2 Watt Class-AB Audio Amplifier by BD139 and BD140

Figure 1 

Here is a 2W Class-AB audio power amplifier which provides low harmonic distortion (≈ 1.5%) and wide frequency response. It is also capable of driving an 8Ω loudspeaker with an output power of 5W but this time distortion increases slightly. Supply voltage is between 12V and 18V.

The 470Ω preset potentiometer controls the quiescent current flowing through the BD139 and BD140 complementary transistors. Adjustment of this resistor is a trade-off between low distortion and low current across the output transistors Q3 and Q4.

Since the amplifier is DC biased, emitters of the BD139 and BD140 transistors are at about half power supply voltage to allow for a max. output swing. Additionally R9 and R10 resistor pair provide a degree temperature stabilisation.

Read More:www.zen22142.zen.co.uk/Circuits/Audio/2wamp.html