A MOSFET common source (CS) amplifier is your definitive topology when you need high input impedance paired with moderate-to-high voltage gain. Unlike bipolar junction transistors (BJTs), which are current-controlled and load down the preceding stage, the MOSFET is voltage-controlled. It draws virtually zero DC gate current, making the CS amplifier ideal for buffering high-impedance sources like piezo pickups, electret microphones, or sensor bridges before feeding them into an ADC or power stage. The trade-off is a 180-degree phase inversion between the gate input and the drain output, and a higher noise floor compared to low-noise BJTs, but for general-purpose hobby and bench work, the MOSFET CS amplifier is unbeatable for simplicity.
The Core Anatomy: Symbol, Pinout, and Operation Regions
Before wiring anything, you must understand the three terminals and how the silicon behaves across different bias voltages. In an N-channel enhancement MOSFET, the pins are the Gate (G), Drain (D), and Source (S). The symbol includes an arrow on the source pointing inward (for N-channel) and a dashed line between drain and source, indicating it is normally off (enhancement mode). Crucially, a parasitic body diode exists internally between the source and drain, which heavily influences how the device handles reverse voltages and how you test it.
For amplification, we must bias the MOSFET into the correct region. A common point of confusion for beginners transitioning from BJTs is the terminology: MOSFET 'saturation' is actually the active amplification region, whereas 'triode' is the closed-switch region.
| Region | Gate-Source Voltage (V_GS) | Drain-Source Voltage (V_DS) | Typical I_D / Behavior | Primary Use |
|---|---|---|---|---|
| Cutoff | < V_GS(th) (e.g., < 2.0V) | Any | ~0 mA (Leakage only) | Open switch, digital logic '0' |
| Triode (Linear) | > V_GS(th) | < (V_GS - V_GS(th)) | 10mA - 50A (Acts as resistor) | Closed switch, PWM motor control |
| Saturation (Active) | > V_GS(th) | > (V_GS - V_GS(th)) | 1mA - 10A (Constant current) | Linear amplification, CS amplifiers |
Biasing for Linear Gain: Setting the Q-Point
To use the MOSFET as a linear amplifier, you must set a DC quiescent point (Q-point) dead center in the saturation region. If you just tie the gate to a voltage divider and ground the source, the circuit will be highly unstable. The threshold voltage (V_GS(th)) varies wildly between individual transistors of the exact same part number (e.g., a 2N7000 might turn on at 0.8V or 3.0V), and it drifts with temperature.
Add a resistor (R_S) between the source and ground. This creates negative feedback. If the MOSFET tries to draw more current due to heat or part variance, the voltage across R_S rises, which effectively lowers the V_GS (since V_GS = V_G - V_S), throttling the current back down. To prevent this resistor from killing your AC voltage gain, you bypass it with a capacitor (C_S) that acts as a short circuit for audio frequencies.
The voltage gain (A_v) of a CS amplifier with a bypassed source capacitor is roughly A_v = -g_m × R_D, where g_m is the transconductance of the MOSFET at your chosen bias current, and R_D is the drain resistor. The negative sign denotes the 180° phase inversion.
Complete Application Circuit: 20kHz Audio Preamp
Here is a complete, bench-tested common source preamplifier designed to amplify a high-impedance acoustic guitar pickup or piezo sensor to line-level (approx. 1V p-p). We are using a standard 9V battery supply and the ubiquitous 2N7000 N-channel MOSFET.
Component Bill of Materials
- Q1: 2N7000 (N-Channel Enhancement MOSFET)
- R1, R2: 1 MΩ (Gate voltage divider, sets V_G = 4.5V)
- R_D: 2.2 kΩ (Drain load resistor)
- R_S: 1 kΩ (Source degeneration resistor)
- R_G: 1 MΩ (Gate pull-down to prevent static buildup on floating inputs)
- C_in, C_out: 1 µF ceramic (AC coupling, blocks DC)
- C_S: 10 µF electrolytic (Source bypass, sets low-frequency roll-off)
- C_filt: 100 nF ceramic (Power rail decoupling)
Numbered Build and Verification Steps
- Establish the Bias Network: Connect R1 from the 9V rail to the Gate. Connect R2 from the Gate to Ground. This sets the Gate DC voltage to exactly 4.5V. Because gate current is zero, there is no voltage drop across the 1MΩ resistors.
- Set the Source Current: Connect R_S (1k) from Source to Ground. The MOSFET will self-bias until V_S is roughly 2.0V to 2.5V (depending on the specific V_GS(th) of your part). This yields a quiescent drain current (I_D) of about 2.0 mA to 2.5 mA.
- Wire the Drain Load: Connect R_D (2.2k) from the 9V rail to the Drain. With ~2mA flowing, the voltage drop across R_D is ~4.4V, leaving the Drain sitting at roughly 4.6V. This leaves roughly 4.6V of headroom for the AC signal to swing up and down without clipping.
- AC Coupling: Connect C_in to the Gate (via R_G to ground) and C_out to the Drain. Connect C_S in parallel with R_S, observing correct polarity (positive toward the Source).
- Verify with a Multimeter: Power the circuit. Measure V_G (should be 4.5V), V_S (should be ~2.2V), and V_D (should be ~4.5V). If V_D is pegged at 9V, your MOSFET is in cutoff. If V_D is near 0V, it is in the triode region. Adjust R1/R2 ratio if your specific 2N7000 is biased too far off-center.
Decision Tree: Topology and Part Selection
Not every application calls for a Common Source topology. Use this decision matrix to lock in the right configuration and part number for your specific bench project.
| If your primary requirement is... | Choose this Topology | Key Characteristic | Safe Default Part Number |
|---|---|---|---|
| High voltage gain, high input impedance (Audio/Sensors) | Common Source (CS) | Inverts phase, moderate bandwidth | 2N7000 or BS170 |
| Impedance matching, unity voltage gain, high current drive | Common Drain (Source Follower) | No phase inversion, V_out follows V_G - V_GS | IRF510 (for power) or 2N7000 |
| High-frequency / RF amplification, low input impedance | Common Gate (CG) | No Miller effect, excellent high-freq response | BF998 (Dual gate RF FET) |
| Switching high currents (PWM, Motor Control) | Triode Region (Switch) | Drive V_GS to 10V+ to minimize R_DS(on) | IRLZ44N (Logic level power FET) |
Failure Modes and Multimeter Troubleshooting
MOSFETs do not fail like BJTs. A BJT usually fails short due to thermal runaway. A MOSFET usually fails because of gate oxide puncture. The silicon dioxide layer separating the gate from the channel is incredibly thin. A static shock (ESD) or exceeding the maximum V_GS (typically ±20V for a 2N7000) will arc through this oxide, permanently shorting the gate to the source or drain.
How to Test a MOSFET with a Digital Multimeter
You can verify the health of an N-channel MOSFET in-circuit (if power is removed and capacitors are drained) or out-of-circuit using the Diode Test mode on your DMM.
- Discharge the Gate: Briefly short the Gate and Source pins together with a piece of wire or your finger to ensure no residual charge is holding the channel open.
- Test the Body Diode: Place the Red probe on the Source and the Black probe on the Drain. You should read a standard diode forward voltage drop (typically 0.4V to 0.6V).
- Test Reverse Block: Swap the probes (Red on Drain, Black on Source). The meter should read 'OL' (Open Loop), indicating the body diode is reverse-biased and the channel is off.
- The Failure Indicator: If the meter reads 0.0V (a dead short) in both directions, or if it reads 'OL' in both directions (meaning the body diode is blown open), the MOSFET is destroyed. Desolder and replace it.
The Verdict: Your Safe Default Picks
When designing a MOSFET CS amplifier for general-purpose audio, sensor buffering, or educational breadboarding, do not overcomplicate your BOM with surface-mount RF FETs or high-power switching MOSFETs. The 2N7000 (TO-92 package) remains the undisputed king of the hobbyist bench. It costs pennies, is available from every major distributor, and its transconductance is perfectly suited for 1mA to 5mA small-signal amplification. If you need slightly higher current handling (up to 500mA) while maintaining the same pinout and TO-92 footprint, step up to the BS170. For anything requiring more than 100mA of continuous drain current, abandon the TO-92 package and move to a TO-220 power MOSFET like the IRF510, ensuring you provide adequate heatsinking and a gate driver capable of pushing V_GS to at least 10V to fully enhance the channel.
