12 Watts Transistor Amplifier Circuit
The circuit shown below is not at all expensive as the amplifier contains only an op-amp and four transistors (easily available from your electronics junk box). Here the op-amp used is uA 741 which produces the required gain. The four transistors are wired as complementary Darlington’s which produces the drive for the speaker.
The voltage drop across the resistors R2 and R3, are used as the input of the Darlington pairs. As the input current to the op-amp depends on the level of the signal op amp is amplifying the voltage drop across the resistors R2 and R3 will be proportional to the input signal. These voltage drops are given to the base of Darlington pairs. The amplification is stabilized as a result of the negative feedback from the junction of collectors of Q2 and Q4. The theory may seem little awkward for you. But its working good. Such a simple but stable circuit as this can produce a reasonable output of 12W on a 4 Ohm speaker.
Jump to: Darlington Pair
Transistor Amplifier Circuit Diagram with Parts List
- Use a well regulated and filtered power supply for noise free operation.
- Assemble the circuit on a good quality PCB or common board.
- Any op amp which can be operated from a 12V dual supply can be used instead of uA 741.
As shown in the transistor amplifier circuit above, the four transistors shown in the circuit are used to form Darlington pairs. A Darlington Pair is mainly used to produce a high current gain and a high input impedance. The current gain may jump up to a thousand times and more in most cases. The total current gain for a transistor pair in Darlington configuration can be calculated as Hfe1xHfe2.
Two transistors are paired to form a Darlington transistor pair. The main principle behind this pair is to connect 2 or 3 transistors with the emitter of one transistor connected to the base of the other, and all these transistors will share the same collector. In the circuit above, two separate transistors are used to form a Darlington pair. The same circuit can be modified using a single chip where the two transistors will be connected in Darlington style. Many such pairs can also be integrated in one chip and can be used for high end applications in audio circuits, power supply circuits, TV & display drivers, and so on.
The Darlington pair is also advantageous in the case of it’s base emitter voltage, which is high when compared to the value of a single transistor. The circuit may show a higher voltage between the input base and output emitter voltage when compared to a single transistor. As two or more than two emitter junctions are available, the turn ON voltage for the whole pair is twice compared to just one transistor.
The frequency response of the pair is very low because the base current for the output transistor can’t be shut off at once. Thus the pair can be used only in low frequency applications.
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