Analysis of MOSFET Amplifier

Because of their high input impedance, low power consumption, and ease of manufacture, MOSFETs are frequently employed in amplifier circuits. MOSFETs are perfect for voltage amplification, particularly in the input stages of analog circuits, because their high input impedance guarantees little loading of the input signal.

The MOSFET in amplifier applications is appropriately biased in the saturation area to generate magnified output signals from little changes in input voltage.

Depending on the terminals used for input and output, MOSFET amplifiers are classified as:

Analysis of Common Source Amplifier

The input signal is fed into the gate of a common source (CS) amplifier, and the output is drawn from the drain. Both input and output share the source terminal.

With a phase inversion between the input and output, this arrangement offers substantial voltage gain.

Small Signal AC Equivalent Circuit (with Bypass Capacitor)

For AC analysis:

• Coupling capacitors act as short circuits
• DC supply is treated as ground
• Source resistor is bypassed (acts as AC ground)

Let:

R_G = R_1 \parallel R_2

(i) Input Impedance (Z_i)

Input impedance is the resistance seen at the input terminal

Z_i = R_G = R_1 \parallel R_2

Since gate current is zero, input impedance is very high.

(ii) Output Impedance (Z_o)

Output impedance is the resistance seen from the output

Z_o = R_D \parallel r_d

If r_d \gg R_D , then:

Z_o \approx R_D

(iii) Voltage Gain (A_v)

A_v = \frac{V_o}{V_i}

For CS amplifier:

A_v = - g_m (R_D \parallel r_d)

If r_d \gg R_D , then:

A_v \approx - g_m R_D

The negative sign indicates a 180° phase shift.

Key Features

• High input impedance
• Moderate output impedance
• High voltage gain
• Provides 180° phase shift (inversion)
• Widely used for voltage amplification

Analysis of Common Drain Amplifier

A common drain (CD) amplifier uses the source as the output and the gate as the input. The drain terminal is typical.

Because the output comes after the input, this setup is often referred to as a source follower.

Small Signal AC Equivalent Circuit

For AC analysis:

• Capacitors are short-circuited
• DC supply is grounded

Let:

R_G = R_1 \parallel R_2

(i) Input Impedance (Z_i)

Z_i = R_G

Input impedance is high due to insulated gate.

(ii) Output Impedance (Z_o)

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

If r_d \to \infty

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

(iii) Voltage Gain (A_v)

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

Since g_m R_S \gg 1

A_v \approx 1

Thus, the common drain amplifier has unity gain.

Key Features

• High input impedance
• Low output impedance
• Used as a buffer (impedance matching)

Analysis of Common Gate Amplifier

The source receives the input and the drain receives the output in a common gate (CG) amplifier. The gate terminal is frequently used.

High-frequency applications make use of this arrangement.

Small Signal AC Equivalent Circuit

For AC analysis:

• Capacitors are shorted
• Gate is at AC ground

(i) Input Impedance (Z_i )

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

Input impedance is low.

(ii) Output Impedance (Z_o)

Z_o = R_D \parallel r_d

If r_d \gg R_D:

Z_o \approx R_D

(iii) Voltage Gain (A_v)

A_v = g_m R_D

This amplifier provides high voltage gain without phase inversion.

Key Features

• Low input impedance
• High output impedance
• High voltage gain
• Used in RF and high-frequency circuits