Here, we put all the theory to work and show you how to make a simple 1.5W power amplifier module with parts that are easy to find. Fig. 1 shows a block diagram of the amplifier. It's like most audio amplifiers, but the circuit is a little different. The audio input, the amplifier, the driver, the output, and the power supply are all parts of a power amplifier. Most of the voltage gain comes from the amplifier section. Between the amp section and the output stage is the driver stage, which acts as a buffer. Most of the time, the output stage has to drive a low-impedance load, like a speaker. Power comes from the power supply, and the signal that comes out of the load should, ideally, be the same as the signal that went in. In other words, the output stage gets power from a DC supply to boost the signal so it can drive a load. Fig. 1: Block diagram of 1.5W power amplifier How a Power Amplifier Circuit Works Figure 2 shows the parts of the circuit that are the amplifier, driver, and output. The JFET VHF/UHF amplifier 2N5484 (T1) and the NPN transistor BC548 make up the amplifier section (T2). The driver part is made up of the transistor BC639 (T3), and the output part is made up of the transistors BD139 and BD140 (T4 and T5). The signal from the input is sent to the volume control VR1 through the capacitor C1. It says that the value of VR1

is 1 megaohm. Since the gate terminal of the FET (T1) can be thought of as an open circuit, the value of VR1 is equal to the circuit's input impedance. Like all audio volume controls, VR1 needs to have a logarithmic taper, which is usually denoted as "type C," so that turning the control and the volume level seem to go in a straight line. Human hearing has a logarithmic response, which means that a change in output power by a factor of 10 sounds like a change by a factor of 2. In the amplifier section, the FET stage is used to make the input impedance high. The next step is a common emitter amplifier, which is made up of transistor T2. Set VR2 is used to change the amount of amplification and keep transistor stages T2 through T5 from getting too close to each other. This means that the collector voltage at T2 sets the DC voltage at T3, T4, and T5. The voltage at the emitters of T4 and T5 is the most important. This can be set to half the supply voltage with the help of VR2. Resistor R13 sends negative feedback from the output to the emitter of transistor T2 to stabilize this and other voltages in the circuit. If you don't include capacitor C8, the feedback will be for both DC and AC voltages. If C8 is added, it will only be for DC. When the voltage at the emitters of transistors T4 and T5 goes up, say because of a change in temperature, the voltage at the emitter of transistor T2 also goes up because of R13. This will make T2 pass less current, which will raise the DC voltage at its collector. So, transistor T3 will carry more current, and the voltage at its collector will drop. The voltages at the bases of T4 and T5 are then lower, and so are the voltages at their emitters.

Fig. 2: 1.5W power amplifier circuit The base circuitry for T4 and T5 is connected to T3's collector load, which is the driver transistor. In effect, T3 is set up as an amplifier with a common emitter. Through diode D1 and the parallel combination of resistor R9 and preset VR3, the output signal from T3 is sent to the base of T4. The collector of T5 is directly connected to the base of T3. So, since the driver stage drives a load with a low resistance, it needs a transistor that can handle a lot of power. The output transistors are the NPN transistor T4 and the PNP transistor T5, which are wired together as a class- AB output stage with complementary symmetry. In this set-up, you need a complementary NPN transistor and a symmetrical PNP transistor with the same amount of current gain. So, if you measure them, the DC current gains of T4 and T5 should be the same. One diode and two resistors connected in parallel make up the DC biasing circuit for T4 and T5. With VR3, you can change the quiescent collector current of T4 and T5 and, by extension, the operation class.

Capacitor C4 is also part of the output stage. It is called a bootstrapping capacitor. Bootstrapping is there so that the output voltage swing can be higher. If capacitor C4 is not used, biasing resistors R7 and R8 are combined into a single resistor. In an ideal situation, the output signal should be able to go from 0V to the supply voltage. But this can't happen because the base-emitter junctions of the output transistors need 0.6V of forward bias and there are losses. For the positive half cycle, the voltage at T4 must be at least 0.6V higher than the supply voltage if the output is to reach the supply voltage. In the same way, T5 can only give off 0V if its base voltage drops to –0.6V. By adding the "bootstrap" capacitor C4, the swing in the output voltage is added to the DC bias voltages. So, during the positive half-cycle, the positive change adds to the bias voltage at T4, making it conduct more current and produce a higher output voltage. In the same way, during the negative half-cycle, the negative-going swing lowers the quiescent bias voltage. This makes T5 turn on harder and produce a lower output voltage. Power supply decoupling is an important part of designing an amplifier. When the output stage is putting out its maximum amount of power, the power supply sends out large peak currents. Under these circumstances, it's possible that some of the sound signals will show up on the power line. So that this signal doesn't affect how the rest of the circuit works, it needs to be taken out of the part of the power supply that goes to the voltage amplifier. So, resistor R6 is added to capacitor C2 and diode ZD1 to keep the supply voltage of the amplifier at a steady 10V. Put the circuit together on a PCB that can be used for many things and put it in a suitable cabinet. Mount the diodes, electrolytic capacitors and transistors with the correct polarity. The metalized side of output transistors T4 and T5 should be facing the middle of the board.

For both of the transistors, you need a heat sink. Either a small piece of aluminum (20 mm2) or a heat sink that can be bought in stores can be used. The heat sinks on T4 and T5 should be separated from the transistors with a piece of Mylar or something similar, since this transistor (and, by extension, the heat sink) connects directly to the power supply. Important Points to Remember Before Testing the 1.5W Power Amplifier Before putting power into the circuit, connect an 8-ohm load (resistor or loudspeaker) to the output and a capacitor C1 between the input terminal and the volume control. Set the volume control to minimum and then apply power—either from the plug pack or an external 12V DC supply. Make sure that when you touch either of the output transistors, they feel cool. If not, change the VR3 preset. The right setting for VR3 should give a current of around 100 mA through T4 and T5 when the collector is at rest.