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 Description of 20W Audio Amplifier Using Ecn149-ECP149 Pair Transistor
A discrete power amplifier circuit is shown in figure 1 employs the 30W silicon output transistor pair ECN149-ECOP49. This amplifier will deliver 20W into 8Ωwith a 450mV input signal.
We have already posted a simple 3W audio amplifier using AC187-AC188 circuit. Both the design are similar. 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 and diode D1 combination into class AB operation. This arrangement reduces crossover distortion and the diode 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 ECN149-ECP149 PAIR
|Resistor (all ¼-watt, ± 5% Carbon)|
|R1 = 27 KΩR2 = 56 KΩR3 = 18 Ω
R4 = 1 KΩ
R5 = 2.2 Ω
R6 = 1.5 KΩ
R7, R8 = 0.47 Ω/2W
R9 = 400 Ω/5W
VR1 = 20 KΩ
|C1, C2 = 10 µF/25VC3 = 100 µF/25VC4 = 100 µF/25V
C5 = 1000 µF/50V
|T1 = BC148BT2 =SK100BT3 = ECN149
T4 = ECP149
D1 = 1N4001
LS1 = 8Ω, 20W Loud Speaker