The MOSFET Amplifier explained

BASIC KNOWLEDGE The MOSFET Amplifier explained

2025-01-02 From Venus Kohli 5 min Reading Time

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All transistors can become amplifiers but all amplifiers are not transistors. MOSFET amplifiers are types of FET small signal linear amplifiers. The article explains MOSFET amplifier types with their circuit diagram and AC equivalent representation. It explains the MOSFET amplifier operating through the common source configuration.

MOSFET amplifiers, leveraging the power of MOSFET transistors, offer a versatile solution for signal amplification across various electronic applications.(Source: Hseyin - stock.adobe.com) — MOSFET amplifiers, leveraging the power of MOSFET transistors, offer a versatile solution for signal amplification across various electronic applications.
(Source: Hseyin - stock.adobe.com)

The three main functions of transistors are switching, amplification, and memory/logic operations in semiconductor chips. All amplifiers perform the amplification function in electronic systems. Amplifiers can use transistors, transformers, vacuum tubes, and many other components to amplify signals. These components are known as active components. Transistor amplifiers use transistors like BJTs, JFETs, and MOSFETs as active components to perform amplification.

What is a MOSFET amplifier?

A Metal Oxide Semiconductor Field Effect Transistor or simply MOSFET amplifier is a type of transistor amplifier that uses a MOSFET transistor to amplify signals. It is a voltage-controlled device. Input voltage modulates the current flow through the device. It results in the output voltage change across the output resistance.

MOSFET amplifier types

There are three main types of MOSFET amplifier types:

Common source (CS)

Common drain (CD)

Common gate (CG)

In such topologies, one of the terminals is common while others function to be input and output. MOSFET amplifiers can use n-channel or p-channel. Similarly, MOSFET amplifiers can be of two types, based on construction/mode of operation: depletion mode and enhancement mode.

Working principle of MOSFET amplifiers: Common Source

AC models describe amplifier operation for small signal analysis. The working principle of MOSFET amplifiers is explained through common source configuration.

The direction of the input and output signal shows that the gate is the input terminal and the drain is the output terminal. A CS MOSFET amplifier operates when the gate-to-source voltage exceeds the threshold voltage. The MOSFET operates in the saturation region to amplify the input signal.

Basic N-channel MOSFET common-source amplifier with active load ID.(Source: Common source with active load /Brews ohare / CC BY-SA 4.0) — Basic N-channel MOSFET common-source amplifier with active load ID.
(Source: Common source with active load /Brews ohare / CC BY-SA 4.0)

The input signal gets superimposed on gate-to-source voltage above the threshold voltage. As a result, we get amplified but inverted output signals during positive and negative half cycles. There is a 180-degree phase shift between input and output signals. Hence, the CS MOSFET amplifier is an inverting amplifier.

Coupling capacitors are used at the output to reduce the DC component to zero. The small signal AC equivalent circuit of the CS MOSFET amplifier is shown below. The gate terminal isolates due to the presence of the silicon dioxide layer.

Small-signal circuit for N-channel MOSFET common-source amplifier.(Source: Small-signal common source with C gd /Brews ohare / CC BY-SA 4.0) — Small-signal circuit for N-channel MOSFET common-source amplifier.
(Source: Small-signal common source with C gd /Brews ohare / CC BY-SA 4.0)

The CS MOSFET amplifier exhibits a high input resistance. The reactance of gate-to-source capacitance is large. Due to gate-to-source voltage being above the threshold voltage, Sometimes source resistance is used to stabilize saturation operation. MOSFET operates as a controlled current source.

Miller’s effect

CS MOSFET amplifiers have a limited high-frequency response. Input capacitance of the transistor increases due to feedback capacitance between input and output terminals. The amplifier gain multiplies The magnitude of parasitic capacitance (gate-to-drain capacitance). It gives rise to an undesirable phenomenon called “the Miller effect”.

For any inverting amplifier, Miller’s effect (increased input capacitance) reduces bandwidth, impedance, slew rate, and overall performance. MOSFET amplifiers combat Miller’s effect in cascode configuration. Compensator networks help to mitigate the increased capacitance for single CS MOSFET amplifier use.

Common Drain

In a CD MOSFET amplifier, the signal is input to the gate terminal, and output is taken out from the source terminal. The drain terminal stays grounded while all voltages are measured across it. The common drain configuration is the same for JFET and MOSFET transistors.

Basic N-channel JFET source follower circuit (neglecting biasing details).(Source: N-channel JFET source follower /Omegatron / CC BY-SA 3.0) — Basic N-channel JFET source follower circuit (neglecting biasing details).
(Source: N-channel JFET source follower /Omegatron / CC BY-SA 3.0)

The CD MOSFET amplifier is a non-inverting amplifier, commonly called a source follower, voltage follower, stabilizer, or drain circuit. There is no feedback capacitance between input and output due to grounding an entire side. As a result, Miller’s effect is absent.

Common Gate

In a CG MOSFET amplifier, the input is the source terminal, and the output is the drain terminal. The gate terminal is common or grounded. A large capacitor connects the gate to the ground for an AC signal. It can also be connected to some DC biasing voltage.

Most of the time, the input given to the CG MOSFET amplifier is a current signal. The AC equivalent circuit shows the current source as the input. As a result, CG MOSFET amplifiers are used as current buffers and in cascode amplifiers and CMOS RF receivers.

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Small-signal low-frequency hybrid-pi model for amplifier driven by a Norton signal source.(Source: Common gate hybrid pi /Brews ohare / CC BY-SA 4.0) — Small-signal low-frequency hybrid-pi model for amplifier driven by a Norton signal source.
(Source: Common gate hybrid pi /Brews ohare / CC BY-SA 4.0)

Comparing MOSFET amplifier types

The table compares CS, CD, and CG MOSFET amplifiers based on operation, voltage gain, impedances, and Miller’s effect.

The minus sign for the CS MOSFET amplifier showcases its inverting nature. However, it doesn’t imply that it offers the lowest gain. CD MOSFET amplifiers offer the lowest gain among all.

Advantages of MOSFET amplifiers

Voltage-controlled behavior.

High input impedance.

Fast switching speed.

Low power consumption.

Small size.

Thermal stability compared to BJT amplifiers.

Disadvantages of MOSFET amplifiers

Unsuitable for high-power applications.

High noise.

Poor high-frequency operation.

Presence of leakage currents.

While both the MOSFET and BJT are transistors, they work differently and exhibit different behaviours. (Source: Yuriy Zahachevskyy)

Cascode amplifier

A cascode amplifier is a type of two-stage amplifier consisting of a common source amplifier stage followed by a common gate stage. The common source stage “feeds” the common gate stage. A cascode amplifier can also function using BJT amplifiers- common emitter and common base configurations.

MOSFET Cascode using ideal voltage sources for DC gate bias and a DC current source as active load. Since each MOSFET transistor has gate and source connected, this configuration is valid only for discrete 3-terminal components.(Source: MOSFET Cascode /brews_ohare / CC BY-SA 4.0) — MOSFET Cascode using ideal voltage sources for DC gate bias and a DC current source as active load. Since each MOSFET transistor has gate and source connected, this configuration is valid only for discrete 3-terminal components.
(Source: MOSFET Cascode /brews_ohare / CC BY-SA 4.0)

MOSFET Cascode amplifiers improve device performance, bandwidth, gain, slew rate, input and output impedance, stable operation, and isolation capabilities. However, cascode amplifiers need high supply voltage due to the presence of two transistor amplifiers.

Applications of MOSFET amplifiers

N-channel MOSFET amplifiers are more common than P-channel MOSFET amplifiers. This section discusses applications of all types of MOSFET amplifiers.

CS MOSFET amplifiers

CS MOSFET amplifiers are used as voltage amplifiers in electrical and electronics applications like amplification of sensor signals, communication systems, and op-amps.

CD MOSFET amplifiers

CD MOSFET amplifiers are used as a voltage buffer— a voltage amplifier that provides a unity gain. Other applications include low-resistance load, speakers, and signal isolation circuits.

CG MOSFET amplifiers

CG MOSFET amplifiers are less common in use compared to common source and common drain MOSFET amplifiers. However, they are used as current buffers and in cascode configurations for CMOS receivers.

Cascode MOSFET amplifiers

Cascode MOSFET amplifiers are used in voltage regulators, high-voltage transistors, mixer circuits, superheterodyne receivers, tuners, amplitude modulators, ICs, and various applications.

References

Boylestad, R., & Nashelesky, L. (2012). Electronic Devices and Circuits Theory (11th ed.). Pearson Education.

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