To increase the level of very weak signals from a microphone you would use:

A microphone is a transducer that converts physical sound waves into weak electrical signals. These signals exist in the Audio Frequency (AF) spectrum, which represents sounds that can be heard by the human ear. To make these tiny microphone signals strong enough to drive the next stage of a transmitter, they must be boosted. An audio amplifier is specifically designed to increase the amplitude of these low-frequency sound signals. An RF amplifier would be useless here because it is tuned for high-frequency radio waves. Similarly, oscillators are incorrect because their purpose is to generate continuous waveforms, not to magnify an external signal like the one coming from a microphone.