5 - ELECTRIC AMPLIFIERS AND FINAL CONTROL ELEMENTS

Abstract

This chapter discusses electric amplifiers and final control elements. The power rating of the preamplifier normally would be low, as its principal function is usually signal amplification and not power amplification. The chapter also focuses on transistor power amplifiers. Transistor power amplifiers normally use direct-coupled transistor stages and are biased to operate in the Class AB mode. Their principal function is to amplify power levels, so that the voltage amplification may be small or even unity. Negative feedback often is incorporated into the amplifier. A frequency converter converts AC at one voltage, and frequency into AC at a different voltage and frequency. The conversion may involve an intermediate DC stage, so that the frequency converter consists of an AC-DC converter followed by a DC-AC inverter. Alternatively, the AC-AC conversion may be direct, and the network then is known as a cycloconverter. The harmonic content of the load voltage waveform is reduced progressively as the pulse number of the cycloconverter network is increased, so that most practical networks have high pulse numbers and generally convert from three phase to three phase.