Frequency Response of FET Amplifier

Low-Frequency Response – The low Frequency Response of FET Amplifier circuits is determined by exactly the same considerations as for BJT circuits. The lower cutoff frequency is normally set by a source bypass capacitor, and it can be affected by coupling capacitors.

High Frequency Response

Unlike BJTs, a device cutoff frequency is not normally specified for a FET. Instead, FETs intended for high-frequency operation have the parameters listed as measured at a specified high frequency. The low-frequency parameters are also normally listed. For example, a 2N5484 JFET has the following parameters:

The device inter-terminal capacitances (C_gs, C_gd, and C_ds) are important quantities in determining the performance of a FET circuit, and these are used in the same way as the BJT capacitances. An input capacitance limited cutoff frequency (f_2(i)), and an output capacitance limited cutoff frequency (f_2(o)) can be calculated. The input capacitance is amplified by the Miller effect in the case of a CS circuit (an inverting amplifier). Equation for the FET circuit,

The FET capacitances are specified as the input capacitance (C_iss), the reverse transfer capacitance (C_rss), and the output capacitance (C_oss).

The input capacitance is the sum of the gate-source capacitance and the gate-drain capacitance. The output capacitance is simply the drain-source capacitance.

Typical input capacitance values for a 2N5484 high-frequency JFET are: C_rss = 1 pF, C_iss = 5 pF, C_oss = 2 pF. Because, these are extremely small, the circuit performance can be easily affected by stray capacitance.