Practical power amplifier.
Audio power amplifier is an amplifier which produces amplification of power between the input and output. Theoretically every audio amplifier produces some amount of power amplification, but in practical scenario every audio amplifier cannot be called a power amplifier. It is the amount of power amplification that makes an amplifier power amplifier or not. A power amplifier must produce a considerable amount of power amplification so as to drive a load.
Generally there is no fixed criteria for the output of a power amplifier even though some designers say 50W RMS, some 20W RMS and some other guys 10W RMS. In practical sense your power amplifier must have enough power output to drive the intended load. For an i-pad a few hundred milli watt amp will do a power amplifier while for a home theater system it might be a few ten watt amplifier.
A simple RC coupled pre-amplifier makes considerable amount of voltage gain but it cannot be considered as a power amplifier. When we connect a speaker to it we can hardly hear anything because such an amplifier has very low current gain. Since power is the product of voltage and current, such an amplifier has a low power gain and so it cannot drive a load like speaker.
Power amplifier stages.
A practical audio power amplifier must have dedicated circuits for producing voltage gain and current gain. Sheer power is not the only factor that is considered while designing a practical power amplifier. Special emphasis is given to factors like performance, reliability, ruggedness etc while designing a practical audio power amplifier. Different power amplifier stages are shown in the block diagram below.
Voltage amplification Stage
The input signal to the amplifier from the source will be generally in the mill volt range and it is very weak to drive the succeeding stages. The purpose of the voltage amplification stage is to provide necessary voltage amplification to the input signal for the succeeding stages to process. Usually the voltage amplification stage consists of two or more RC coupled Class A amplifiers. These Class A amplifiers coupled together will make the necessary voltage gain.
Driver stage is the stage that is standing between the voltage amplification stage and the output stage. The output stage will usually have low input impedance and so the voltage amplification stage cannot drive the output stage alone. The purpose of the driver stage is to produce enough current gain in order to drive the output stage. Since there is sufficient current gain, the driver stage produces considerable amount of power gain too.
Output stage is the stage that is connected to the loudspeaker. The output stage gives further improvement to the power gain and transfers this power to the loud speaker with minimum loss. Push-pull arrangement consisting of two transistors is often employed in this stage. Efficiency and impedance matching are the two important parameters considered while designing this stage.
Push-pull arrangement is a very common type of output configuration used in power amplifiers. In push-pull arrangement a pair of active devices (transistors) alternatively source or sink the load current. Advantages of push-pull configuration over single transistor output are better efficiency, higher power output, cancellation of even harmonics, cancellation of DC current at the output etc.
Power amplifier stages in a real circuit.
Circuit diagram of a three stage practical audio power amplifier is shown in the figure below.
Small signal transistor Q1 and its associated components form the voltage amplification stage. R1 and R2 are biasing resistors of Q1. C4 is the input coupling capacitor which blocks DC components from the input signal. Resistor R7 limits the current through the biasing network. C1 is a filter capacitor. Output of the voltage amplification stage is taken from the collector of Q1.
Medium power transistor Q2 forms the driver stage. Output of the voltage amplification stage is directly coupled to the base of Q2. Output of the driver stage is taken from the base of Q2.
Dedicated power transistor Q3 and Q4 wired in the push-pull mode forms the output stage. Collector of Q2 is connected to the base of Q3 and emitter of Q2 is connected to the base of Q4. Audio output is taken from the emitter-base junction of the output transistors.