A MOSFET amplifier is an electronic circuit that uses a Metal-Oxide-Semiconductor Field-Effect Transistor to increase the amplitude (voltage, current, or power) of an input signal. Unlike Bipolar Junction Transistors (BJTs) which are current-controlled, a MOSFET is a voltage-controlled device. A small AC input voltage applied to the high-impedance Gate modulates the current flowing from the Drain to the Source. This effectively carves out a larger, high-fidelity replica of the input signal across a load resistor, powered by a DC supply. Because the Gate draws virtually zero steady-state current, MOSFET amplifiers offer exceptionally high input impedance, making them ideal for buffering high-impedance sources like piezo pickups, condenser microphones, and sensor outputs.
The Anatomy and Symbol of a MOSFET
To understand how the amplifier works, you must first understand the physical terminals and the schematic symbol. A discrete MOSFET typically has three accessible pins, with a fourth internal connection:
- Gate (G): The control terminal. It is physically separated from the semiconductor channel by a microscopically thin layer of silicon dioxide (SiO2). This insulation is what gives the MOSFET its near-infinite DC input impedance.
- Drain (D): The terminal where majority carriers exit the channel. In an N-channel device, this is connected to the positive supply via a load resistor.
- Source (S): The terminal where majority carriers enter the channel. It is typically tied to ground or a source-degeneration resistor.
- Body/Substrate (B): In discrete 3-pin MOSFETs, the body is internally shorted to the Source. This creates an intrinsic body diode between Drain and Source, which is crucial to remember during testing and inductive load switching.
Schematic Symbol: For an N-channel enhancement-mode MOSFET (the most common type used in amplifiers), the symbol features three lines for D, S, and B, with an arrow on the Source pointing inward toward the channel. The Gate is drawn as a separate line parallel to the channel, indicating insulation. The channel line between Drain and Source is broken (dashed), signifying that the device is 'normally off' and requires a positive Gate-to-Source voltage (Vgs) to turn on.
MOSFET Operation Regions for Amplification
A MOSFET operates in three distinct regions depending on the applied voltages. For linear amplification, you must bias the transistor strictly in the Saturation (Active) Region. If it drifts into the Triode region, your audio or signal will clip and distort heavily.
| Region | Condition (N-Channel) | Behavior | Typical Application |
|---|---|---|---|
| Cutoff | Vgs < Vth | Channel is pinched off. Ids = 0A. | Switching (OFF state) |
| Linear (Triode) | Vgs > Vth AND Vds < (Vgs - Vth) | Acts as a voltage-controlled resistor. Ids increases linearly with Vds. | Switching (ON state), analog multipliers |
| Saturation (Active) | Vgs > Vth AND Vds ≥ (Vgs - Vth) | Ids is controlled almost entirely by Vgs and is largely independent of Vds. | Linear Amplification, current mirrors |
Designing a Common-Source MOSFET Amplifier
The Common-Source (CS) configuration is the MOSFET equivalent of the BJT Common-Emitter. It provides high voltage gain and inverts the phase of the input signal by 180 degrees. Let's design a basic Class-A voltage amplifier using a 2N7000 N-channel MOSFET.
Target Specifications and Biasing Math
We will use a Vdd = 12V DC supply. To maximize the symmetrical output voltage swing without clipping, we must bias the Drain-to-Source voltage (Vds) to exactly half of Vdd. Therefore, Target Vds = 6V. We will target a quiescent Drain current (Id) of 5mA.
Complete Application Circuit Component Values
- MOSFET (Q1): 2N7000 (Threshold voltage Vth is typically ~2.1V).
- Drain Resistor (Rd): Sets the gain and DC bias. Rd = (Vdd - Vds) / Id = (12V - 6V) / 5mA = 1.2kΩ. Use a standard 1.2kΩ 1/4W resistor.
- Source Resistor (Rs): Provides DC stability (negative feedback) against temperature shifts and part-to-part Vth variations. Let's drop 0.5V across it. Rs = 0.5V / 5mA = 100Ω. Use a standard 100Ω 1/4W resistor.
- Gate Bias Network (R1, R2): We need the Gate voltage (Vg) to be Vgs + Vs. If Vth is ~2.1V and we want 5mA, a Vgs of roughly 2.5V is required. Since Vs = 0.5V, Vg needs to be 3.0V. We use a voltage divider from the 12V rail. Let R2 = 1MΩ. R1 = R2 * ((Vdd/Vg) - 1) = 1M * (12/3 - 1) = 3MΩ. Use R1 = 3.3MΩ and R2 = 1MΩ (yields Vg ≈ 2.8V, close enough for small-signal with Rs feedback).
- Input Coupling Capacitor (Cin): Blocks DC from the signal source. Use 1µF film or ceramic.
- Output Coupling Capacitor (Cout): Blocks the 6V DC bias from the load. Use 10µF electrolytic (polarity: positive to Drain).
- Source Bypass Capacitor (Cs): Placed in parallel with Rs to restore AC gain that Rs would otherwise suppress. Use 100µF electrolytic.
Selecting Safe Default Part Numbers
Choosing the right MOSFET is critical. A power MOSFET optimized for switching (low Rds(on)) often has massive input capacitance (Ciss), making it a poor choice for high-frequency or small-signal linear amplification due to the Miller effect. Here are three safe, widely available defaults depending on your power requirements.
| Part Number | Type / Use Case | Vds (Max) | Id (Continuous) | Rds(on) @ 10Vgs | Package |
|---|---|---|---|---|---|
| 2N7000 | Small-signal, pre-amps, sensors | 60V | 200mA | 5.0Ω | TO-92 |
| IRF520 | Medium power, headphone amps, drivers | 100V | 9.2A | 0.27Ω | TO-220 |
| IRFZ44N | High power, Class D switching, motor loads | 55V | 49A | 0.0175Ω | TO-220 |
For more detailed parameter matching, refer to the Vishay MOSFET device selector or standard electronics tutorials on MOSFET amplifier configurations.
Failure Modes and Multimeter Testing
The most common way a MOSFET fails in a linear amplifier is Gate oxide punch-through. The SiO2 insulating layer is incredibly thin (often less than 100 nanometers). A static discharge (ESD) of just 20V can arc across this layer, permanently shorting the Gate to the Source or Drain. Once punctured, the MOSFET loses its high impedance and acts as a dead short or a leaky resistor.
How to Test a MOSFET with a Digital Multimeter (DMM)
- Discharge the Gate: Touch a finger across the Gate and Source pins, or use a 10kΩ resistor to short G to S. This bleeds off any trapped charge in the gate capacitance.
- Set DMM to Diode Test Mode: This mode applies a small voltage and reads the forward voltage drop.
- Test the Body Diode: Place the Red probe on the Source and the Black probe on the Drain (for an N-channel). You should read a standard diode forward voltage drop, typically between 0.4V and 0.6V.
- Reverse Bias the Diode: Swap the probes (Red to Drain, Black to Source). The meter should read OL (Overload / Open Loop), confirming the diode blocks reverse current.
- Test Gate Insulation: Place one probe on the Gate and the other on the Drain. It must read OL. Repeat for Gate to Source. It must also read OL. If you read any resistance or a diode drop between the Gate and the other pins, the oxide layer is destroyed. Throw the part away.
For a deeper dive into semiconductor physics and failure analysis, the All About Circuits semiconductor textbook provides excellent foundational theory.
Frequently Asked Questions
What is the difference between a BJT and a MOSFET amplifier?
The primary difference lies in how they are driven. A BJT (Bipolar Junction Transistor) is a current-controlled device; it requires a continuous base current to maintain amplification, resulting in a relatively low input impedance (typically a few kilo-ohms). A MOSFET is voltage-controlled; it requires virtually zero steady-state gate current, yielding an input impedance in the mega-ohm or giga-ohm range. Additionally, BJTs suffer from negative temperature coefficients (they draw more current as they get hotter, leading to thermal runaway), while power MOSFETs generally have a positive temperature coefficient for Rds(on), making them easier to parallel for high-power applications without one device hogging all the current.
Why does my MOSFET amplifier keep blowing the gate?
If your MOSFETs are repeatedly failing with a shorted Gate, you are likely experiencing ESD (Electrostatic Discharge) damage or voltage spikes exceeding the maximum Vgs rating (usually ±20V for standard MOSFETs). To fix this, always add a 10kΩ to 100kΩ pull-down resistor directly between the Gate and Source pins on the PCB. This provides a safe path for static charges to dissipate when the signal source is disconnected. If your circuit involves long cables or inductive loads, add a 12V Zener diode across the Gate and Source to clamp transient voltage spikes before they can punch through the silicon dioxide layer.
Can I use a power MOSFET like the IRFZ44N for small audio signal amplification?
While it will technically amplify a signal, it is highly discouraged for high-fidelity small-signal audio. Power MOSFETs (especially vertical trench types like the IRFZ44N) are optimized for hard switching, not linear operation. They possess massive input and Miller capacitances (often >1000pF), which will severely roll off your high-frequency response and cause phase shifts. Furthermore, their transconductance (gm) curve is highly non-linear near the threshold voltage, leading to severe crossover distortion in Class AB push-pull audio stages. For audio, stick to small-signal MOSFETs (like the 2N7000), lateral MOSFETs specifically designed for audio (like the Exicon EC10N16), or standard BJTs.
