What is an AF amplifier?

AF means audio frequency. Audio Amplifiers can be subgrouped as voltage amplifiers (or pre-amplifiers) and power amplifiers. The voltage amplifier will raise the level of level of the output of a microphone from say 1 mV to 500mV or 1V. The power amplifier will be the interface between this and the load, which is a speaker. The resistance of the speaker is often 4 ohm or 8 ohm. You cannot have a speaker with 1k as resistance! As such to get say 8W output, an amplifier providing 8V will have to pump 1 amp into an 8 ohm speaker. Thus power amplifiers will have to drive a lot of current into the load. Voltage amplifiers cannot do this. A power amplifier sometimes just has unity voltage gain, but has ability to drive a lot of current into the load.

Typical power amplifiers are operated in class B, where for 50% of the signal swing, one transistor provides current, and for the other 50%, the other transistor in a class B provides current into load. Class B amplifiers have cross over distortion, But this can be overcome by pre-biasing, or making the stage operate as class AB. The power amplifiers can also be used to power low resistance loads such as a step up transformer.