Engineering Notes
RF POWER AMPLIFIERS TECHNICAL REFERENCE MATERIAL
Gain
Normaly expressed in decibels, dB
Power Gain (dB) = 10 Log _{10} (RF Output Power / RF Input Power)
Gain is defined as the ratio of the output power to the input power in dB. Assume that the input power is 10 mW (+10 dBm) and the output power is 1 W (1000 mW, +30 dBm). The ratio will be 1000/10 = 100, and the gain will be 10 * log 100 = 20 dB.
It is much easier to calculate gain by converting the power to dBm first, so the gain of the above amplifier will be 3010 = 20 dB. A simple reduction will reveal the gain. In tradition, all power is read in dBm and the gain is in dB.
Example:
A low noise amp (30 dB gain) cascades with a driver amp (20 dB gain) and a power amp (10 dB gain). The total gain will be 30+20+10 = 60 dB.
Frequently Used Decibel Conversions 

dB  Power Ratio 
O 1 (1) 3 (3) 6 (6) 10 (10) 13 17 20 30 
1 1.26 (0.8) 2 (1/2) 4 (1/4) 10 (0.1) 20 50 100 1000 