A MOSFET amp uses a Metal-Oxide-Semiconductor Field-Effect Transistor to amplify voltage or current signals. Unlike BJTs, which are current-controlled, MOSFETs are voltage-controlled devices with near-infinite gate impedance, making them ideal for high-impedance sensor buffering and audio power stages. If you need a quick answer for your next build: use a 2N7000 for small-signal voltage amplification under 200mA, and step up to an IRF510 or IRFP240 for audio power output stages.
This guide moves past textbook physics and gets straight to the bench. We will cover how to bias a MOSFET into its linear region, provide a complete common-source amplifier schematic with exact component values, and walk through a real-world failure scenario so you know exactly what to look for when the magic smoke escapes.
The Anatomy of a MOSFET Amp: Pinouts and Safe Default Parts
Before you breadboard anything, you need to know your pinouts and select the right silicon. The standard symbol for an N-channel enhancement MOSFET shows three terminals: the Gate (G), Drain (D), and Source (S). The arrow on the source terminal points inward for N-channel devices.
Physical pinouts vary by package, which is a common trap for beginners:
- TO-92 (e.g., 2N7000): With the flat side facing you and pins pointing down, the order left-to-right is Source, Gate, Drain.
- TO-220 (e.g., IRF510): With the metal tab facing away from you and pins pointing down, the order left-to-right is Gate, Drain, Source. (Note: The metal tab is internally connected to the Drain).
| Part Number | Application | Vds (Max) | Id (Continuous) | Rds(on) | Typ. Cost |
|---|---|---|---|---|---|
| 2N7000 | Small-signal / Sensor buffering | 60V | 200mA | 5.0 Ω | $0.15 |
| IRF510 | Low-power audio / RF drivers | 100V | 5.6A | 0.54 Ω | $0.80 |
| IRFP240 | High-power audio output stages | 200V | 20A | 0.18 Ω | $2.50 |
| ECX10N20 | Lateral audio (Exicon) | 200V | 8A | N/A (Lateral) | $8.50 |
Operation Regions and Biasing for Linear Amplification
The most confusing aspect of MOSFET theory for hobbyists is the terminology. In BJT land, "saturation" means the transistor is fully ON (acting as a closed switch). In MOSFET land, Saturation (often called the Active region) is where the device acts as an amplifier. The region where it acts as a closed switch is called the Triode or Ohmic region.
To use a MOSFET as an amp, you must bias it firmly in the Saturation region. This requires two conditions: Vgs > Vth (to turn it on) and Vds > Vgs - Vth (to pinch off the channel and create a constant current source).
| Region | Gate Condition | Drain-Source Condition | Behavior | Typical Voltages (IRF510) |
|---|---|---|---|---|
| Cutoff | Vgs < Vth | Any | Open switch (Id ≈ 0) | Vgs < 2.0V |
| Triode (Ohmic) | Vgs > Vth | Vds < Vgs - Vth | Closed switch / Resistor | Vgs=5V, Vds=1V |
| Saturation (Active) | Vgs > Vth | Vds > Vgs - Vth | Linear Amplifier | Vgs=4V, Vds=5V |
How to bias it for the job: The most stable way to bias a discrete MOSFET amp is using a voltage divider on the Gate combined with a Source resistor (source degeneration). The Source resistor provides negative DC feedback. If the MOSFET heats up and tries to draw more current, the voltage drop across the Source resistor increases, which effectively reduces the Vgs (since Vg is fixed by the divider), throttling the current back down.
Complete Common-Source MOSFET Amp Circuit and Component Values
Let’s build a practical common-source voltage amplifier using the 2N7000. We will target a quiescent drain current (Id) of 5mA from a 9V battery supply, maximizing the voltage swing without clipping.
Design Targets:
- Vdd = 9.0V
- Target Id = 5mA
- 2N7000 Vth (typical) ≈ 2.1V
- Required Vgs for 5mA ≈ 2.5V (derived from datasheet transfer characteristics)
Calculated Component Values:
- Source Resistor (Rs): We want about half of Vdd dropped across the Drain-Source path and resistors. Let's set Vs = 2.5V.
Rs = Vs / Id = 2.5V / 5mA = 500Ω. Use standard 510Ω. - Gate Voltage (Vg):
Vg = Vs + Vgs = 2.55V + 2.5V = 5.05V. - Voltage Divider (R1, R2): To get 5.05V from 9V with high input impedance, use R2 = 1MΩ (Gate to GND).
R1 = R2 * ((Vdd/Vg) - 1) = 1M * ((9/5.05) - 1) = 782kΩ. Use standard 820kΩ for R1 (Vdd to Gate). - Drain Resistor (Rd): We want Vd to sit at roughly half the remaining supply to allow symmetrical AC swing. Let's drop 2.55V across Rd.
Rd = 2.55V / 5mA = 510Ω. Use standard 510Ω. - Coupling Capacitors: Use 1µF film or ceramic caps for Cin and Cout to block DC while passing audio (down to ~300Hz with a 10k load).
Bench Scenario: When the Magic Smoke Escapes
Theory is clean; the bench is not. Here is a real-world scenario from a recent Class AB audio MOSFET amp build that highlights the dangers of parasitic oscillation and thermal runaway in power stages.
The Setup: I was building a 100W audio power amp using a complementary pair of vertical MOSFETs (IRFP240 N-channel and IRFP9240 P-channel) on a ±40V dual rail supply. I used a standard Vbe-multiplier bias spreader to set the quiescent current, targeting 100mA per output device to eliminate crossover distortion.
The Numbers: I connected my multimeter across the 0.1Ω source degeneration resistors. I adjusted the bias trim pot until I read exactly 10mV across each resistor, confirming 100mA of idle current. I connected an 8-ohm dummy load and fed a 1kHz sine wave from my function generator.
The Outcome: The amp sounded pristine for about 45 seconds. Then, a sharp pop came from the dummy load, followed immediately by the harsh smell of burning epoxy. The 5A main rail fuse blew violently. Inspecting the board, the N-channel IRFP240 had shorted Drain-to-Source, and its 100-ohm gate stopper resistor was charred black.
What Went Wrong: Two distinct failures compounded here. First, vertical power MOSFETs like the IRFP240 have a massive gate capacitance and are highly prone to parasitic RF oscillation at VHF frequencies (often 10MHz+). This oscillation doesn't show up on a standard audio oscilloscope but causes massive internal die heating. Second, at low quiescent currents, vertical MOSFETs exhibit a negative temperature coefficient for Vth. As the die heated from the RF oscillation, its Vth dropped, causing it to draw more DC current, which created more heat—a classic thermal runaway loop.
The Fix: I replaced the blown parts and added 4.7Ω gate stopper resistors physically soldered directly to the gate pins (not on the PCB) to kill the RF resonance. I also increased the source resistors to 0.33Ω (5W) to provide stronger local DC feedback, and upgraded the bias spreader to use a thermistor mounted directly to the heatsink to track device temperature.
Failure Modes and Multimeter Diagnostics
MOSFETs generally fail in three ways: Gate punch-through (ESD destroys the ultra-thin oxide layer, shorting Gate to Source), Thermal runaway (melts the silicon, shorting Drain to Source), and Avalanche breakdown (exceeding Vds with an inductive load, puncturing the die).
You can diagnose a suspect MOSFET on the bench using your digital multimeter's Diode Test mode. Here is the exact sequence for an N-channel device:
- Discharge the Gate: Briefly short the Gate and Source pins with a piece of wire or your finger. This bleeds off any trapped charge in the gate capacitance.
- Test 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).
- Check for Gate Shorts: Measure resistance between Gate and Source, and Gate and Drain. Both must read "OL". If you read anything less than a megaohm, the gate oxide is punctured. Trash the part.
- Force the Channel ON: Keep the Black probe on the Source. Momentarily touch the Red probe to the Gate to charge it positive. Move the Red probe back to the Drain. The meter should now read a near-short (0.00V to 0.05V), indicating the channel is enhanced and conducting.
- Force the Channel OFF: Short the Gate to the Source again. Move the Red probe back to the Drain; it should return to reading "OL".
If the device passes all five steps, the silicon is healthy. For a deeper dive into MOSFET internal parasitics and safe operating areas (SOA), the All About Circuits MOSFET amplifier guide provides excellent breakdown graphs.
Scaling Up: Transitioning to Audio Power MOSFET Amps
If you are moving from small-signal sensor amps to high-fidelity audio power amps, the part selection changes dramatically. Audio enthusiasts favor MOSFET output stages because their square-law transfer function mimics the soft clipping of vacuum tubes, and unlike BJTs, they do not suffer from secondary breakdown.
However, you must choose between Vertical MOSFETs (like the IRFP series) and Lateral MOSFETs (like the Exicon ECX series). Vertical MOSFETs are cheap, have low Rds(on), and are great for switching power supplies, but their high gate capacitance and negative thermal coefficient at low currents make them notoriously difficult to stabilize in linear audio amps without heavy compensation.
Lateral MOSFETs, originally designed specifically for audio, have a positive temperature coefficient across their entire operating range. This means they are inherently immune to the low-current thermal runaway that plagues vertical parts. While an Exicon lateral pair might cost $17.00 compared to $5.00 for a vertical pair, the reduction in required protection circuitry, the elimination of source degeneration resistors in some designs, and the sheer peace of mind on the bench make them the undisputed default for high-end discrete audio power stages.
