If you are asking what is a MOSFET amp, the direct answer is: it is an amplifier circuit that uses a Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) to control a large output current using a small input voltage. Unlike Bipolar Junction Transistors (BJTs) which are current-controlled devices, MOSFETs are voltage-controlled. This gives them an exceptionally high input impedance, meaning they draw virtually zero steady-state current from the preceding signal source. In practical bench and jobsite applications, MOSFET amplifiers dominate both high-fidelity linear audio output stages (Class AB) and high-efficiency switching power stages (Class D).

Below is a complete guide to understanding MOSFET operation regions, selecting the right part numbers, biasing a stable common-source circuit, and testing these components when they inevitably fail.

MOSFET Pinout, Symbol, and Operating Regions

A standard discrete power MOSFET (typically in a TO-220 or TO-247 package) has three primary terminals, plus a fourth that is internally tied:

  • Gate (G): The control terminal. Voltage applied here relative to the Source creates an electric field that forms a conductive channel. It is capacitively coupled to the rest of the device, drawing only transient charging current.
  • Drain (D): The terminal where the main current enters (in an N-channel device). In the schematic symbol, this is the top connection.
  • Source (S): The terminal where the main current exits. This is the reference point for the Gate voltage (VGS).
  • Body/Substrate: Internally shorted to the Source in discrete power MOSFETs. This creates an intrinsic body diode pointing from Source to Drain (in N-channel), which is critical for inductive load clamping but can complicate bidirectional switching.

To use a MOSFET as an amplifier, you must bias it in the Saturation (Active) Region. This is where the device acts as a voltage-controlled current source, providing linear amplification. Do not confuse MOSFET "saturation" with BJT saturation; in a BJT, saturation means fully ON (acting as a switch), whereas in a MOSFET, the fully ON switching state is called the Triode or Linear Region.

Operating Region VGS Condition VDS Condition Typical VDS / ID Example Primary Application
Cutoff VGS < Vth Any VDS = 24V, ID ≈ 0A Switch OFF / Idle state
Triode (Linear) VGS > Vth VDS < (VGS - Vth) VDS = 0.5V, ID = 20A Switching (Class D amps, PWM motor control)
Saturation (Active) VGS > Vth VDS ≥ (VGS - Vth) VDS = 12V, ID = 100mA Linear Amplification (Class A/AB audio, RF)

Note: Vth (Gate-Source Threshold Voltage) is typically between 2.0V and 4.0V for standard power MOSFETs, and 1.0V to 2.0V for logic-level variants.

How to Select and Bias a MOSFET for the Job

Selecting a MOSFET for amplification requires looking past the headline current rating on the box. A 100A MOSFET will melt if you try to push 5A through it in the linear (saturation) region without checking the Safe Operating Area (SOA) graph in the datasheet. For linear audio amps, you need devices specifically rated for linear mode (often older planar designs), whereas modern trench MOSFETs are optimized for switching and suffer from thermal instability in linear mode.

Bench Tip: Always add a small gate stopper resistor (10Ω to 100Ω) physically close to the Gate pin. This damps high-frequency parasitic oscillations caused by the Miller capacitance and trace inductance, which can instantly destroy the gate oxide.

Safe Default Part Numbers for Amplifiers

When prototyping or repairing, stick to these proven, widely available part numbers. They are paired with their complementary opposites for push-pull output stages.

Part Number Polarity VDS (Max) ID (Continuous) RDS(on) @ 10V Best Application
IRF540N N-Channel 100V 33A 44 mΩ General purpose, low-power linear audio, motor drivers
IRF9540N P-Channel -100V -23A 117 mΩ Complementary match for IRF540N in push-pull stages
IRFP240 N-Channel 200V 20A 180 mΩ High-voltage linear audio output stages (excellent SOA)
IRFP9240 P-Channel -200V -12A 250 mΩ Complementary match for IRFP240 in high-end audio
IRF3205 N-Channel 55V 110A 8 mΩ Class D switching amps, DC-DC converters (avoid for linear)

A Complete Common-Source MOSFET Amplifier Circuit

Let us build a stable, bench-testable Common-Source linear amplifier using an IRF540N. We will use a voltage-divider bias network with a source degeneration resistor to prevent thermal runaway. According to Electronics Tutorials, source degeneration provides negative feedback that stabilizes the quiescent drain current against temperature variations and part-to-part Vth mismatches.

Component List and Values

  • Q1: IRF540N (N-Channel MOSFET)
  • VDD: 24V DC Power Supply
  • R1: 2.2 MΩ (Gate to VDD)
  • R2: 1.0 MΩ (Gate to GND) — Sets VG ≈ 7.5V
  • RS: 47 Ω, 1W (Source to GND) — Source degeneration
  • RD: 100 Ω, 2W (Drain to VDD) — Drain load
  • RG: 47 Ω (Gate stopper, placed between R1/R2 junction and Gate pin)
  • CIN: 1 µF film capacitor (Input coupling)
  • COUT: 10 µF electrolytic (Output coupling, polarity matters)

Circuit Math and Bias Point (Q-Point)

Assuming a typical VGS(th) of 3.5V for the IRF540N at our operating current:

  1. Gate Voltage (VG): The R1/R2 divider sets VG = 24V × (1.0 / 3.2) = 7.5V.
  2. Source Voltage (VS): VS = VG - VGS(th) = 7.5V - 3.5V = 4.0V.
  3. Drain Current (ID): Since ID ≈ IS, ID = VS / RS = 4.0V / 47Ω ≈ 85 mA.
  4. Drain Voltage (VD): VD = VDD - (ID × RD) = 24V - (0.085A × 100Ω) = 15.5V.
  5. Drain-Source Voltage (VDS): VD - VS = 15.5V - 4.0V = 11.5V.

Because VDS (11.5V) is well above VGS - Vth (4.0V), the MOSFET is firmly in the Saturation (Active) Region. The power dissipated by the MOSFET is VDS × ID = 11.5V × 0.085A ≈ 0.97W. A TO-220 package can dissipate ~2W in free air, but adding a small clip-on heatsink is recommended for bench reliability.

How MOSFETs Fail and How to Test Them

MOSFETs rarely fail gracefully. When they do, they typically short Drain-to-Source, taking the power supply and load with them. Understanding the failure modes helps you diagnose a blown amp channel quickly.

Common Failure Modes

  • Gate Oxide Puncture (ESD): The silicon dioxide layer between the Gate and Source is incredibly thin. A static discharge exceeding ±20V can punch a microscopic hole through it, creating a permanent Gate-to-Source short. The device will no longer turn on.
  • Avalanche Breakdown: If an inductive load (like a speaker crossover coil) generates a voltage spike exceeding the VDS max rating, the MOSFET enters avalanche. If the energy exceeds the device's Single Pulse Avalanche Energy (EAS) rating, the silicon melts internally.
  • Thermal Runaway (Linear Mode): While RDS(on) has a positive temperature coefficient (which helps parallel switching MOSFETs share current), the VGS(th) has a negative temperature coefficient. In linear mode at low currents, as the die heats up, Vth drops, causing the MOSFET to draw more current, heat up further, and destroy itself. This is why source degeneration (RS) is mandatory in linear amps.

Bench Testing with a Digital Multimeter

You do not need a curve tracer to verify a MOSFET. A standard DMM in Diode Test Mode can check the body diode and the gate capacitance. Here is the exact sequence for an N-Channel MOSFET:

Safety First: Ensure the MOSFET is completely removed from the circuit. In-circuit testing will yield false readings due to parallel resistances and capacitances.
  1. Discharge the Gate: Touch your finger across all three pins (Gate, Drain, Source) or use a 10kΩ resistor to short G to S. This bleeds off any trapped gate charge.
  2. Test the Body Diode: Place the Red probe on the Source and the Black probe on the Drain. The meter should read a forward voltage drop of 0.4V to 0.6V. Reverse the probes (Black on Source, Red on Drain); it should read OL (Over Limit). If it reads 0.0V or a dead short in both directions, the Drain-Source channel is blown.
  3. Charge the Gate: Leave the Black probe on the Source. Touch the Red probe to the Gate for one second. The DMM's internal battery (usually 3V to 9V) will charge the Gate capacitance, turning the MOSFET ON.
  4. Verify the Channel: Move the Red probe back to the Drain (keep Black on Source). The meter should now read a very low voltage (near 0.0V), indicating the channel is fully enhanced and conducting.
  5. Discharge to Turn Off: Short the Gate to the Source with your finger or a resistor. Move the Red probe to the Drain again; it should return to OL, proving the device turns off correctly.

BJT vs. MOSFET Amplifiers: Which Should You Build?

While BJTs (like the classic 2N3055 or MJ15024) dominated audio amplifier designs in the 1970s and 80s, MOSFETs (and later, lateral DMOSFETs) took over high-end audio and modern Class D designs. As noted in application notes from All About Circuits, the choice between the two fundamentally changes your driver stage requirements.

Characteristic BJT Amplifier (e.g., Class AB) MOSFET Amplifier (e.g., Class AB / D)
Control Mechanism Current-controlled (requires base current) Voltage-controlled (requires gate charge)
Input Impedance Low to Moderate (kΩ range) Extremely High (MΩ to GΩ range)
Driver Stage Complexity High (needs Darlington pairs or Sziklai pairs for current gain) Low for linear; High for Class D (needs fast gate drivers to overcome Miller capacitance)
Thermal Stability Prone to thermal runaway (requires VBE multiplier bias spreaders) Highly stable in switching; requires source degeneration in linear mode
Audio Characteristic Warm, soft clipping (gradual saturation) Clean, hard clipping (resembles tube triode curves in lateral types)
Cost & Availability Very cheap, ubiquitous Moderate; high-current linear types are becoming niche/expensive

The Verdict: Choose a BJT design if you are repairing vintage gear, operating on a strict budget, or need soft-clipping characteristics for guitar preamps. Choose a MOSFET design if you are building a modern high-efficiency Class D subwoofer amp, a high-fidelity Class AB audiophile amplifier, or if you need to buffer a high-impedance sensor signal without loading it down.