What a MOSFET amplifier circuit does and why you'd draw one
A MOSFET amplifier takes a small electrical signal and makes it larger. The MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) is the component that does the amplifying work. When you draw the circuit on paper or in software, you're mapping out how the MOSFET connects to resistors, capacitors, and a power supply so that a weak input signal becomes a strong output signal.
You draw a MOSFET amplifier circuit because you need to understand how the parts work together before you build it, test it, or troubleshoot it. The drawing shows you where current flows, which components control the signal, and where voltage drops occur. Calculating the circuit tells you whether your design will actually amplify the signal the way you want, or whether you need to change resistor values, capacitor sizes, or the power supply voltage.
The most common type is the common-source amplifier, where the input signal goes to the gate of the MOSFET, the output comes from the drain, and the source connects to ground. This configuration gives you voltage gain — meaning the output voltage swings are larger than the input voltage swings.
Key Takeaways
- A MOSFET amplifier circuit has five main parts: the MOSFET itself, a drain resistor, a gate resistor, coupling capacitors, and a power supply.
- The drain resistor value determines how much the output voltage changes when the MOSFET turns on and off, directly affecting your voltage gain.
- Coupling capacitors block DC voltage while passing AC signals, so they must be large enough that they don't weaken the signal at your lowest frequency of interest.
- You calculate voltage gain by dividing the output voltage swing by the input voltage swing, or by using the formula gain equals transconductance times drain resistance.
- Biasing the gate correctly — usually with a voltage divider made from two resistors — keeps the MOSFET in the active region where it amplifies instead of switching on and off.
The five essential components and what each one does
A basic common-source MOSFET amplifier needs a MOSFET, a drain resistor, a gate bias network, coupling capacitors, and a DC power supply. The MOSFET is the active element — it's the transistor that amplifies. The drain resistor (often called RD) connects between the positive power supply and the drain pin of the MOSFET. When the MOSFET conducts more current, more voltage drops across this resistor, so the drain voltage drops. This voltage change is your amplified output signal.
The gate bias network is usually two resistors in series between the power supply and ground, with the connection point between them going to the gate of the MOSFET. This voltage divider sets the DC voltage at the gate so the MOSFET sits in the middle of its active region — not fully off, not fully on. Without proper biasing, the MOSFET either won't amplify or will distort the signal badly.
The coupling capacitors block DC voltage while letting AC signals pass through. One capacitor sits between your input signal source and the gate resistor. Another sits between the drain and your output load. These capacitors prevent the DC bias voltages from reaching the input and output, which would damage equipment or cause the signal to sit at the wrong voltage level.
The power supply provides the DC voltage that the MOSFET needs to operate. Common values are 5 volts, 12 volts, or 15 volts, depending on what signal levels you need and what other circuits you're connecting to.
How to draw the circuit on paper or in software
Start by drawing the power supply rail at the top of your page and the ground rail at the bottom. Between them, draw a vertical line representing the drain resistor RD. At the bottom of RD, draw the MOSFET symbol — a rectangle with three lines coming out: the gate (usually on the left), the drain (top), and the source (bottom). Connect the source directly to ground.
To the left of the gate, draw a capacitor symbol (two parallel lines) — this is your input coupling capacitor. To the left of that capacitor, draw a resistor symbol and label it Rin — this represents your signal source impedance or an external resistor. The signal you want to amplify connects to the left side of Rin.
Between the power supply and ground, draw two resistors in series — these form your gate bias network. Label the top one R1 and the bottom one R2. Connect the junction between R1 and R2 to the gate of the MOSFET through the input coupling capacitor. At the drain of the MOSFET, draw another capacitor — your output coupling capacitor. To the right of that, draw a resistor labeled RL (your load resistor) connecting to ground. The amplified signal appears between the output capacitor and RL.
If you're using software like LTspice, Multisim, or KiCad, the process is similar: place the components from the library, connect them with wires, set the component values, and run a simulation. Most software will let you see the voltage at every node and plot how the output changes with the input.
Calculating the DC operating point and bias voltages
The DC operating point is the voltage and current at each part of the circuit when there's no signal — just the steady DC from the power supply. You calculate this first because it determines whether the MOSFET will amplify or distort.
Start with the gate voltage. Using the voltage divider formula, the gate voltage VG equals the power supply voltage VDD times R2 divided by (R1 + R2). For example, if VDD is 12 volts, R1 is 100 kilohms, and R2 is 100 kilohms, then VG = 12 × (100 / 200) = 6 volts.
Next, find the drain current. This depends on the MOSFET's transconductance parameter, which you find in the datasheet. The drain current ID is roughly proportional to (VG − VT)², where VT is the threshold voltage — the gate voltage at which the MOSFET just starts to conduct. For an enhancement-mode MOSFET, VT is typically 1 to 3 volts. If VG is 6 volts and VT is 2 volts, the MOSFET is conducting.
Once you know ID, calculate the drain voltage: VD = VDD − (ID × RD). If ID is 5 milliamps and RD is 1 kilohm, then VD = 12 − (0.005 × 1000) = 12 − 5 = 7 volts. This is your DC operating point at the drain. For good amplification, you want VD to be roughly in the middle of the power supply range — not too close to 0 volts or to VDD — so the signal can swing up and down without hitting the limits.
Calculating voltage gain and output swing
The voltage gain is how much larger the output signal is compared to the input signal. For a MOSFET amplifier, the voltage gain AV is approximately equal to the transconductance gm times the drain resistance RD. The transconductance is found in the MOSFET datasheet or calculated from the drain current and gate-source voltage difference.
As a practical example: if the transconductance is 0.02 siemens (20 millisiemens) and the drain resistor is 1 kilohm, then the voltage gain is 0.02 × 1000 = 20. This means if you put in a 100-millivolt AC signal at the gate, you get a 2-volt AC signal at the drain. Larger drain resistors give higher gain, but they also limit how much current can flow, which reduces the maximum output swing.
The output swing is the range of voltages the output can reach. It's limited by the power supply voltage and the DC operating point. If your drain voltage is sitting at 7 volts and your power supply is 12 volts, the output can swing up by about 5 volts (to 12 volts) or down by about 7 volts (to 0 volts). The smaller of these two numbers is your maximum output swing before the signal clips and distorts. In this case, you can swing up 5 volts, so your maximum output signal is about 5 volts peak-to-peak before clipping occurs.
Choosing coupling capacitor values so the signal passes through
The coupling capacitors must be large enough that they don't block your signal at the frequencies you care about. A capacitor blocks low frequencies and passes high frequencies. The cutoff frequency where the capacitor starts to significantly block the signal is f = 1 / (2π × R × C), where R is the resistance in series with the capacitor and C is the capacitance.
For the input coupling capacitor, the series resistance is usually the input source resistance plus the gate resistor R2. For the output coupling capacitor, it's the load resistor RL. If you want to pass signals down to 100 hertz, and your series resistance is 100 kilohms, then C = 1 / (2π × 100,000 × 100) ≈ 16 nanofarads. In practice, you'd choose the next standard capacitor value larger than this — perhaps 22 nanofarads or 47 nanofarads — to ensure the signal isn't weakened at your lowest frequency.
If you're amplifying audio (20 hertz to 20 kilohertz), you need larger capacitors than if you're amplifying radio-frequency signals (megahertz range). Electrolytic capacitors are cheap and come in large values, but they have polarity — the positive side must connect toward the higher voltage. Film capacitors have no polarity and are more stable, but cost more and take up more space.
Simulating the circuit to verify your calculations
After you've drawn the circuit and calculated the component values, run a simulation to check your work. In LTspice, you can set up a transient analysis to see how the output responds to a sine-wave input, or a DC sweep to see how the drain voltage changes as you vary the gate voltage.
Look for three things: first, does the DC operating point match your hand calculations? Second, does the output signal have the gain you predicted? Third, does the output signal clip or distort at the signal levels you expect? If the gain is too low, increase the drain resistor. If the output clips too easily, lower the drain resistor or reduce the input signal level. If the signal is weak at low frequencies, increase the coupling capacitors.
Simulation also lets you see things that are hard to calculate by hand, like how the gain changes across a range of frequencies, or how the circuit behaves when you change the MOSFET type or the power supply voltage. Most simulators are free — LTspice is free from Analog Devices, and many online circuit simulators let you build and test circuits in a web browser without installing software.
Frequently Asked Questions
What's the difference between a common-source and a common-drain amplifier?
In a common-source amplifier, the output comes from the drain and you get voltage gain — the output voltage swings are larger than the input. In a common-drain amplifier (also called a source follower), the output comes from the source and you get unity gain or less — the output voltage follows the input but doesn't amplify it. Common-drain amplifiers are used for impedance matching and buffering, not for amplification.
Why does the MOSFET need a gate resistor if it draws almost no gate current?
The gate resistor is part of the bias network that sets the DC voltage at the gate. It also limits the current that flows when you're charging and discharging the gate capacitance during switching. Without it, the gate voltage would be undefined or unstable.
Can I use a smaller drain resistor to get more current and less voltage gain?
Yes. A smaller drain resistor lowers the voltage gain but allows more current to flow through the MOSFET, which can increase the maximum output current available to drive a load. This is a trade-off — you gain current capability but lose voltage amplification.
What happens if I don't bias the gate correctly?
If the gate voltage is too low, the MOSFET won't conduct and you get no output signal. If the gate voltage is too high, the MOSFET conducts too hard and the output signal clips on both peaks, creating severe distortion. Correct biasing puts the MOSFET in the middle of its active region so the signal can swing equally in both directions.
Do I need to include a source resistor or source bypass capacitor?
A source resistor (between the source and ground) can improve biasing stability and linearity, but it reduces gain. A source bypass capacitor (a capacitor in parallel with the source resistor) passes AC signals around the resistor so they don't lose gain, while the resistor still provides DC biasing. These are optional refinements — a basic amplifier works without them, but adding them can improve performance.