Here is a schematic for a 100W MOSFET power amplifier circuit utilizing IRFP240 and IRFP9240 MOSFETs. This amplifier is designed to operate with a dual power supply of +45V/-45V DC and is capable of delivering 100 watts RMS into an 8-ohm speaker or 160 watts RMS into a 4-ohm speaker. This high-fidelity amplifier circuit is versatile and suitable for various applications, including general-purpose amplification, guitar amplification, and keyboard amplification. Additionally, it can serve as a subwoofer amplifier, but the inclusion of a subwoofer filter stage before the input stage is necessary. Notably, the amplifier boasts low distortion, measuring just 0.1%, a high damping factor exceeding 200, an input sensitivity of 1.2V, and a bandwidth spanning from 4Hz to 4 KHz.
About the circuit.
Capacitor C8 serves as the input DC decoupling capacitor, effectively blocking any DC voltage that may come from the input source. Without this blocking, any DC voltage could potentially disrupt the bias settings of subsequent stages in the circuit. To limit the input current to Q1, resistor R20 is in place, while C7 acts as a bypass to eliminate high-frequency noise from the input.
The input differential pair is composed of transistors Q1 and Q2, and a constant current source circuit, utilizing Q9 and Q10, sources a stable 1mA. Preset R1 is used to fine-tune the voltage at the amplifier’s output. The gain of the amplifier is established by resistors R3 and R2.
Moving on to the second differential stage, transistors Q3 and Q6 are employed, and transistors Q4 and Q5 form a current mirror arrangement, ensuring that the second differential pair drains an identical current. This technique enhances linearity and overall gain.
The power amplification stage is built around Q7 and Q8, and it operates in the class AB mode. For adjustment of the amplifier’s quiescent current, preset R8 is provided. The network comprising capacitor C3 and resistor R19 plays a crucial role in enhancing high-frequency stability and mitigating the risk of oscillation.
Lastly, F1 and F2 are safety fuses integrated into the circuit for protection purposes.
Begin by positioning R1 at the midpoint before applying power, and then gradually fine-tune it to achieve a minimal voltage output, ideally less than 50mV. The subsequent step involves configuring the quiescent current by setting preset R8 to its lowest resistance setting. Connect a multimeter across the designated points labeled as X and Y on the circuit diagram. Proceed to adjust R8 until the multimeter indicates a reading of 16.5mV, which corresponds to a quiescent current of 50mA.
- Assemble the circuit on a good quality PCB.
- Use a +45/-45 V DC, 3A dual supply for powering the circuit.
- Power supply voltage must not exceed +55/-55 V DC.
- Before connecting the speaker, check the zero signal output voltage of the amplifier and in any case it should not be higher than 50mV. If it is higher than 50mV, check the circuit for any error. Replacing Q1, Q2 with another set could also solve the problem.
- Fit Q7 and Q8 to a 2°C/W heat sink. Both Q7 and Q8 must be isolated from the heat sink using mica sheets. Heat sink mounting kits for almost all power transistors/ MOSFETs of almost all package styles are readily available in the market.
- All resistors other than R10, R11 and R19 are 1/4 watt metal film resistors. R10 and R11 are 5W wire wound type while R19 is a 3W wire wound type.