LNA,PA,Opamp
OpAmp is a differential amplifier with a high open-loop gain, high input and low output impedances, which is the optimum for building analog signal processing blocks, such as integrators, amplifiers, filters, instrumentation amplifiers, gyrators, etc. There are two main types: VFA and CFA.
Power amplifier is optimized for power gain. It does not have to be differential, for example. Types: audio amplifiers, video amplifiers, buffers, RF, etc.
Low-noise amplifier (LNA) is optimized for low input noise, either voltage noise Vn or current noise In. Main types: Vn from 0.5 to 3 nV (optimized for <1k source resistances), In in the fA range (optimized for >1k source resistances).