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🎛 MOSFET as an Amplifier (Introduction)

Although MOSFETs dominate switching applications in embedded systems, they can also operate as linear amplifiers. Understanding this mode is important—not because you’ll use it every day in embedded work, but because it explains what a MOSFET really does between OFF and ON.

🧠 MOSFET Amplifier Basics​

When the gate voltage is:

VGS(th)<VGS<VGS(on)V_{GS(th)} < V_{GS} < V_{GS(on)}

the MOSFET operates in the linear (triode) region.

In this region:

  • The MOSFET is partially ON
  • It behaves like a voltage-controlled resistor
  • Small changes in gate voltage cause large changes in drain current

This proportional control of current is what allows amplification.

🚗 Analogy

The gate voltage is like a car’s accelerator pedal.
A small press (tiny voltage change) produces a large change in engine power (current).

📉 Linear Region Operation​

In the linear region:

  • Drain–Source voltage VDSV_{DS} is significant
  • The MOSFET is not saturated
  • Power is dissipated inside the transistor

Power dissipation:

P=VDS×IDP = V_{DS} \times I_D

This is why heat management matters in amplifier designs.

Drain current depends on:

  • Gate–Source voltage VGSV_{GS}
  • Drain–Source voltage VDSV_{DS}
  • Load resistance

🔌 Common-Source MOSFET Amplifier​

The most basic MOSFET amplifier is the common-source configuration:

  • Input → Gate
  • Output → Drain
  • Source → Ground
  • Drain resistor converts current change into voltage change

Small gate-voltage variations → large drain-voltage variations.

Example:​

  • Input signal: 10 mV
  • Output signal: 100 mV
Av=VoutVin=10A_v = \frac{V_{out}}{V_{in}} = 10

That’s voltage amplification.
Power amplification is even higher because current is also controlled.

🔄 Two Regions, Two Jobs​

MOSFET RegionPurposeBehavior
Cutoff / SaturationSwitchingON / OFF
Linear (Triode)AmplificationProportional control

Understanding this explains why:

  • Embedded systems use MOSFETs as switches
  • Analog circuits use MOSFETs as amplifiers

🆚 MOSFET vs BJT as Amplifiers​

MOSFET Advantages:​

  • Extremely high input impedance
  • Gate draws almost zero current
  • Does not load weak signal sources

Ideal for:

  • Sensor outputs
  • High-impedance signal sources

BJT Disadvantages:​

  • Requires base current
  • Can load and distort weak signals

🎯 Bias Point – The Critical Concept​

For clean amplification, the MOSFET must be biased in the middle of the linear region.

  • Too close to cutoff → signal clips on one side
  • Too close to saturation → clips on the other side
  • Centered bias → maximum symmetrical output swing

Biasing is the heart of analog design.

⚠️ Practical Limitations of MOSFET Amplifiers​

  • Internal parasitic capacitances limit high-frequency performance
  • Lower gain at very high frequencies
  • BJTs often outperform MOSFETs in RF and high-speed amplifiers
  • Specialized MOSFETs are required for HF applications

✅ When MOSFET Amplifiers Make Sense​

MOSFET amplifiers are ideal for:

  1. High-impedance sensor signals
  2. Low-frequency analog amplification
  3. Power amplification
  4. Very low input current applications
  5. Voltage-controlled current sources

🚀 Key Takeaway​

MOSFETs are not just digital switches.

They are:

  • Switches in saturation
  • Amplifiers in linear operation

The circuit design, not the device itself, decides the role.

⚡ This dual nature is what makes MOSFETs one of the most powerful building blocks in electronics.