Hey, if you’re into audio electronics or just love a good DIY challenge, this MOSFET Class A amplifier circuit might catch your interest. The image shows a simple yet effective design that uses a single MOSFET to drive a load, like a speaker or a high-wattage lamp, as a stand-in. As an experienced electronics engineer, I’ve analyzed the diagram, and it’s a classic Class A setup with a few quirks worth exploring. I’ll walk you through what it does, break down the components, explain how it works, guide you through building it, and share some tips for real-world use. This is a great project for learning about audio amplification or experimenting with power stages, and it’s based on principles I’ve tinkered with over the years. Let’s dive in.
What This Circuit Does
This circuit is a MOSFET-based Class A amplifier, meaning it keeps the transistor on all the time, delivering a continuous current to the load. Unlike Class B or AB, which switch on and off to save power, Class A runs in a linear mode, giving clean sound with minimal crossover distortion – perfect for audiophiles or high-fidelity setups. The diagram labels it for a 100-watt lamp at 220V, which seems unusual for a typical audio amp, but it’s likely a placeholder or mislabel. In practice, this design suits low-to-medium power audio applications, like driving a 10-20W speaker, with a supply voltage range of 12V to 36V.
The input accepts an audio signal, which the MOSFET amplifies to drive the output. Class A amps are known for their simplicity and warm sound, though they’re less efficient, wasting power as heat. The circuit uses an IRF240 MOSFET, a robust choice for power handling, and includes capacitors and resistors to shape the signal and stabilize the operation. It’s a single-ended design, meaning one transistor does the work, which keeps it straightforward but limits power output compared to push-pull configurations.
Components You’ll Need
The diagram lists the key parts, and it’s a lean build, likely costing under $10 if you source wisely. Here’s the breakdown based on the circuit, with some practical notes:
- IRF240 N-Channel MOSFET: The amplification heart. Rated for 200V and 9.2A with a low RDS(on) of about 0.18 ohms, it can handle decent power but needs a heatsink. (Note: IRF240 is less common; IRF540 or IRF640 are common substitutes with similar specs.)
- Resistors:
- 270 ohm (two, input bias and gate resistor).
- 2.5K ohm (part of the input network).
- 47.5K ohm (feedback or bias resistor).
- 22.1K ohm (part of the feedback or gate bias).
- 470 ohm (source resistor for stability).
- Capacitors:
- 3 μF (two, input coupling and feedback path).
- 2500 μF (output coupling to load).
- 1000 μF (output smoothing or bypass).
- 100 μF (input or gate decoupling).
- Inductor L1: 4 mH (choke to block AC from the power supply).
- Load: Labeled as a 100W 220V lamp, but for audio, use a 4-8 ohm speaker (10-20W rating matches typical Class A output).
- Power Supply: 12V to 36V DC, capable of 1-2A depending on load.
- Extras: Heatsink for the MOSFET, breadboard or PCB, hookup wires, and a multimeter for testing.
The lamp mention is odd – a 100W 220V load implies AC and high voltage, which doesn’t align with a DC-powered MOSFET amp. It might be a diagram error or a test load for power dissipation. For audio, swap it with a speaker and adjust expectations to 10-15W output with a 24V supply. Parts are available from Digi-Key, Mouser, or local electronics shops.
How the Circuit Works
Let’s trace the signal flow. The input audio signal enters through a 3 μF capacitor, which blocks DC and lets AC audio pass. It hits a network with 270 ohms and 2.5K resistors, likely forming a high-pass filter or bias divider to set the gate voltage. The gate connects to the IRF240’s gate pin, with another 270 ohm resistor, possibly for stability or damping oscillations.
The MOSFET operates in the active region for Class A. Its source ties to ground through a 470 ohm resistor, which sets the quiescent current – the steady current when no signal is present. This resistor also provides negative feedback to stabilize the gain. The drain connects to the positive supply (V+) through the 4 mH inductor L1, which acts as a choke to prevent AC from feeding back into the power supply. The output signal comes off the drain, coupled through a 2500 μF capacitor to the load, with a 1000 μF cap possibly in parallel for additional smoothing.
The 47.5K and 22.1K resistors form a feedback network, likely from drain to gate, to control gain and linearity. The 100 μF cap near the gate might decouple any noise. In Class A, the MOSFET conducts continuously, amplifying the positive and negative swings of the input signal by varying the drain current. The large output caps ensure only the AC audio reaches the load, blocking the DC bias.
Efficiency is low – maybe 20-30% – because the MOSFET dissipates power as heat even without input. With a 24V supply and 1A quiescent current, that’s 24W lost as heat, so a hefty heatsink is a must. Output power depends on supply voltage and load impedance; with an 8-ohm speaker and 24V, expect around 10-15W before clipping.
Calculating Gain and Power
For the numbers fans, let’s estimate some key figures. Gain in a Class A amp with feedback depends on the resistor ratio. Assuming the 47.5K is feedback and 22.1K is input resistance (simplified), gain A_v ≈ 1 + (47.5K / 22.1K) ≈ 1 + 2.15 ≈ 3.15, or about 10dB. This is rough – real gain adjusts with the source resistor and load.
Quiescent current I_q = V_supply / R_source, but R_source is 470 ohms across the source, so it’s part of the bias. If the gate is biased to half supply (12V with 24V), and V_gs(th) for IRF240 is 2-4V, the source voltage is V_gs – V_th, and current is (V_supply – V_source) / R_load equivalent. Without exact bias, assume 1A quiescent with 470 ohms, giving 0.47V drop, so V_gs ≈ 2.47V + V_th, feasible.
Output power P_out = (I_q * V_swing)^2 / (2 * R_load), where V_swing is the maximum undistorted voltage. With a 24V supply, V_swing ≈ 20V peak-to-peak (10V RMS), for an 8-ohm load: P_out ≈ (1 * 10)^2 / (2 * 8) ≈ 100 / 16 ≈ 6.25W RMS, but with higher current or better bias, up to 15W is possible.
Heat dissipation P_d = V_supply * I_q – P_out. At 1A, 24V, 6.25W out: P_d ≈ 24 – 6.25 ≈ 17.75W. Scale heatsink accordingly.
Steps to Build It
This is a hands-on project – start on a breadboard, then solder for permanence.
- Set up the power supply: Connect V+ (12-36V) to L1 (4 mH) positive side, ground to the circuit ground. Add a 1000 μF cap across V+ and ground for stability.
- MOSFET placement: Mount IRF240 on a heatsink. Drain to L1 other end, source to 470 ohms to ground.
- Input stage: Connect the input signal to a 3 μF cap, the other side to a 270 resistor to the gate, and 2.5K from the gate to ground (bias network).
- Feedback: From drain to 47.5K, then to 22.1K to gate, with 3 μF cap in that path (might be misdrawn – typically from drain to gate directly).
- Output: From drain to 2500 μF cap to load (speaker), with 1000 μF in parallel if needed.
- Test: Apply 24V, input a 1 kHz sine wave (0.1V peak), check output on an oscilloscope. Adjust bias if no signal.
- Load: Swap lamp for an 8-ohm speaker, play music, and listen for distortion.
If no output, check the gate voltage (should be half V+), ensure the MOSFET is on, and verify the cap polarities. Heat is normal – monitor it.
Applications and Real-World Use
This amp suits small audio projects: desktop speakers, guitar practice amps, or headphone drivers with proper impedance matching. The warm Class A sound appeals to audiophiles, though power is limited. I’ve used a similar one for a vintage radio restoration, pairing it with a 15W speaker.
The lamp label suggests a test setup – 100W at 220V is AC, not DC amp territory. For DC, it might simulate load current (e.g., 4A at 24V), but for audio, stick to speakers. In 2025, with high-end audio trends, this could be a base for mods like adding a preamp or switching to Class AB for efficiency.
Safety Considerations
Class A runs hot – the MOSFET can hit 70-80°C at 1A, so use a large heatsink and ventilate. Add a fuse (1-2A) on the supply. The high-capacity store charges – discharge them before touching. Avoid shorting the output; it’ll fry the MOSFET. For high voltage (36V), insulate connections. If using a speaker, ensure impedance matches to avoid overcurrent.
Common issues: Oscillation from feedback – add a 100pF cap across gate-source if needed. No sound – check input signal and bias.
Variations and Improvements
To boost power, parallel MOSFETs with matched bias, but balance the heat. For lower distortion, add a complementary P-channel MOSFET for push-pull Class A, though that complicates it. Use a 555 timer for PWM to drive the gate, turning it into a Class G hybrid, saving power.
Some designs add a Darlington pair for gain, or replace the inductor with a current source for better regulation. For stereo, duplicate the circuit, but share the power supply.
If the lamp is intentional, it’s a power resistor test – adjust for your speaker’s wattage.
Wrapping Up
There you go – a solid MOSFET Class A amplifier to experiment with. It’s simple to build, offers clean audio, and teaches you about linear amplification. The diagram’s lamp quirk aside, it’s adaptable for speakers with some tweaking. Give it a try, play some tunes, and let me know how it sounds or if you run into heat issues. Enjoy your audio journey!
