Advanced JFET Amplifiers

The most popular JFET amplifier configuration is the common source (CS) amplifier because of its straightforward design and high voltage gain. In this arrangement, the source terminal is shared by the input and output, the input is applied at the gate terminal, and the output is obtained from the drain terminal.

Depending on whether the source resistor bypass capacitor is present, there are two kinds of common source amplifiers:

• Common source amplifier with bypass capacitor
• Common source amplifier with unbypassed

Common Source Amplifier with Bypass Capacitor

This circuit establishes the gate voltage using a voltage divider bias network made up of resistors and . Only the AC component can travel through the coupling capacitors and , which separate DC biasing from the AC signal.

A bypass capacitor is linked across the source resistor . In AC analysis, this capacitor is crucial.

Working

• Capacitor is used to apply the input signal.
• Variations in the input signal modify to the gate terminal
• This regulates the drain current .
• A fluctuating voltage across is caused by the fluctuating drain current.
• Capacitor is used to take the output.

For mid-frequency analysis:

• Short circuits are created by coupling capacitors and .
• The bypass capacitor acts similarly to a short circuit.
• Consequently, is successfully taken out of the AC route.

Small Signal Analysis

Input impedance:

Output impedance:

Voltage gain:

Key Behavior

Since is bypassed by , there is no degeneration effect, which results in:

• Higher voltage gain
• No negative feedback in AC operation
• Phase inversion of output ( shift)

Common Source Amplifier with Unbypassed

The source resistor in this setup is not circumvented by a capacitor. As a result, it stays in the AC equivalent circuit and has an immediate impact on amplifier performance.

Working

• Capacitor is used to apply the input signal to the gate.
• Changes in regulate the drain current
• Capacitor provides the output at the drain.

Given that is found in the AC path, it creates negative feedback.

Effect of Unbypassed

• When rises, the voltage across increases,
• Results in more negative
• As a result, decreases

The circuit is stabilized by this feedback.

Small Signal Analysis

The presence of modifies the gain expression.

Voltage gain:

assuming

Output impedance:

Input impedance:

Key Behavior

Due to the presence of in AC analysis:

• Voltage gain is reduced
• Stability is improved
• Distortion is reduced
• Negative feedback is introduced

Comparison