A (Metal-Oxide-Semiconductor Field-Effect Transistor) MOSFET as an amplifier is widely used in analog and digital circuits. Amplifiers are essential in electronics because they increase the amplitude of electrical signals without altering the information contained in the signal.
MOSFETs are ideal for amplification because of their high input impedance, fast switching speeds, and low power consumption. In this blog, we’ll dive into how a MOSFET operates as an amplifier, focusing on the n-channel MOSFET in common-source configuration, the most frequently used setup in amplification circuits.
Table of Contents
Basics of MOSFET Operation
A MOSFET has three terminals: Gate (G), Drain (D), and Source (S). It controls the flow of current between the drain and source by applying a voltage to the gate.
The MOSFET operates in different modes depending on the voltage between the terminals:
- Cut-off Region: The MOSFET is OFF when the gate-to-source voltage VGS is below the threshold voltage VTH. No current flows between drain and source.
- Ohmic Region (Linear Region): When VGS exceeds VTH, and the voltage between drain and source VDS is small, the MOSFET behaves like a resistor, allowing current to flow from drain to source.
- Saturation Region: If VGS > VTH and VDS is larger than a certain threshold, the MOSFET enters saturation mode, where the current is relatively constant. In this region, the MOSFET functions as an amplifier.
For amplification purposes, we typically operate the MOSFET in the saturation region. Here, the drain current ID is mostly independent of the drain-source voltage VDS, and is controlled by VGS.
MOSFET as an Amplifier (Common-Source Configuration)
The most common MOSFET amplifier configuration is the common-source amplifier. In this configuration, the source is connected to a common ground, the input signal is applied to the gate, and the output is taken from the drain.
Circuit Setup:
- Input Signal: Applied to the gate terminal.
- Output Signal: Taken from the drain terminal.
- Source Terminal: Grounded.
- Load Resistor RD: Connected between the drain and the supply voltage VDD.
This setup allows the MOSFET to amplify small AC signals. Let’s break down the amplification process:
Amplification Process
1. Biasing the MOSFET: To operate the MOSFET as an amplifier, we first need to apply a DC bias to the gate. This is done by applying a constant voltage VGS to the gate, ensuring that the MOSFET operates in the saturation region. The DC bias voltage keeps the MOSFET in an “active” state, allowing small input signals to be amplified.
2. Small-Signal Input: Once the MOSFET is biased, a small AC input signal is superimposed on the DC bias voltage at the gate. The gate voltage VG varies according to the input signal, which modulates the gate-to-source voltage VGS.
3. Control of Drain Current: Changes in VGS alter the drain current ID flowing from the drain to the source. Since the MOSFET is in the saturation region, small variations in VGS lead to proportional changes in ID. The relationship between ID and VGS in the saturation region is approximately quadratic:
ID = (K / 2) ⋅ (VGS − VTH )2
Here, 𝐾 is a constant dependent on the MOSFET’s physical parameters, and VTH is the threshold voltage.
4. Voltage Gain: The changes in ID result in corresponding changes in the output voltage at the drain VD, because of the voltage drop across the load resistor RD. The output voltage Vout is given by:
Vout = VDD − ID ⋅ RD
When ID increases due to a higher VGS, the voltage drop across RD increases, causing Vout to decrease, and vice versa. This is how the MOSFET amplifies the input signal: small variations in the input gate voltage result in larger variations in the output drain voltage.
→ The voltage gain AV of the amplifier is approximately:
AV = − gm ⋅ RD
Where: gm is the transconductance of the MOSFET, which indicates how effectively the MOSFET converts gate voltage changes into drain current changes.
The negative sign indicates that the output is inverted with respect to the input, a characteristic of common-source amplifiers.
Why MOSFETs Are Ideal for Amplification
MOSFETs are widely used in amplifiers for several reasons:
- High Input Impedance: MOSFETs have very high input impedance because of the insulated gate structure. This minimizes the loading effect on preceding stages, allowing them to be easily driven by other circuits.
- Low Power Consumption: Since the gate current is negligible (almost zero), MOSFETs consume very little power, making them ideal for low-power applications.
- High Gain: MOSFETs can provide significant voltage gain, especially in common-source configurations, making them suitable for analog signal amplification.
Applications of MOSFET Amplifiers
MOSFET amplifiers are used in a wide range of applications, including:
- Audio Amplifiers: To boost weak audio signals in sound systems.
- Radio Frequency Amplifiers: Used in communication systems to amplify high-frequency signals.
- Analog Signal Processing: Where accurate amplification of signals is needed, such as in sensors and instrumentation circuits.
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Frequently Asked Questions
Enhancement MOSFETs function as amplifiers in non-saturated or ohmic regions. The ohmic zone continues until the drain-to-source voltage reaches a threshold value known as the threshold voltage (VTP). Following this voltage, the p-channel E-MOSFET operates in a saturated state.
Common-source, Common-gate, and Common-drain (also known as source follower) are the three different configurations of MOSFET transistor amplifiers. Every one of these configurations has unique qualities that make them superior to the others in particular circuit applications.
Common Source MOSFET Amplifier Operating Principles:
- First, the MOSFET transistor’s gate receives a tiny input voltage (AC signal).
- The quiescent operating point is then established by the biasing network by setting an appropriate DC voltage at the gate terminal.
Power MOSFETs, especially vertical ones, have the following main benefits:
- They are incredibly fast, with equivalent ft’s of about 300MHz.
- There is no secondary breakdown in them, they just require the most basic kind of short circuit protection.
