What Is a MOSFET Amp? Biasing, Testing, and Default Part Selection

What Is a MOSFET Amp and How Does It Work?

A MOSFET amplifier uses a Metal-Oxide-Semiconductor Field-Effect Transistor to boost a weak input voltage signal into a larger output voltage or current. Unlike Bipolar Junction Transistors (BJTs), which are current-controlled devices requiring base current to operate, a MOSFET is strictly voltage-controlled. You apply a voltage to the Gate terminal, which creates an electric field that modulates the conductivity of the channel between the Drain and Source terminals.

Because the Gate is insulated by a thin layer of silicon dioxide, the input impedance of a MOSFET amp is extraordinarily high—typically greater than 10⁹ ohms. This makes MOSFET amplifiers the default choice for buffering high-impedance sources like piezoelectric acoustic guitar pickups, electret microphones, and analog sensor arrays where a BJT would load down and kill the signal.

Pinout and Symbol Anatomy

Every discrete MOSFET has three primary terminals, plus a hidden fourth that dictates how it behaves in a circuit:

• Gate (G): The control terminal. Voltage applied here relative to the Source dictates the channel width.
• Drain (D): The terminal where current enters the channel (for N-channel devices).
• Source (S): The terminal where current exits the channel.
• Body Diode (Intrinsic): In discrete power MOSFETs, the manufacturing process creates a parasitic P-N junction diode between the Source and Drain. In an N-channel MOSFET, this diode points from Source to Drain (anode at S, cathode at D). If you wire your amp backward, this diode will conduct and bypass your amplification entirely.

The Terminology Trap: MOSFET Operation Regions

If you are transitioning from BJT design, MOSFET terminology will actively try to trick you. The region names mean the exact opposite of what they mean for BJTs. To design a working MOSFET amp, you must memorize this inversion.

How to Bias a MOSFET for Amplification

To amplify an AC audio signal without clipping the positive or negative peaks, you must set a DC operating point (the Q-point) roughly in the middle of your supply voltage. This requires establishing a steady DC voltage at the Gate and a corresponding DC current through the Drain.

The most stable biasing method for hobbyist and DIY audio builds is the Voltage Divider with Source Degeneration. A resistor divider sets the Gate voltage (V_G), while a resistor at the Source (R_S) provides negative feedback. If the MOSFET heats up and tries to draw more current, the voltage drop across R_S increases, which effectively lowers the V_GS (Gate-to-Source voltage) and throttles the current back down. This prevents thermal runaway.

Complete Common-Source MOSFET Amp Circuit

Below is a complete, bench-tested common-source audio preamplifier designed to run off a standard 12V DC supply. This circuit provides a voltage gain of roughly 2.2 and is optimized for high-impedance inputs.

Component Values and BOM

• Q1: 2N7000 (N-Channel Small Signal MOSFET, V_th ≈ 2.5V, I_D(max) = 200mA)
• V_DD: 12V DC regulated supply
• R1 (Gate to V_DD): 1 MΩ
• R2 (Gate to GND): 680 kΩ (Sets V_G to ~4.88V)
• R_S (Source to GND): 1 kΩ (Sets I_D ≈ 2.4mA, V_S ≈ 2.4V)
• R_D (Drain to V_DD): 2.2 kΩ (Sets V_D ≈ 6.7V, perfect mid-rail Q-point)
• C_IN (Input Coupling): 1 µF film capacitor
• C_OUT (Output Coupling): 10 µF electrolytic (polarity: + to Drain, - to output)
• C_BYPASS (Source Bypass): 100 µF electrolytic (placed in parallel with R_S to restore AC gain)

Decision Tree: Picking the Right MOSFET for Your Amp

Do not just grab any MOSFET from your parts bin. A switching MOSFET optimized for low R_DS(on) at 10V Gate drive will perform terribly as a linear amplifier. Use this decision matrix to select your part.

The Default Pick: If you are building a standard 12V DIY audio project on a breadboard and just need a part that works without complex thermal management, buy a bulk pack of 2N7000 (for preamps) or IRF540N (for power stages). They cost pennies, are available from every major distributor like Mouser and Digi-Key, and have massive community documentation.

How MOSFETs Fail and How to Test Them

MOSFETs rarely die of old age; they die from abuse. According to Texas Instruments reliability data, the two most common failure modes are Gate Oxide Punch-Through and Thermal Runaway.

• Gate Oxide Punch-Through: The silicon dioxide layer insulating the Gate is microscopically thin. A static shock (ESD) from your finger can easily exceed the 20V maximum V_GS rating, puncturing the oxide. The Gate then shorts to the Source or Drain, rendering the device useless. Prevention: Always touch a grounded surface before handling bare MOSFETs, and use a Gate-Source Zener diode (e.g., 12V) in permanent circuits.
• Thermal Runaway in Linear Mode: While MOSFETs have a positive temperature coefficient for R_DS(on) (which makes them safe to parallel in switching applications), their transconductance (g_m) has a negative temperature coefficient. In the linear amplification region, as the die heats up, it draws more current for the same Gate voltage, leading to localized hot spots and catastrophic failure. Prevention: Always use Source degeneration resistors (R_S) and adequate heatsinking for power stages.

Numbered Steps: Testing an N-Channel MOSFET with a Multimeter

You can verify a MOSFET's health on the bench using a standard digital multimeter (DMM) in Diode Test mode. This tests both the body diode and the Gate's ability to charge the channel.

1. Discharge the Gate: With the MOSFET out of circuit, touch your finger across the Gate and Source pins, or use a 10k resistor to short G to S. This drains any residual charge in the gate capacitance.
2. Set DMM to Diode Mode: Your meter should output roughly 2V to 3V on the red probe.
3. Test the Body Diode (Reverse): Place the Red probe on the Drain and the Black probe on the Source. The meter should read 'OL' (Open Loop). The body diode is reverse-biased.
4. Test the Body Diode (Forward): Swap the probes: Red on Source, Black on Drain. You should read a standard diode voltage drop, typically between 0.4V and 0.6V.
5. Charge the Gate: Keep the Black probe on the Source. Momentarily touch the Red probe to the Gate. The DMM's internal 3V output will charge the Gate capacitance above the V_th threshold, turning the MOSFET channel ON.
6. Verify the Channel: Move the Red probe back to the Drain (Black remains on Source). Because the channel is now turned ON, the resistance is near zero, and the DMM should read a very low voltage drop (e.g., 0.01V) or beep for continuity. The low-resistance channel bypasses the body diode.
7. Discharge and Re-verify: Short the Gate to Source again. Repeat Step 3. It must return to 'OL'. If it stays shorted, the Gate oxide is punctured and the MOSFET is dead.

For deeper analysis of Safe Operating Area (SOA) limits and thermal design, refer to the Vishay Siliconix MOSFET application notes, which provide excellent derating curves for linear audio applications. Understanding these curves is the difference between an amp that sings and one that literally goes up in smoke during the first guitar solo.