The apparent impedance of the primary winding (Z P) will be the actual impedance of the loudspeaker (Z LS) multiplied by the square of the turns ratio.Īlthough the impedance of the transformer primary winding is high, its DC resistance (at 0Hz) is practically zero ohms. The transformer primary winding has high apparent impedance (Z P) at audio frequencies because of the action of the transformer in ‘magnifying’ the impedance of the loudspeaker. The circuit shown in fig 5.2.2 is a class A power output stage, but its efficiency is improved by using an output transformer instead of the resistor as its load. This was not a great problem in class A voltage amplifiers, where the collector current was very small, but in power amplifiers output currents are thousands of times larger, so efficient use of power is crucial. The standing bias current (the Quiescent Current) is sufficient to make the collector voltage fall to half the supply voltage, and therefore power (P = I C x V CC/2) is being dissipated by the transistor whether any signal is being amplified or not. This method causes a standing bias current to be flowing during the whole waveform cycle, and even when no signal is being amplified. The output power they produce is theoretically 50%, but practically only about 25 to 30%, compared with the DC power they consume from the power supply.Ĭlass A power amplifiers use the biasing method illustrated in Fig. Class A design produces good linear amplifiers, but are wasteful of power.
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The purpose of class A bias is to make the amplifier relatively free from distortion by keeping the signal waveform out of the region between 0V and about 0.6V where the transistor’s input characteristic is non linear. In either of these states the transistor is dissipating little or no power. Classes D to H are used in switch mode amplifiers where power is saved by having the output transistors switched rapidly between fully on and fully off. The classes A, B, AB and C refer to the way the amplifiers are biased, although class C is mainly used in oscillator circuits. Although Class A may be used for power output stages (usually low to medium power), it is less used for higher power output stages, as more efficient classes of amplifier such as Classes B, AB or even classes D, E, F, G and H are available.
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