PHYS 252 Lecture Notes - Lecture 3: Low-Pass Filter, Digital Image Processing, Audio Power Amplifier
4 Apr 2016
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Document Summary
In low level modulation a small audio stage is used to modulate a low power stage. The output of this stage is then amplified using a linear rf amplifier. The great disadvantage of this system is that the amplifier chain is less efficient, because it has to be linear to preserve the modulation. Hence high efficiency class c amplifiers cannot be employed, unless a doherty amplifier, eer (envelope elimination and restoration) or other methods of predistortion or negative feedback are used. High level modulation uses class c amplifiers in a broadcast am transmitter and only the final stage or final two stages are modulated, and all the earlier stages can be driven at a constant level. When modulation is applied to the plate of the final tube, a large audio amplifier is needed for the modulation stage, equal to 1/2 of the dc input power of the modulated stage. Traditionally the modulation is applied using a large audio transformer.
