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Circuit
Diagram > Class A-Designed Headphone Amplifier Using BC308
Transistor

Class A-Designed Headphone Amplifier Using BC308 Transistor

Date: 2012-7-19Author: anonymousReading(542)Collection
this page

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 noise

BC237 – 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 package

Class 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.

Source:users.otenet.gr/~athsam/headphone_amplifier_class_a.htm

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