The high wattage power transistor with matched complementary pair is now readily available. Using such a transistor, an economical high power amplifier using a minimum of components and giving a good performance can easily be constructed by an audio enthusiast. Here in this article, we are going to build a 3watt audio amplifier by using complementary pair of transistors.
Circuit Diagram of 3watt Audio Amplifier Using AC187-AC188 Pair
A discrete 3watt Audio power amplifier circuit is shown in figure 1 employs a complementary output stage featuring the AC 187-AC188 (T3 and T4) germanium output transistor pairs. This amplifier can deliver a maximum of 3W into an 8Ω loads. Input sensitive for rated output is 50mV.
Transistors T1 & T2 work as common emitters with transistor T1 driving transistor T2. T1 function to transfer the DC bias developed by the potential divider VR1, R1, R2, and R3 to the output through DC feedback resistor R6, so as to establish the midpoint voltage at about half the supply voltage. T2 provides the necessary current drive for the complementary output pair T3 and T4. T2 collector loads resistor R9 is bootstrapped by the output coupling capacitor so that sufficient drive is available at the base of the lower transistor. Transistor T2 provides most of the amplifier open-loop gain.
Transistors T3 and T4 are operated as emitter followers and biased by the resistor R5 into class AB operation. This arrangement reduces crossover distortion and the resistor stabilizes the temperature coefficient of the base-emitter diode of T3 and T4Â so that they do not fall prey to the thermal runways.
The only adjustment required with this amplifier is for VR1 so that the DC voltage at the positive terminal of the output capacitor equals half the supply voltage.
PARTS LIST OF 3W AUDIO AMPLIFIER USING AC187-AC188 PAIR
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Resistor (all ¼-watt, ± 5% Carbon) |
|
R1 = 27 KΩ R2 = 56 KΩ R3, R5 = 18 Ω R4, R6 = 2.2 KΩ R7, R8 = 1 Ω R9 = 470 Ω R10 = 68 Ω VR1 = 20 KΩ |
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Capacitors |
| C1, C2 = 10 µF, 16V (Electrolytic Capacitor)
C3 = 100 µF, 12V (Electrolytic Capacitor) C4 = 2.2kPF (Ceramic Disc) C5 = 500 µF, 12V (Electrolytic Capacitor) |
| Semiconductors |
| T1 = BC148B (general-purpose, silicon NPN bipolar junction transistor)
T2 = BC158BÂ (general-purpose, silicon PNP bipolar junction transistor) T3 = AC187Â (general-purpose, germanium NPN bipolar junction transistor) T4 = AC188Â (general-purpose, germanium PNP bipolar junction transistor) |
| Miscellaneous |
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LS1 = 8Ω, 3W Loud Speaker |
Hey! Im really interested in building your amp, looks really nice!
But i’m not that pro and i’m having problem to get things right on the circuit
What should go wired to what?
thanks in advance and happy new year!
Please go through the circuit description as given above, and your problem will be definitely solved.
Hi, thank you for your schematic, it was a great help to repair an old “imperator” radio. But I think there is a little mistake : on my circuit, R10 is in fact a capacitor (and R5 a 150 ohm variable resistor).
Regards,
RG