High wattage power transistor is matched complementary pair are now readily available. Using such transistor, an economical high power amplifier using a minimum of components and giving a good performance can easily by constructed by an audio enthusiast.
Circuit Diagram of 3W Audio Amplifier Using AC187-AC188 Pair
A discrete 3W 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.
Transistor T1 & T2 work as common emitters with T1 driving 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. T2 provides most of the amplifier open-loop gain.
Transistor 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 T4, 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
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
|C1, C2 = 10 µF/16VC3 = 100 µF/12VC4 = 2.2kPFC5 = 500 µF/12V|
|T1 = BC148BT2 = BC158BT3 = AC187T4 = AC188|
LS1 = 8Ω, 3W Loud Speaker