Input Resistance C CE A o I V V y r 22 1 Output Resistance Y-parameter Model Hybrid-pi Model v 1 ECE 3040 - Dr. Alan Doolittle Georgia Tech
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Alternative Representations b o b m be m i i r g v g Voltage Controlled Current urce version of Hybrid i
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This note was uploaded on 08/23/2011 for the course ECE 3040 taught by Professor Doolittle during the Spring '11 term at University of Florida.

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Lecture20-BJT Small Signal Model - Lecture 20 Lecture 20...

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