**Low Frequency Response of FET Amplifier:**

The analysis of the Low Frequency Response of FET Amplifier is quite similar to that of the BJT amplifier. Though here common-source configuration is considered but the results can be applied to most FET configurations. For the circuit given in Fig. 15.23, the capacitors C_{in}, C_{out} and C_{S} will determine low frequency response. Now we will examine the impact of each independently.

**Effect of C**_{in} on Frequency Response of FET Amplifier:

_{in}on Frequency Response of FET Amplifier:

For the coupling capacitor C_{in} connected between the applied source and the active device, the general form of the R-C combination is established by the network shown in Fig. 15.24.

The cutoff frequency is given by the equation

For network given in Fig. 15.23

Typically R_{G} ≫ R_{signal} and lower cutoff frequency is determined primarily by R_{G }and C_{in}. The fact that R_{G }is so large permits a relatively low level of C_{in} while maintaining a low frequency level for f_{Li}.

**Effect of C**_{out} on Frequency Response of FET Amplifier:

_{out}on Frequency Response of FET Amplifier:

The output capacitor C_{out} is normally connected between the output of the active device and the load, therefore, the R-C configuration determining the lower cut-off frequency due to C_{out} will appear as shown in Fig. 15.25.

From Fig. 15.25, the total series resistance is R_{out} + R_{L} and the cutoff frequency due to C_{out} is given by the equation

The value of R_{out} for the circuit given in Fig. 15.23 is given by equation

**Determination of Effect of C**_{s} on The Low-Frequency Response:

_{s}on The Low-Frequency Response:

For determination of frequency f_{Ls}, the network “seen” by C_{s} must be determined as illustrated in Fig. 15.26.

Once the level of R_{eq} is determined the cutoff frequency due to C_{s} may be computed from the equation

For Fig. 15.23, the resulting value of R_{eq} is given by the equation

which for r_{d} ≡ α Ω becomes