The objective of the circuit is to create a medium power amplifier based on the functionality of a HEXFET to produce a good quality of sound while providing 65W of power output.
Terminology BC550 – an NPN general purpose transistor with low current and low voltage used for low noise stages in audio frequency equipmentBD139 – NPN power transistor used for driver stages in hi-fi amplifiers and television circuits because of its low voltage at 80 V maximum and high current at 1.5 A maximumIR9540 – a fifth generation HEXFET from International Rectifier that utilizes advanced processing techniques to achieve extremely low on-resistance per silicon area combined with the fast switching speed and ruggedized device design that HEXFET power MOSFETs are well known for, and provides the designer with an extremely efficient and reliable device for use in a wide variety of applications due to its fully avalanche rating, 175ºC operating temperature, and dynamic dv/dt rating Circuit Explanation
With the technology of HEXFET, the transistors in the output stage are of good quality, highly available in the market, and are being controlled by voltage rather than by current as compared to the classic bipolar transistors. The harmonic distortion is resolved the by the symmetrical design of the circuit. A matched pairing of the differential amplifiers Q1-2 and Q3-4 should be implied between them like BC550C and BC560C to ensure the same characteristics and uniform behavior in temperature changes and other important factors, which can be done with the use of a multimeter.
The power amplifier in the 6.5 MHz and the bandwidth of differential amplifiers can be decreased by the RC networks from R7/C3 and R12/C4. To improve the linearity, the resistors R8-9-10-11 are used as a local feedback in the differential amplifier while the constant current is being supplied by Q5 and Q6. The combination of D1, D2, and R20 provides the biasing of current sources while the combination of transistor/LED provides thermic stability. The bias current of output power stage is regulated by the trimmer TR1. Due to this activity, Q8 should have suitable heatsink to maintain thermic stability in the bias so that it does not change with the change in temperature.
In the output stage, the resistors R32-33 forms the local feedback as they function as voltage amplifier while the DC offset voltage of amplifier output is adjusted to near zero by the combination of TR1, C14, and R3-4. The inductor L1 consists of 6 coils of insulated copper wire around a 1.5mm diameter and 16mm of length.
During the supply of voltage to the amplifier, TR2 regulates it in the middle of its journey while TR1 provides the greater resistance. A multimeter is connected in the J4 output for the 200mV DC measurement range while providing voltage to the amplifier and the DC voltage in the output is taken near to zero by the adjustment of TR2. The supply is removed and the miltimeter is connected in line with one of the supply cables for the range of 1A to 2A current measurement. The supply is given again and TR1 regulates the available current to an approximate value of 330mA which stabilizes at 230mA after 10 minutes.
The medium power amplifiers may be used in several applications such as oscillator amplifier in the power amplifier, IF/RF buffer amplifier, high linearity transmit, variable gain and high output power, drivers for CATV amplifier, small signal high linearity amplifiers, wireless data SONET, cellular, 3G, WiBro, & WiMAX applications, and broadband medium power gain blocks.