Power amplifiers are intended.. hmm.. to amplify power of outgoing signal from transmitter to antenna. I would categorize power amplifiears, very broadly into three categories: low power QRP (to 20 W), normal power (to 100-200 W) and high power QRO (all above), sometimes you may see QROO (which stands for very high power and, usually, amount power which is being delivered is not asked, but silently observed from power meter. :D