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 R_S

Common Source Amplifier with Bypass Capacitor

This circuit establishes the gate voltage using a voltage divider bias network made up of resistors R_1 and R_2. Only the AC component can travel through the coupling capacitors C_1 and C_2, which separate DC biasing from the AC signal.

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

Working

• Capacitor C_1 is used to apply the input signal.
• Variations in the input signal modify V_{GS} to the gate terminal
• This regulates the drain current I_D.
• A fluctuating voltage across R_D is caused by the fluctuating drain current.
• Capacitor C_2 is used to take the output.

For mid-frequency analysis:

• Short circuits are created by coupling capacitors C_1 and C_2.
• The bypass capacitor C_S acts similarly to a short circuit.
• Consequently, R_S is successfully taken out of the AC route.

Small Signal Analysis

Input impedance:

Z_i = R_G = R_1 \parallel R_2

Output impedance:

Z_o = r_d \parallel R_D

Voltage gain:

A_v = -g_m (r_d \parallel R_D)

Key Behavior

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

• Higher voltage gain
• No negative feedback in AC operation
• Phase inversion of output (180^\circ shift)

Common Source Amplifier with Unbypassed R_S

The source resistor R_S 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 C_1 is used to apply the input signal to the gate.
• Changes in V_{GS} regulate the drain current
• Capacitor C_2 provides the output at the drain.

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

Effect of Unbypassed R_S

• When I_D rises, the voltage across R_S increases,
• Results in V_{GS} more negative
• As a result, I_D decreases

The circuit is stabilized by this feedback.

Small Signal Analysis

The presence of R_S modifies the gain expression.

Voltage gain:

A_v = \frac{-g_m R_D}{1 + g_m R_S}

assuming r_d \gg R_D

Output impedance:

Z_o = r_d \parallel R_D

Input impedance:

Z_i = R_G = R_1 \parallel R_2

Key Behavior

Due to the presence of R_S in AC analysis:

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

Comparison