JFET Amplifiers

Because a JFET can regulate drain current using the gate-to-source voltage, it can be utilized as an amplifier. A slight change in input voltage causes a large change in drain current, which leads to amplification because the gate current is almost zero.

The JFET's voltage-controlled design serves as the foundation for the amplification process. The drain current I_D is altered when an input signal is applied at the gate due to changes in V_{GS} . The output voltage is increased as a result of the voltage drop across the drain resistor caused by the fluctuating drain current.

JFET amplifiers are frequently utilized because

• Elevated input impedance
• Minimal sound
• High voltage gain

Common Source Amplifier

Most used setup for JFET amplifiers. The input signal is applied between the gate and the source in this circuit, and the output is obtained between the drain and the source.

Both input and output share the source terminal.

Working

• A change in the input signal applied at the gate V_{GS}
• The drain current I_D is controlled by this variation.
• A fluctuating voltage across R_D results from the fluctuating drain current.
• At the drain terminal, the output voltage is measured.

There is a 180^\circ phase shift between the input and output due to the amplification and inversion of the output signal.

Small Signal Model

For analysis at mid-frequency:

• Ground is used in place of the DC supply V_{DD}.
• Capacitors that couple are regarded as short circuits.
• Its tiny signal model takes the place of the JFET.

Included in the tiny signal model are:

• Transconductance g_m
• Resistance to drain r_d
• Source of controlled current g_m v_{gs}

These characteristics are used to examine the amplifier's impedance and voltage gain.

Common Drain Amplifier (Source Follower)

The drain terminal of the common drain (CD) amplifier, sometimes referred to as a source follower, is shared by the input and output. The output is taken between the source and the drain, while the input signal is applied between the gate and the drain.

Working

• The input signal regulates V_{GS}
• The source resistor R_S is used to create the output voltage
• The input signal is followed by the output

Characteristics

• The voltage increase is roughly equivalent to 1
• There is no phase reversal between the input and output
• Elevated input impedance
• Minimal output impedance

Analysis

Input impedance:

Z_i = R_G

Output impedance:

Z_o = \frac{1}{g_m} \parallel R_S

Voltage gain:

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

If g_m R_S \gg 1, then:

A_v \approx 1

This configuration is mainly used as a buffer stage.

Common Gate Amplifier

The gate terminal in the common gate (CG) design is shared by the input and output. The output is taken between the drain and the gate, while the input signal is applied between the source and the gate.

Typically, the potential of the gate is constant.

Working

• The source is where the input signal is applied
• The device's current is altered by variations in the source voltage
• At the drain, the discharge is collected

There is no phase shift between the input and output, in contrast to a common source amplifier.

Characteristics

• Low input impedance
• High output impedance
• High voltage gain
• No phase inversion

Analysis

Input impedance:

Z_i = R_S \parallel \frac{1}{g_m}

Output impedance:

Z_o = r_d \parallel R_D

Voltage gain:

A_v = g_m R_D

assuming r_d \gg R_D

This configuration is suitable for high-frequency applications due to its low input impedance and reduced Miller effect.