mga master ito po pala ang circuit. .nagkamali lang ako sa paggawa ng diagram. .
This amp is designed to drive audio alarms. Not for listening pleasure. Most probable source of audio signal is a 555 circuit oscillator or variations of that to produce a distinct alarm sound.
A simple way of looking at it is -
Q3 is a collector biased common emitter stage. Its collector voltage at zero input signal sets the operating point. The voltage therefore at the emitter of Q1 and Q2 is just one Vbe higher than Q3's collector voltage. We cannot expect the voltage at the the emitter of Q1 and Q2 to be exactly 1/2 the supply rail. It will be +/- 1 Volt (or more) around that figure because the collector voltage of Q3 will depend primarily on the temperature and Q3's hfe. For this circuit, Q1 and Q2 remains off when there is no input signal due to the single diode D1. So the biasing of Q3 and the load formed by D1 and R2 determines the collector voltage of Q3. This simplicity in this setup also allows it to operate over a wide range of supply rail voltage. Typically the supply will be 24V in industrial applications.
Q1 is a follower for the upper half of the signal while Q2 is a follower for the lower half.
Q1 and Q2 operate in class B mode due to the single diode D1 that prevents them from conducting at the same time.