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Small Signal Amplifier

We are now in a position to analyse a small signal amplifier circuit with source and load. The amplifier in Figure 110 is called a common emitter amplifier.   The term small signal refers to relatively small excursions about the DC operating point Q. 

The component values we use in this section are: $R_{B1}=68k\Omega$, $R_{B2}=15k\Omega$, $R_C=2.2k\Omega$, $R_E=1.0k\Omega$, $R_S = 10k\Omega$, $R_L = 2.7k\Omega$, C_S = 0.1 $\mu$F, C_E=22 $\mu$F, C_C=1 $\mu$F, and V_CC=25 V.

  

Figure 110: Small signal common-emitter BJT transistor amplifier.

The amplifier has a voltage divider bias network with a bypass capacitor  C_E across R_E. The value of C_E is chosen so that its reactance at AC frequencies of interest is very small, so that it provides an AC short circuit. The load is modelled by a single resistance R_L, connected to the output of the amplifier (collector terminal) via a coupling capacitor   C_C. The source is modelled by a Thevenin circuit consisting of a voltage source v_s and a resistor R_S, and is connected to the input of the amplifier by a coupling capacitor C_S. The coupling capacitors are chosen to provide an AC short circuit. The capacitors provide a blocking of DC (open circuit).

We will analyse the circuit in steps, beginning with an analysis of the amplifier alone, and later taking into account the effect of the load and source.

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