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300 watt Mosfet Amplifier Circuit Explained

Updated on January 31, 2014
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Circuit Description


In the circuit schematic shown below, the 300 watt power amplifier could be seen in the form of a Class B configuration. The input resistance make sure that the input impedance is significantly high for nullifying any possibility of a high current input via the music content.

The components which are coupled with ground are basically all high frequency suppressor components, introduced for neutralizing the unwanted bad frequencies inside the music content.

The above consideration ensures that only legit inputs between 10Hz and 20Khz are allowed into the amplifier, rest are ignored or cancelled.

To be precise the above protection help to safeguard the amplifier from TIM distortions or transient intermodulation disturbances.

The couple of transistors which can be seen after the input filter stage are arranged in a differential amplifier mode for boosting the input to some reasonably stronger levels, and these also function as a constant current source for the coming stages.

Next comes the driver stage consisting the BD139/BD140 complementary paired devices. This stage effectively pulls the signal level received from the preceding differential amplifier stage to higher voltage levels such that the following mosfet output stage is able to get the required trigger potential across their gate pinouts.

The output state devices are actually not crucial. Here we can see the incorporation of the pairs K1058 ad J162, however any relevant high power mosfets can be used such as IRF540 and IRF9540 or other similar types will also work.


The mosfets here are basically connected in the push-pull mode so that the loudspeaker coil is able to achieve a compete two way amplification exploiting the applied dual power supply across the circuit.

One thing to be noted here is that, although the discussed power amplifier circuit is rated at 300 watts it may be exceeded to higher levels simply by using a higher voltage to the circuit, this would also call for a lower current requirement from the power supply.

However if a higher voltage is used, the various transistors will need to be upgraded accordingly for tolerating the higher specs.

To be precise the BD139/BD140 will need to be replaced with MJE340/350 pairs which are rated to accept well above 100V in contrast to the BD series which can handle not more than 60V.

The shown mosfets in the present diagram need not be changed in this case, as these are already well suited to voltages over the 100V mark.

If you see the circuit carefully you will find the inclusion of 1N4148 diodes somewhere at the center of the design.

These are extremely crucial and in fact make the design very convenient for the new hobbyists, because these diodes ensure an optimal saturating voltage for the associated devices and the user can be sure of an automatic optimal quiescent current setting within the circuit when power is first switched ON.

Normally these diodes would be replaced with a preset for a manual adjustment of the quiescent current, but with these diodes included ensures an auto adjust for the same.

The proposed 300 watt power amplifier can be upgraded even up to 1000 watts by raising the power supply and the devices appropriately as discussed in the previous sections.

This amplifier when built would specifically suit the subwoofers, and with an additional tone enhancement circuit attached before the input stage you could achieve an outstanding bass levels with this little monster.



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