Analysis of CE amplifier
Voltage gain:
To find voltage gain, consider an unloaded CE amplifier. The ac equivalent circuit is shown in the figure 1. The transistor can be replaced by its collector equivalent model that is a current source and emitter diode which offers AC resistance r'e.
![67_voltage gain.png](https://www.expertsmind.com/../CMSImages/67_voltage gain.png)
Figure 1
The input voltage appears across the emitter diode.
Hence emitter current ie = Vin / r'e.
Since, collector current approximately equals emitter current and iC = ie and vout = - ie RC (The minus sign is used here to indicate phase the inversion)
Further vout = - (Vin RC) / r'e
Therefore voltage gain A = vout / vin = -RC / r'e
The AC source driving an amplifier has to supply alternating current to amplifier. The input impedance of the amplifier determines how much current the amplifier takes from the AC source.
In the normal frequency range of an amplifier, where all the capacitors look like AC shorts and other reactance are negligible, the AC input impedance can be defined as
zin= vin/ iin
Here vin, iin are peak to peak values or RMS values
The impedance looking directly into the base can be symbolized zin (base) and can be given by
Z in(base) = vin / ib ,
Since,v in = ie r'e
» bi b r'e
zin (base) = b r'e.
From AC equivalent circuit, the input impedance zin is parallel combination of R1 , R2 and b r'e.
Zin = R1 || R2 || b r'e
The Thevenin voltage appearing at the output is
vout = A vin
The Thevenin impedance is parallel combination of the RC and internal impedance of the current source. The collector current source is the ideal source; thus it has an infinite internal impedance.
zout = RC.
Simplified AC equivalent circuit is shown in the figure 2.
![1322_voltage gain1.png](https://www.expertsmind.com/CMSImages/1322_voltage%20gain1.png)
Figure 2
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