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Electronics-I Lecture 29

# Electronics-I Lecture 29 - EquivalentCircuits 1 2 3 4 5...

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Lecture 29 Electronics I 1 Application of the Small-Signal  Equivalent Circuits 1. Determine the DC operating point of the BJT and the  dc collector current I C 2. Calculate small-signal model parameters g m  = I C /V T , r π  =  β /g m , r e  =  α /g m 3. Eliminate DC sources Short circuit DC voltage sources Open circuit DC current sources 4. Replace the BJT with one of the four small-signal  equivalent circuit models 5. Analyse the resulting circuit to determine the required  quantities Voltage gain, Input Resistance etc.

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Lecture 28 Electronics I 2 Example 5.14 Voltage Gain = ? 2200 β  = 100 DC Operating Point I B  (v = 0 assumed) = (3 – 0.7)/100  = 0.023mA  I C  = I B β  = 2.3 mA V C  = 10 – I C R C  = 3.1 V Active region Small-Signal Parameters   g m  = I C /V T  = 2.3/25 = 92 mA/V r π  =  β /g m  = 100/92 = 1.09 K r e  =  α /g = 0.99/92 = 10.8
Lecture 28 Electronics I 3 Example 5.14 (cont’d) Eliminate DC sources and  replace BJT with equivalent

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Electronics-I Lecture 29 - EquivalentCircuits 1 2 3 4 5...

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