Si CMOS power amplifier based on source common circuit for millimeter wave super broadband wireless systems using beam forming antenna has been designed. Requirement of output power of each beam forming antenna elements can be reduced to less than 3dBm because of power combining at the air after radiation. Instead of reduction of output power requirement, power added efficiency (PAE) has been improved in our design. Maximum PAE of 18.3% and PAE at operation point of 15.9% have been achieved by simulation. Measured PAE has been evaluated to be 12.5% at the operation point. AM-AM and AM-PM conversion of designed power amplifier has been modeled using modified Rapp model for system simulation. Degradation of Eb/N0 at bit error rate of 10-3 using QPSK has been evaluated to be less than 0.5 dB considering with AM-AM and AM-PM conversion.