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6. BJT small signal

6. BJT small signal - Chapter 5 BJT Small-Signal Analysis...

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Sem I 0809/rosdiyana Chapter 5: BJT Small-Signal Analysis
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Contents Common-Emitter fixed-bias configuration Voltage divider bias CE Emitter bias Emitter-follower configuration Common-base configuration Collector-feedback configuration Hybrid equivalent circuit and model
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r e transistor model – employs a diode and controlled current source to duplicate the behavior of a transistor in the region of interest. The r e and hybrid models will be used to analyze small- signal AC analysis of standard transistor network configurations. Ex: Common-base, common-emitter and common-collector configurations. The network analyzed represent the majority of those appearing in practice today. BJT Small Signal Analysis
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AC equivalent of a network is obtained by: 1. Setting all DC sources to zero 2. Replacing all capacitors by s/c equiv. 3. Redraw the network in more convenient and logical form
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Common-Emitter (CE) Fixed-Bias Configuration The input (Vi) is applied to the base and the output (Vo) is from the collector. The Common-Emitter is characterized as having high input impedance and low output impedance with a high voltage and current gain.
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Removing DC effects of VCC and Capacitors Common-Emitter (CE) Fixed-Bias Configuration
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re Model Determine β , re, and ro: β and ro: look in the specification sheet for the transistor or test the transistor using a curve tracer. r e : calculate r e using dc analysis: E e I 26mV r = Common-Emitter (CE) Fixed-Bias Configuration
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Impedance Calculations Input Impedance: Output Impedance: e B i r || R Z β = e B e i r 10 R r Z β β 2245 O r || R Z C o = c o 10 ro Z R R c 2245 Common-Emitter (CE) Fixed-Bias Configuration
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Gain Calculations Voltage Gain (Av): Current Gain (Ai): Current Gain from Voltage Gain: e o C i o v r ) r || (R V V A - = = C o e C v 10R r r R A - = ) r )(R R (r r R I I A e B C o o B i o i β β + + = = e B C o i r 10 R , 10R r A β β 2245 C i v i R Z A A - = Common-Emitter (CE) Fixed-Bias Configuration
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Voltage Gain e C v C o e o C e b o C b v e b i o C b O i O v r R A 10R or r if r ) r || (R βr I ) r || (R βI A βr I V ) r || (R βI V V V A = = = - = = - = = Common-Emitter (CE) Fixed-Bias Configuration
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Current gain ( 29 ( 29 ( 29 ( 29 C i v i B o B o i o i e B C o e B C o B o i o i e B B C o o i b b o i o i e B B i b e B i B b C o o b o C o b o o R Z A A oo equation t this use can or we β R r βR r I I A , βr 10 R and 10R r if βr R R r βR r I I A βr R R R r β r I I I I I I A βr R R I I and βr R I R I R r β r I I and R r βI r I circuits output and input the to rule divider - current the applying by determined is gain current The - = = 2245 = + + = = + + = = = + = + = + = + =
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