An enhancement-mode MOSFET (E-MOSFET) amplifier uses a normally-off transistor biased into its saturation region to amplify small AC signals. Unlike JFETs or depletion-mode MOSFETs, an E-MOSFET requires a gate-to-source voltage ($V_{GS}$) that exceeds its specific threshold voltage ($V_{GS(th)}$) to conduct. For small-signal audio and sensor amplification, the 2N7000 (N-channel) and BS170 are the safest, most widely available default parts, while the IRF510 serves as the standard for low-power audio output stages.
E-MOSFET Basics, Pinouts, and Safe Default Part Numbers
Before designing the circuit, you must understand the physical layout and the schematic symbol. In a standard TO-92 package (like the 2N7000), holding the flat side facing you with the leads pointing down, the pins from left to right are Source (S), Gate (G), and Drain (D). The schematic symbol for an N-channel E-MOSFET features a broken vertical line between the Drain and Source, indicating the normally-off channel, with an arrow on the Source terminal pointing inward toward the channel.
Selecting the right part prevents thermal runaway and ensures your bias network can actually turn the device on. The table below lists the benchmark E-MOSFETs for hobbyist and bench amplifier designs, complete with their critical ratings.
| Part Number | Type | $V_{DS(max)}$ | $I_{D(max)}$ | $V_{GS(th)}$ Range | $R_{DS(on)}$ | Package |
|---|---|---|---|---|---|---|
| 2N7000 | N-Channel | 60V | 200mA | 0.8V to 3.0V | 5.0 Ω @ 10V | TO-92 |
| BS170 | N-Channel | 60V | 500mA | 0.8V to 2.0V | 5.0 Ω @ 10V | TO-92 |
| IRF510 | N-Channel | 100V | 5.6A | 2.0V to 4.0V | 0.54 Ω @ 10V | TO-220 |
| IRF9540N | P-Channel | -100V | -23A | -2.0V to -4.0V | 0.117 Ω @ -10V | TO-220 |
Operation Regions and Biasing for Linear Amplification
To function as a linear amplifier, the E-MOSFET must be biased in the Saturation region (often called the Active region in BJT terminology). If it drifts into the Triode region, it acts as a voltage-controlled resistor and your signal will clip severely. If it drops below threshold, it enters Cutoff and the signal cuts out entirely.
| Region | Gate Condition | Drain-Source Condition | Behavior & Typical Use |
|---|---|---|---|
| Cutoff | $V_{GS} < V_{GS(th)}$ | Any $V_{DS}$ | No channel, $I_D \approx 0$. Used for digital switching (OFF). |
| Triode (Ohmic) | $V_{GS} > V_{GS(th)}$ | $V_{DS} < V_{GS} - V_{GS(th)}$ | Acts as a voltage-controlled resistor. Used for analog switches. |
| Saturation (Active) | $V_{GS} > V_{GS(th)}$ | $V_{DS} \ge V_{GS} - V_{GS(th)}$ | Constant current source behavior. Required for linear amplification. |
The most stable biasing topology for an E-MOSFET amplifier is the voltage-divider bias with source degeneration. A high-impedance resistor divider sets the Gate voltage ($V_G$), while a Source resistor ($R_S$) develops a voltage drop ($V_S$) proportional to the Drain current. This creates negative feedback: if temperature rises and $I_D$ tries to increase, $V_S$ increases, which reduces $V_{GS}$ (since $V_{GS} = V_G - V_S$), automatically throttling the current back down.
Complete Common-Source E-MOSFET Amplifier Circuit
Below is a complete, bench-tested common-source amplifier design using the 2N7000. This circuit targets a quiescent drain current ($I_{DQ}$) of 5mA from a 12V DC supply, providing a voltage gain of approximately -15.
Component Values and Bill of Materials
- Q1: 2N7000 (N-Channel E-MOSFET)
- R1: 1 MΩ (Gate pull-up to $V_{DD}$)
- R2: 470 kΩ (Gate pull-down to GND)
- RD: 1 kΩ (Drain load resistor, 1/4W)
- RS: 220 Ω (Source degeneration resistor, 1/4W)
- CIN: 1 μF (Input coupling capacitor, 25V)
- COUT: 10 μF (Output coupling capacitor, 25V)
- CS: 100 μF (Source bypass capacitor, 25V)
Design Math and Verification
- Set Gate Voltage ($V_G$): The divider sets $V_G = 12V \times [470k / (1M + 470k)] = 3.83V$.
- Establish Source Voltage ($V_S$): With a target $I_D = 5mA$, the drop across $R_S$ is $V_S = 5mA \times 220\Omega = 1.1V$.
- Verify $V_{GS}$: $V_{GS} = V_G - V_S = 3.83V - 1.1V = 2.73V$. This sits perfectly inside the 2N7000's 0.8V-3.0V threshold window.
- Set Drain Voltage ($V_D$): The drop across $R_D$ is $5mA \times 1k\Omega = 5V$. Therefore, $V_D = 12V - 5V = 7V$.
- Confirm Saturation: $V_{DS} = V_D - V_S = 7V - 1.1V = 5.9V$. The overdrive voltage ($V_{GS} - V_{GS(th)}$) is roughly 0.7V. Since $V_{DS}$ (5.9V) is much greater than 0.7V, the MOSFET is firmly in the saturation region.
The source bypass capacitor ($C_S$) shorts $R_S$ at AC signal frequencies, preventing AC negative feedback and maximizing voltage gain. The gain is determined by $A_v = -g_m \times R_D$, where transconductance $g_m \approx 15 mS$ at this bias point, yielding a gain of roughly -15.
Failure Modes and Multimeter Testing Procedures
E-MOSFETs fail differently than BJTs. The most common failure mode is gate oxide puncture caused by Electrostatic Discharge (ESD). The silicon dioxide layer separating the gate from the channel is incredibly thin; a static shock from your finger can arc through it, permanently shorting the Gate to the Source or Drain. Thermal runaway is less common than in BJTs due to the MOSFET's positive temperature coefficient at high currents, but localized heating in power stages (like the IRF510) can still destroy the die if the Safe Operating Area (SOA) is exceeded.
You can test an E-MOSFET on the bench using a standard digital multimeter (DMM) in Diode Test mode. This exploits the intrinsic body diode present between the Drain and Source.
- Discharge the Gate: Touch the DMM probes across the Gate and Source pins simultaneously to bleed off any trapped capacitive charge. The MOSFET is now in Cutoff.
- Test the Body Diode (Reverse): Place the red probe on the Drain and the black probe on the Source (for an N-channel). The meter should read 'OL' (open loop) because the body diode is reverse-biased and the channel is off.
- Test the Body Diode (Forward): Swap probes: red on Source, black on Drain. You should read a standard diode drop, typically between 0.450V and 0.650V. If it reads 0.00V or a dead short, the device is blown.
- Charge the Gate: Leave the black probe on the Source. Briefly touch the red probe to the Gate. This injects positive charge into the gate capacitance, turning the channel ON.
- Verify Channel Conduction: Move the red probe back to the Drain (black still on Source). The meter should now read a very low voltage drop (often near 0.00V or a low resistance), indicating the channel is conducting and bypassing the body diode.
- Discharge to Turn Off: Short Gate to Source again. Re-test Drain-to-Source forward bias; it should revert to the 0.5V body diode reading, proving the gate control works.
E-MOSFET vs. JFET and BJT Amplifiers
When deciding whether to use an E-MOSFET, a Junction FET (JFET), or a Bipolar Junction Transistor (BJT) for your amplifier stage, the choice hinges on input impedance requirements, biasing complexity, and available headroom. The following comparison matrix outlines the practical trade-offs for audio and sensor preamplifiers.
| Criterion | E-MOSFET (e.g., 2N7000) | JFET (e.g., J201) | BJT (e.g., 2N3904) |
|---|---|---|---|
| Input Impedance | Extremely High ($>10^9 \Omega$) | Very High ($>10^8 \Omega$) | Moderate ($1k\Omega - 5k\Omega$) |
| Biasing Complexity | Moderate (Requires $V_{GS} > V_{th}$) | Simple (Self-biasing with $R_S$) | Moderate (Requires $V_{BE} \approx 0.7V$) |
| Transconductance ($g_m$) | Medium (10 - 50 mS) | Low (1 - 5 mS) | High (50 - 200 mS) |
| Thermal Stability | Good (with $R_S$ degeneration) | Excellent (Zero Temp Co point) | Poor (Prone to thermal runaway) |
| Best Application | High-Z sensor interfaces, electret mic preamps | Guitar preamps, RF front-ends | General purpose voltage gain, low-Z drivers |
Choose the E-MOSFET when you need massive input impedance to prevent loading down high-impedance sources like piezoelectric sensors or capacitive microphones. Choose the JFET if you want the simplest possible self-biasing circuit without worrying about threshold voltage variations. Choose the BJT if you need maximum voltage gain per stage and are driving lower impedance loads, provided you implement proper thermal compensation. For a comprehensive look at small-signal MOSFET characteristics, refer to the ON Semiconductor 2N7000 datasheet and the foundational theory outlined in All About Circuits' semiconductor textbook.
