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Simple voltage aplifier stage followed by power stage, and current source. The problem of a transistor as amplifier is that the gain is limited by the collector load. Using the current source, which has a huge output impedance, greatly increase the possibility to have a voltage swing without a current change; by definition this correspond to a very big impedance, which implies in turn a big voltage gain (impedance at collector divided by impedance at the emitter).
The problem is that a huge gain leads to big harmonic distorsion, that must be fixed with a feedback network. It reduces the gain; but why designing a high gain amplifier and then damping the gain ? because feedback also produces some positive effects: stabilizes the amplifier wrt beta change and temperature change, lessens much the distorsion, increases the bandwidth of the entire system.
So, the idea is to push much the open-loop (i.e. without feedback) amplification, and reduce it with feedback network to get back some beneficial effect.
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