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Class A-Designed Headphone Amplifier Using BC308 Transistor

2015-01-03 21:28  
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Figure 1 

The circuit was constructed around the functionality of a Class A amplifier to create a headphone circuit.

BC308 – an epitaxial planarPNPtransistor used for general purpose and low noise amplifier application due to its features such as high voltage and low noiseBC237 – anNPNgeneral purpose transistor used for general purpose switching and amplification due to its low current with maximum 100 mA, low voltage with maximum 45 V, and enclosed in a T0-92 & SOT54 plastic packageClass A – refers to an output stage where the bias current is greater than the maximum output current thus, all output transistors are always conducting current due to its linearity and low distortion

Although the circuit had a simple design, all the issues regarding the frequency response and distortion have been compensated. In the input stage, the resistance for the current has a value of 250K ohms and the load that can be driven by the circuit can range between 100R and 2K ohms. The circuit performs negative coupling during the operation where the energy is transferred from one medium to another medium.

The output of the amplifier operates in Class A mode where the output signal is an exact scaled-up replica of the of the input signal without any clipping. Typically they are used for implementing small-signal amplifiers since they are less complex, more linear and very efficient. It consists of the resistor 39R and the actively charged BC308 transistor. The frequency response can range from 37 Hz to 470 KHz or -1 dB with a load of 200R.

During the operation, the bias current is approximately 14 mA to 15 mA while the gain of the unit stands at 25. The supply of the circuit comes from a 6 V to 24 Vdc. The output voltage will produce a value of 1.5 V or 11 mW with a distortion of 0.5%.

The headphone amplifier is mainly designed to drive the speakers of headphones but is not suitable for use with loudspeakers since they have low power sources. Most of them are embedded in some electronic devices like the portable music players and televisions. They can also be utilized for the speakers used in desktops or workstations as they function as a preamplifier stage.


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