L21_trans_amplifier

L21_trans_amplifier - ESC102 Introduction to Electronics...

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Transistor Amplifiers ESC102 : Introduction to Electronics A R Harish Dept. of EE, IIT Kanpur Sept 16, 2010

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Transistor Circuit Analysis Circuit V O +v o V IN (dc) v in V IN (dc) V O dc Circuit small signal Circuit v in v o dc analysis Small signal analysis
Small Signal Model or ac Model c b r π e + v π - g m v π T T BQ CQ V V r I I π β = = CQ m T I g V = The small signal model is valid only when 26 300 T kT v V mV at K q π << = = This model is valid for both npn and pnp transistors I B1 I B2 I B4 I B3 r 0 not completely flat v π

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Complete Analysis: dc +ac 1. Dc analysis Capacitor is like an open circuit under dc v in v 0 V 0
1. Dc analysis 0.7 CC BQ B V I R - = CQ BQ I I β = × ( ) O CC CQ C V dc V I R = - × mA I BQ 0215 . 0 = mA I CQ 15 . 2 = V V 85 . 2 0 = Can you plot the load line? β =100 V 0

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DC voltage sources are shorted and dc current sources are open circuited Analysis is done at frequencies for which impedance due to capacitor is small so that capacitor can be considered as short.
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