Circuit diagram of CE and CS amplifier is shown in Figure 1 and frequency response in Figure 2.

Input signal of constant amplitude is applied at the base and gate of transistor and JFET & output is obtained at collector and drain of transistor and JFET.

From the frequency response it is seen that voltage gain decreases at low frequency. This decrease in gain is due to coupling capacitor i.e. Cin, Cout, Ce (for BJT) & Cg, Cc,Cs (for JFET).

Let us consider effect of Cin with Cout=Ce=∞ (for BJT). Hence ac equivalent circuit using h-model becomes,

Rin=R1||R2||hie and Rc’ = Rc||RL

Where,

$Vbe=\frac{Vin Rin}{\sqrt{Xcin^2+Rin^2}}$--------------(1) using Voltage Divider Rule at Input

$Xcin=\frac{1}{2 \pi f Cin}$

As input signal frequency decreases, Xcin increases hence Vbe decreases. Due to this Ib will also decrease, hence this result in decrease in Ic.This decrease in Ic passes through Rc’ which results in decrease in output voltage (Vo= - IcRc’) which decreases gain of CE amplifier (Av=Vo/Vin).