The output/collector current of the amplifier circuit is given by I_L=I_C=β×I_B , where I_B is the base current and . α is the fraction of emitter current received at the collector. This means that I_C=α×I_E and value of is about 0.95 to 0.99. Let us put in equation of and therefore . The power gain at the output is directly proportional to β². Putting β=99 then power gain is proportional to 99²=9801 Hence the output power is several times greater than the input power. The power gain or extra power is coming from the relationship between α and β parameters.