A common source MOSFET amplifier is the field-effect equivalent of a BJT common-emitter stage. It provides high input impedance, moderate-to-high voltage gain, and a 180-degree phase inversion between the gate input and drain output. Unlike bipolar transistors that require continuous base current to maintain bias, a MOSFET's insulated gate draws virtually zero steady-state DC current, making it ideal for buffering high-impedance sensors, piezo elements, or microphone preamps without loading the source.
For small-signal audio or sensor amplification on the bench, the 2N7000 (N-channel, 60V, 200mA) is the safest, most accessible default. For higher-power RF applications or driving heavier loads, the IRF510 (100V, 5.6A) is the standard choice. This guide covers the exact biasing math, a complete 1kHz preamp circuit, and how to verify your transistor with a multimeter before soldering.
Symbol, Pinout, and Safe Default Part Numbers
Before wiring anything, you must verify the physical pinout. The schematic symbol for an N-channel enhancement MOSFET features three terminals: the Gate (G) drawn as an isolated vertical line, the Drain (D) at the top, and the Source (S) at the bottom with an arrow pointing inward. The dashed line between the drain and source indicates it is normally off (enhancement mode).
Physical pinouts vary drastically by package. Never assume the pin order based on the package shape alone; always verify against the manufacturer's datasheet. However, here are the industry-standard bench defaults and their typical TO-92 / TO-220 pinouts (viewed from the front, pins pointing down):
| Part Number | Type & Package | V_DS (Max) | I_D (Max) | V_GS(th) Range | Best Application |
|---|---|---|---|---|---|
| 2N7000 | N-Ch / TO-92 (S-G-D) | 60V | 200mA | 0.8V - 3.0V | Mic preamps, sensor buffering, logic level shifting |
| BS170 | N-Ch / TO-92 (D-G-S) | 60V | 500mA | 0.8V - 2.5V | Higher current small-signal, LED drivers |
| IRF510 | N-Ch / TO-220 (G-D-S) | 100V | 5.6A | 2.0V - 4.0V | RF amplifiers, Class D outputs, ham radio |
| CD4007UB | CMOS Array / DIP-14 | 15V | 10mA | ~1.5V | Matched pair differential amps, current mirrors |
Operation Regions and Biasing for Linear Amplification
To use a MOSFET as a linear amplifier, you must bias it in the Saturation Region. This terminology is a notorious trap for engineers transitioning from BJTs: in a BJT, 'saturation' means the switch is fully ON (low voltage drop). In a MOSFET, 'saturation' means the channel is pinched off at the drain end, creating a constant-current source ideal for amplification. The fully-ON switching state for a MOSFET is actually called the Triode or Ohmic region.
| Operation Region | Gate-Source Voltage (V_GS) | Drain-Source Voltage (V_DS) | Drain Current (I_D) Behavior | Primary Use Case |
|---|---|---|---|---|
| Cutoff | V_GS < V_th | Any | I_D = 0 (Leakage only) | Switch OFF state |
| Triode (Ohmic) | V_GS > V_th | V_DS < (V_GS - V_th) | I_D depends on both V_GS and V_DS (acts as resistor) | Switch ON state, analog multiplexers |
| Saturation (Active) | V_GS > V_th | V_DS ≥ (V_GS - V_th) | I_D depends only on V_GS (constant current) | Linear Amplification, current sources |
How to Bias for the Job:
Because the threshold voltage (V_th) of a MOSFET can vary by ±1V even within the same manufacturing batch, relying on a fixed gate voltage will result in unpredictable drain currents and severe clipping. The standard solution is source degeneration. By placing a resistor (R_S) between the source and ground, you create negative DC feedback. If the transistor tries to draw more current, the voltage across R_S increases, which effectively reduces the V_GS (since V_GS = V_G - V_S), stabilizing the quiescent operating point (Q-point).
Complete Application Circuit: 1kHz Audio Preamp
Let us design a practical common source amplifier using a 2N7000 to amplify a 1kHz AC signal from a high-impedance source. We will target a 12V DC supply (V_DD), a quiescent drain current (I_DQ) of 2mA, and a mid-rail drain voltage (V_DSQ) of 6V to maximize symmetrical voltage swing.
Component Selection and Biasing Math
- Drain Resistor (R_D): We need a 6V drop at 2mA. R_D = (12V - 6V) / 0.002A = 3000Ω. Select standard value: 3.3kΩ. (Actual I_D will settle slightly lower, around 1.8mA).
- Source Resistor (R_S): To provide good DC stability without eating too much voltage headroom, let us drop about 1V across R_S. R_S = 1V / 0.002A = 500Ω. Select standard value: 470Ω.
- Gate Bias Network (R1, R2): The source will sit at roughly V_S = 1.8mA * 470Ω = 0.85V. Assuming a typical V_GS(th) of 2.0V for our specific 2N7000, the gate needs to be at V_G = 0.85V + 2.0V = 2.85V. Using a voltage divider from 12V with R2 = 1MΩ (to maintain high input impedance), R1 calculates to roughly 3.3MΩ. Select R1 = 3.3MΩ, R2 = 1MΩ.
- Coupling Capacitors (C_IN, C_OUT): To pass 1kHz without attenuating the signal, we need an RC high-pass cutoff well below 100Hz. With a 1MΩ gate impedance, a 0.1μF film capacitor yields a cutoff around 1.6Hz. Select 0.1μF for C_IN and 1μF for C_OUT.
- Source Bypass Capacitor (C_S): R_S stabilizes DC but kills AC gain. Placing a capacitor in parallel with R_S shorts it out for AC signals. For 1kHz, a 10μF electrolytic capacitor provides an impedance of ~16Ω, effectively bypassing the 470Ω resistor. Select 10μF (observe polarity).
For a deeper theoretical breakdown of the small-signal model and transconductance derivations, refer to the All About Circuits guide on MOSFET amplifiers or the Electronics Tutorials MOSFET amplifier module.
Failure Modes and Multimeter Testing
MOSFETs do not fail like BJTs. While BJTs typically fail short due to thermal runaway, MOSFETs usually fail due to Gate Oxide Puncture from Electrostatic Discharge (ESD) or exceeding the maximum V_GS rating (typically ±20V). The gate oxide layer is only nanometers thick; a static shock from your finger can easily arc through it, creating a permanent short between the gate and source. Secondary failure modes include avalanche breakdown from inductive kickback (exceeding V_DS) and thermal destruction from sustained high-current switching without adequate heatsinking.
How to Test an N-Channel MOSFET with a Digital Multimeter
You can verify the health of a MOSFET on the bench using the Diode Test mode on your DMM. This tests the intrinsic body diode and the gate's capacitive ability to turn the channel on.
- Discharge the Gate: Touch your finger across the Gate and Source pins (or use a jumper wire) to bleed off any stored static charge in the gate capacitance.
- Test the Body Diode (Reverse): Set DMM to Diode mode. Place the Black (common) probe on the Drain and the Red probe on the Source. The meter should read 'OL' (Open Loop) because the body diode is reverse-biased.
- Charge the Gate: Keep the Black probe on the Drain. Momentarily touch the Red probe to the Gate pin. This applies the DMM's internal ~3V battery to the gate, charging the oxide capacitance and turning the channel ON.
- Test the Channel (Forward): Move the Red probe back to the Source pin (Black still on Drain). The meter should now read a very low voltage drop (typically 0.00V to 0.2V), indicating the channel is conducting and bypassing the body diode.
- Discharge and Verify: Short the Gate to the Source again to discharge it. Repeat Step 2; the meter should return to 'OL', confirming the transistor successfully turned off.
If the meter reads 'OL' in both directions regardless of gate charging, the channel is blown open. If it reads a short (0.00V or beeps continuously) in both directions even after discharging the gate, the gate oxide is punctured or the channel is shorted. Discard the part.
Frequently Asked Questions
Why does my common source MOSFET amplifier have so much distortion?
Distortion in a common source stage usually stems from two issues: Q-point drift or transconductance (g_m) non-linearity. If your DC bias is too close to the supply rail or ground, the AC signal will clip asymmetrically (hard clipping). Measure your DC drain voltage with no signal applied; it should be roughly V_DD / 2. If the DC bias is correct but you still see soft, rounded distortion on your oscilloscope, the issue is the inherent square-law transfer characteristic of the MOSFET. Unlike BJTs which have a relatively linear exponential curve over small ranges, MOSFET g_m varies heavily with drain current at low V_GS levels. Adding an unbypassed source resistor (leaving a small portion of R_S un-bypassed by C_S) introduces local negative feedback, which linearizes the gain at the cost of overall amplification.
Can I use a power MOSFET like the IRF520 for small-signal audio amplification?
Technically yes, but practically it is a poor choice. Power MOSFETs like the IRF520 or IRF540 are optimized for switching high currents, not linear amplification of millivolt signals. They suffer from three major drawbacks in small-signal audio: First, they have massive gate capacitance (C_iss), which rolls off high-frequency response unless driven by a low-impedance buffer. Second, their threshold voltage spread is wide, making DC biasing a guessing game without heavy source degeneration. Third, their transconductance at low quiescent currents (e.g., 5mA) is abysmal, resulting in extremely low voltage gain and high crossover distortion. Stick to small-signal FETs like the 2N7000, BS170, or JFETs like the J201 for audio preamps.
How do I calculate the voltage gain of a common source amplifier?
The small-signal voltage gain (A_v) of a common source amplifier with a fully bypassed source resistor is calculated as A_v = -g_m * (R_D || R_L), where g_m is the transconductance of the MOSFET at your specific quiescent drain current, R_D is the drain resistor, and R_L is the external load resistance. The negative sign denotes the 180-degree phase inversion. To find g_m, use the formula: g_m = 2 * √(K * I_DQ), where K is the device-specific transconductance parameter found in the datasheet (often listed as g_fs or forward transconductance). If your source resistor is not bypassed by a capacitor, the gain formula changes to A_v = -(g_m * R_D) / (1 + g_m * R_S), which is much lower but significantly more stable and linear.
