Frequency Response of JFET Amplifier

A JFET amplifier's frequency response explains how the amplifier's gain changes with frequency. At all frequencies, the amplifier's gain is not consistent. Rather, the gain falls at low and high frequencies and stays about constant in the mid-frequency region.

• Coupling and bypass capacitors have an impact on the gain at low frequencies.
• The JFET's internal capacitances control the circuit is behavior at high frequencies.

Low Frequency Analysis

Capacitors reactance greatly increases at low frequencies. Consequently, coupling and bypass capacitors start to influence the signal instead of functioning as short circuits.

Two RC networks affect the low-frequency response of a typical JFET amplifier:

• Input coupling capacitor and input resistance combine to produce the input RC network.
• Output coupling capacitor and output resistance combine to produce the output RC network.

The amplifier's lower cutoff frequency is determined by these RC networks.

Input RC Network

The input coupling capacitor and the amplifier's input resistance combine to generate the input RC network.

Working

• The capacitor receives the input signal
• The reactance of rises at low frequencies
• As a result, less input signal reaches the gate
• Consequently, the gain reduces

Equivalent Resistance

Since the gate current is very small, the input resistance is usually very high.

Lower Cutoff Frequency

This frequency establishes the threshold below which the input network causes the gain to begin declining.

Output RC Network

The circuit is output resistance and output coupling capacitor combine to form the output RC network.

Working

• Capacitor receives the output signal
• The reactance of rises at low frequencies
• As a result, the output voltage across the load is decreased
• Hence, gain decreases

Equivalent Resistance

Lower Cutoff Frequency

Dominant Low-Frequency Network

• The low-frequency response is dominated by the input and output RC networks with the higher cutoff frequency.
• This network establishes the amplifier's effective lower cutoff frequency.

High Frequency Analysis

The coupling and bypass capacitors function as short circuits at high frequencies and have no effect on the behavior of the circuit. Rather, the JFET's intrinsic capacitances start to matter.

The following internal capacitances are crucial:

• Gate-to-source capacitance ()
• Gate-to-drain capacitance ()

The high-frequency response is impacted by the additional RC networks introduced by these capacitances.

Miller Effect

Between the input and output terminals is the capacitance (). Miller's theorem significantly increases this capacitance due to amplification.

The capacitances that are equivalent are:

As a result, the high-frequency performance is decreased and the effective input capacitance is increased.

Input RC Network

The high-frequency input RC network is created by:

• Source resistance ()
• Gate resistance ()
• Capacitances () and ()

Cutoff Frequency

This determines the upper cutoff frequency due to the input side.

Output RC Network

The output RC network is formed by:

• Drain resistance ()
• Load resistance ()
• Output capacitance ()

Cutoff Frequency

Dominant High-Frequency Network

• The RC network with the lowest cutoff frequency controls the response at high frequencies.
• The amplifier's top cutoff frequency is set by this network.