Small Signal Analysis of Bjts

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  • Topic: Transistor, Electronic amplifier, Amplifier
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  • Published : December 18, 2012
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Chapter 6 6.1 Small signal analysis
    The term small signal amplifier refers to the use of a signal that takes up a relatively small percentage of an amplifier operational range. With small signal the BJT can be replaced with small signal linear model. This model is also called small signal equivalent circuit. For small ac signals the junction capacitances should be considered in the analysis of the BJT. These junction capacitances can be ignored for DC signals but for actual AC signals they need to be considered & modeled. The figure bellow shows a practical CE transistor amplifier .It consist of different circuit components.

Figure 6.1 Practical single stage CE amplifier circuit

Biasing Circuit
 R1, R2, RE forms of the voltage divider biasing circuit for CE amplifier. It sets up proper operating point for the BJT.

Input Capacitor C1
  It couples the signal (small AC signal, vs) to the base of the transistor. It blocks any DC component present in the signal and passes only ac signal for amplification. Because of this biasing conditions are maintained constant.

Emitter by pass capacitor, CE
   Connected in parallel with RE to provide a low reactance path to the amplified ac signal. If it is not inserted, the amplified signal passes through RE will cause voltage drop across it. This will reduce the output voltage, reducing the gain of the amplifier.

Output coupling capacitor C2
  It couples the output of the amplifier to the load or to the next stage of the amplifier. It blocks DC and passes only AC part of the amplified ac signal.

Phase reversal
      The phase relationship b/n input and output can be determined by considering the effect of positive half cycle & negative half cycle separately. For the positive half cycle of the input signal, ac and dc signals add up at the base increasing forward bias on EB junction and base current increases. Thus due to this effect collector current increases (b/c IC= IB). This...
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