A MOSFET amplifier is an electronic circuit that uses a Metal-Oxide-Semiconductor Field-Effect Transistor to increase the voltage, current, or power of an input signal. Unlike Bipolar Junction Transistors (BJTs), which are current-controlled devices, a MOSFET is voltage-controlled. By applying a small AC voltage to the high-impedance Gate terminal, you modulate the resistance of the channel between the Drain and Source, thereby varying a much larger DC current flowing from your power supply through a load resistor. This translates the small input voltage swings into large output voltage swings.

Because the Gate is insulated by a thin layer of silicon dioxide, the input impedance of a MOSFET amplifier is practically infinite (often >100 MΩ). This makes it the ideal front-end for high-impedance sources like piezoelectric guitar pickups, condenser microphones, and precision sensor bridges, where a BJT would load down the signal and cause severe attenuation.

The Anatomy of a MOSFET: Pins, Symbols, and Operating Regions

Before wiring anything on the bench, you need to understand the physical terminals and the symbolic representation. For the standard N-channel enhancement-mode MOSFET (the most common type used in discrete amplifiers), the symbol features three primary terminals:

  • Gate (G): The control terminal. The vertical line in the symbol represents the insulated gate. No steady-state DC current flows into this pin.
  • Drain (D): The terminal where conventional current enters the channel. In a common-source amplifier, the load resistor connects here to the positive supply.
  • Source (S): The terminal where current exits the channel, usually tied to ground or a source-degeneration resistor.
  • Body/Substrate (B): Often internally tied to the Source in discrete 3-pin TO-92 or TO-220 packages. In IC design or 4-pin packages, it must be explicitly biased to the lowest potential in the circuit.

To use a MOSFET as a linear amplifier, it must be biased into the Saturation region (often confusingly called the 'Active' region in BJT terminology). Here is the breakdown of the three operating regions for an N-channel enhancement MOSFET:

Operating Region Gate-Source Voltage ($V_{GS}$) Drain-Source Voltage ($V_{DS}$) Drain Current ($I_D$) Primary Application
Cutoff $V_{GS} < V_{th}$ Any 0 A (Leakage only) Switching (OFF state)
Triode (Ohmic) $V_{GS} > V_{th}$ $V_{DS} < V_{GS} - V_{th}$ Proportional to $V_{DS}$ Switching (ON state), Variable Resistor
Saturation (Active) $V_{GS} > V_{th}$ $V_{DS} \ge V_{GS} - V_{th}$ Constant (Controlled by $V_{GS}$) Linear Amplification, Current Sources
Bench Tip: $V_{th}$ (Threshold Voltage) is the minimum Gate-to-Source voltage required to form a conductive channel. If your $V_{GS}$ is below $V_{th}$, your amplifier is completely off and you will measure 0V AC at the output.

Selecting and Biasing the Right MOSFET for Amplification

Selecting a MOSFET for linear amplification requires looking past the 'switching' specs on the datasheet. You need a part with a well-defined transconductance ($g_m$) and a safe operating area (SOA) that can handle continuous DC power dissipation, not just microsecond switching pulses.

Safe Default Part Numbers for the Bench

Part Number Type / Package Max $V_{DS}$ Max $I_D$ Typical $V_{th}$ Range Best Use Case
2N7000 N-Channel / TO-92 60V 200mA 0.8V - 3.0V Small-signal preamps, sensor buffering
BS170 N-Channel / TO-92 60V 500mA 0.8V - 2.0V Low-noise audio front-ends
IRF510 N-Channel / TO-220 100V 5.6A 2.0V - 4.0V Power audio amps, RF output stages
IRFP240 N-Channel / TO-247 200V 20A 2.0V - 4.0V High-fidelity class-AB audio outputs

How to Bias for Linear Operation:
To keep the MOSFET in the Saturation region, we use a voltage divider at the Gate to set a fixed DC bias voltage ($V_G$). However, because $V_{th}$ varies wildly from part to part (even within the same manufacturing batch), relying solely on a fixed Gate voltage is a recipe for thermal runaway. We solve this by adding a Source Degeneration Resistor ($R_S$). As Drain current increases, the voltage drop across $R_S$ increases, which reduces the effective $V_{GS}$ ($V_{GS} = V_G - V_S$), creating negative DC feedback that stabilizes the bias point.

Build It: A Common-Source MOSFET Audio Preamp Circuit

Let's build a practical common-source voltage amplifier using the ubiquitous 2N7000 to amplify a weak AC signal (like a piezo pickup) to line-level audio.

Target Specs: $V_{DD} = 9V$, Target $I_D \approx 1mA$, Voltage Gain $\approx -4.7$

Component Bill of Materials

  • Q1: 2N7000 N-Channel MOSFET
  • R1, R2: 1 MΩ (Gate voltage divider, sets $V_G = 4.5V$)
  • R3 (Drain Load, $R_D$): 4.7 kΩ
  • R4 (Source Degeneration, $R_S$): 1 kΩ
  • C1 (Input Coupling): 1 µF film or ceramic
  • C2 (Output Coupling): 10 µF electrolytic
  • C3 (Source Bypass): 10 µF electrolytic (parallel with $R_S$)

Wiring and Assembly Steps

  1. Establish the Bias Network: Connect R1 between the 9V rail and the Gate (pin 2 of the TO-92 package, facing you with the flat side forward). Connect R2 between the Gate and Ground. This establishes $V_G = 4.5V$.
  2. Wire the Source Feedback: Connect R4 (1 kΩ) from the Source (pin 3) to Ground. This sets the DC Source voltage to roughly 1V, meaning the actual $V_{GS}$ will stabilize around 3.5V, yielding our target ~1mA Drain current.
  3. Add the AC Bypass: Connect C3 (10 µF) in parallel with R4. This is critical: it shorts the Source resistor to ground for AC signals, preventing AC negative feedback and restoring your voltage gain.
  4. Wire the Drain Load: Connect R3 (4.7 kΩ) from the Drain (pin 1) to the 9V rail.
  5. Couple the I/O: Connect C1 between your AC input source and the Gate. Connect C2 between the Drain and your output load (e.g., an oscilloscope or audio mixer). Ensure the positive stripe of C2 faces the Drain.

For a deeper theoretical breakdown of the small-signal model and transconductance calculations used in this topology, refer to the Electronics Tutorials guide on MOSFET amplifiers.

Bench War Story: When the Bias Drifts and the Magic Smoke Escapes

Theory is clean; the workbench is not. A few years ago, I was tasked with designing a high-current headphone amplifier using the IRF510 power MOSFET. The schematic called for a 12V rail, targeting a quiescent Drain current ($I_D$) of 200mA and a $V_{DS}$ of 6V to allow maximum symmetrical voltage swing.

The Setup: I used a rigid voltage divider to feed exactly 3.2V to the Gate of the IRF510, assuming a typical $V_{th}$ of 2.5V. I omitted the source degeneration resistor to 'maximize gain'.

The Numbers: According to the idealized math, $V_{GS} = 3.2V$ should yield roughly 200mA. Power dissipation would be $P = V_{DS} \times I_D = 6V \times 0.2A = 1.2W$, easily handled by the TO-220 package without a heatsink.

The Outcome: I built three prototype channels. Channel 1 sounded great but clipped early on positive peaks. Channel 2 was dead silent. Channel 3 played audio for exactly four seconds before the MOSFET hissed, the TO-220 package scorched my finger, and the 12V bench supply tripped its overcurrent protection.

What Went Wrong: I ignored the datasheet's $V_{th}$ spread. For the IRF510, $V_{th}$ can range from 2.0V to 4.0V.
Channel 2 had a $V_{th}$ of 3.8V; at a 3.2V Gate drive, it was in Cutoff (0mA).
Channel 3 had a $V_{th}$ of 2.0V. With 3.2V on the Gate, it turned on hard, pulling over 1.5A. The voltage across the Drain dropped to near zero, and the device entered the Triode region, dissipating over 5W and destroying the silicon.
The Fix: I added a 1 Ω source degeneration resistor. While 1 Ω drops very little voltage at normal currents, at 1.5A it dropped 1.5V, instantly choking the Gate drive down to an effective 1.7V and forcing the transistor back into a safe operating point. Always use source feedback for power MOSFETs.

Troubleshooting: How MOSFETs Fail and How to Test Them

MOSFETs generally fail in two ways: Gate Oxide Puncture from Electrostatic Discharge (ESD), which creates a permanent short between Gate and Source, and Thermal Runaway, which melts the silicon and shorts the Drain to the Source.

You can diagnose a suspected dead MOSFET right on the bench using a standard digital multimeter (DMM) in Diode Test mode. Here is the definitive sequence for an N-channel MOSFET:

  1. Discharge the Gate: Touch the black (common) probe to the Gate and the red probe to the Source. This bleeds off any stored charge in the Gate capacitance.
  2. Check the Body Diode: Place the red probe on the Source and the black probe on the Drain. You should read a standard silicon diode drop (typically 0.4V to 0.6V). Reverse the probes (red on Drain, black on Source); the meter should read 'OL' (Open Loop). If it reads 0.00V or a dead short in both directions, the Drain-Source channel is blown.
  3. Charge the Gate: Leave the black probe on the Source. Momentarily touch the red probe to the Gate. This applies the DMM's internal ~3V battery to the Gate, charging the internal capacitance and turning the MOSFET on.
  4. Verify Channel Conduction: Move the red probe back to the Drain (keeping black on Source). The meter should now read a very low voltage drop (near 0.00V) or beep in continuity mode, indicating the channel is actively conducting.
  5. Discharge and Verify Cutoff: Touch the Gate to the Source again with a piece of wire or your finger. Re-test Drain-to-Source; it should return to 'OL'. If it stays shorted, the Gate oxide is punctured and the part is trash.
Safety Warning: When testing high-voltage power MOSFETs (like those in switching power supplies or motor drives), ensure the circuit is fully de-energized and all high-voltage bus capacitors are discharged with a bleeder resistor before applying your multimeter. For more on safe high-power testing, consult All About Circuits' safety guidelines for discrete semiconductors.

Understanding what a MOSFET amplifier is goes far beyond memorizing the saturation equations. It requires respecting the physical realities of threshold voltage spreads, managing thermal feedback via source degeneration, and knowing how to verify the health of the silicon with a DMM before you solder it into your final PCB.