The powerful DJ MOSFET amplifier circuit design provided in this article is reasonably easy to build and will produce a thumping 250 watts of music into a 4 ohm loudspeaker. Use of HEXFETs at the output ensures monstrous current and voltage amplification.
The involvement of MOSFETs or rather HEXFETs at the output stage of this 250 watt mosfet amplifier circuit promises high and efficient amplification of both voltage and current. The circuit particularly exhibits impressing features like low distortion and external offset voltage and quiescent current adjustments.
Amplifier Input Stage
Amplifier Power Output Stage
How the Circuit Functions
This outstanding 250 watt mosfet amplifier circuit can be used as a DJ amplifier in concerts, parties, open grounds etc. The design being symmetrical produces negligible distortions. Let’s try to analyze the circuit details:
Referring to the circuit diagram, we see that the input stages primarily consists of two differential amplifiers. The blocks T1 and T2 are actually matched paired dual transistors in one package, but you may go for discrete transistors, just make sure their hFes are properly matched. Use a couple of BC 547 and BC 557 for the NPN and the PNP types respectively.
A differential configuration is probably the perfect way of integrating two signals, for example here the input and the feedback signals are mixed so efficiently.
Typically the ratio of the collector/emitter resistances of T1 determines the amplification of this stage.
The DC operating reference for T1 and T2 is received from a couple of transistors T3 and T4 along with the associated LEDs.
The above LED/ Transistor network also helps to provide a constant current source to the input stage as it virtually remains unaffected to ambient temperature variations, but preferably the LED/ transistor pair should be attached together by gluing them together or at least soldered very close to each other over the PCB.
Immediately after the coupling capacitor C1, the network comprising of R2, R3 and C2 forms an effective low pass filter and helps maintaining a bandwidth to a level suitable for the amplifier.
Another small network at the input, involving a 1M preset and a couple of 2M2 resistors helps adjusting the off-set voltage so that the DC component at the output of the amplifier stays at zero potential.
After the differential stage an intermediate driver stage is introduced comprising T5 and T7. The configuration consisting of T6, R9 and R17 forms a kind of variable voltage regulator, which is used to set the quiescent current consumption of the circuit.
The boosted signal from the above stage goes to the driver stage consisting of T8 and T9 which are effectively used to drive the output power stage involving the HEXFETs T10 and T11 where the signals ultimately undergoes a massive current and voltage amplification.
From the diagram it is clearly identifiable that T10 is a p-channel and T11 is an n-channel FET. This configuration allows efficient amplification of both current and voltage at this stage. The overall amplification is though limited to 3 due to the feedback wiring of R22/R23 and also with R8/C2. The limitation ensures low distortion at the output.
Unlike bipolar transistors, here the outputs stage incorporating HEXFETs have a distinct advantage over its age old counter part. HEXFETs being positive temperature coefficient devices are equipped with the inherent property of limiting their drain source as the case temperature tend to get too hot, safeguarding the device from thermal runaway situations and getting burnt off.
Resistor R26 and the series capacitor compensate the rising impedance of the loudspeaker at higher frequencies. Inductor L1 is placed to safeguard the loudspeaker from instantaneous rising peak signals.
- R1 = 100K
- R2 = 100K
- R3 = 2K
- R4,5,6,7 = 33 E
- R8 = 3K3,
- R9 = 1K PRESET,
- R10,11,12,13 = 1K2,
- R14,15 =470E,
- R16 = 3K3,
- R17 = 470E,
- R18,19,21,24 = 12E,
- R22 = 220, 5 WATT
- R20,25 = 220E,
- R23 = 56E, 5 WATTS
- R26 = 5E6, ½ WATT
- C1 = 2.2uF, PPC,
- C2 = 1nF,
- C3 = 330pF,
- C6 = 0.1uF, mkt,
- T3 =BC557B,
- T4 = BC547B,
- T7,9 =
- T5,6,8 = TIP31,
- T10 = IRF9540,
- T11 = IRF540,
An alternate version of the above explained 250 watt power amplifier can eb seen in the following diagram having all the details regarding the components:
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