1. The common emitter (CE) amplifier circuit uses an NPN transistor with a signal (Vs) to be amplified, a source resistance (Rs), and a load resistance (Rl) across which the amplified output voltage is obtained. 2. The dependent variables of the CE amplifier (Vbe and Ic) can be expressed in terms of the independent variables (Ib and Vce) using hybrid parameters. 3. The key equations for the CE amplifier are: current gain (Aie) equals -hfe/(1+hoeRl), input impedance (Zie) equals hie + hreAieRl, where hfe, hie, hre, and hoe are

1. B.Sc II Sem III
-Prof D.B.Bobade

2. COMMON EMITTER AMPLIFIER
 The CE amplifier circuit using npn transistor is as shown in fig:

3.  Here, Vs is the signal which is to be amplified. Rs is source resistance, Rl is
load resistance across which amplified o/p voltage is obtained. VBB is base
supply voltage and VCC is collector supply voltage for the biasing of transistor.
 Here, Vbe and Ic are dependent variables and Ib and Vce are independent
variables. Hence, dependent variables can be expressed as:
Vbe = hie.Ib + hre.Vce ------(1)
Ic = hfe.Ib + hoe.Vce ------(2)
 Hence, using equations 1 and 2, hybrid equivalent circuit of CE amplifier is as
shown:

4.  Now, find out the equation for current gain, voltage gain, i/p impedance and
o/p admittance of the amplifier.
1. CURRENT GAIN (Aie):
We know that Aie = IL/Ib = -Ic/Ib ----(3) as current flows into the transistor.
But from equation 2, Ic = hfe.Ib + hoe.VCE and VCE = -Ic.Rl.
Hence, Ic = hfe.Ib + hoe(-Ic.Rl).
Hence, Ic + hoe.Ic.Rl = hfe.Ib
Ic(1+hoe.Rl) = hfe.Ib
Hence, Aie = -Ic/Ib = -hfe/(1+hoe.Rl) -----(4)
This is the required equation of Aie of CE amplifier.
2. INPUT IMPEDANCE (Zie):
It is the ratio of i/p voltage to i/p current and given by
Zie = Vbe/Ib -----(5)
But from equation 1, Vbe = hie.Ib + hre.Vce and Vce = -Ic.Rl
Hence, Vbe = hie.Ib + hre(-Ic.Rl)

5. Divide this equation by Ib. This implies Vbe/Ib = hie + hre(-Ic/Ib)Rl. But Ic/Ib= -
Aie. Hence, Zie = hie + hre.Aie.Rl -----(7)
This is the required equation of i/p impedance of CE amplifier. Thank you