Frequency Response of Common Source Amplifier

Frequency Response of Common Source Amplifier:

Let us consider a typical common source amplifier as shown in the above figure.

From above figure, it shows the high frequency equivalent circuit for the given amplifier circuit. It shows that at high frequencies coupling and bypass capacitors act as short circuits and do not affect the amplifier high frequency response. The equivalent circuit shows internal capacitances which affect the high frequency response.

Using Miller theorem, this high frequency equivalent circuit can be further simplified as follows:

The internal capacitance Cgd can be splitted into Cin(miller) and Cout(miller) as shown in the following figure.

From simplified high frequency equivalent circuit, it has two RC networks which affect the high frequency response of the amplifier. These are,

1.                 Input RC network

2.                 Output RC network

Input RC network:

This network is further reduced as follows since Rs << RG

Output RC network:

Fig. Output RC network

The critical frequency for the above circuit is,

It is not necessary that these frequencies should be equal. The network which has lower critical frequency than other network is called dominant network.

The phase shift in high frequency is