This paper presents a wideband transformer-based power amplifier applicable for the millimeter-wave (MMW) fifth generation (5G) mobile communication. The proposed power amplifier is manufactured in 28-nm HPC-plus CMOS process. The power stage and driver stage amplifiers are biased in deep class-AB to reduce the dc power dissipation in the linear power region and provide a linear operation close to the saturated output power (PSAT) of the PA. In the output matching network, a transformer with two coupled resonant capacitors is designed for broadband power matching. The proposed PA achieves the peak small-signal power gain (Gp) of 20.4-dB at 23 GHz and a measured 3dB small-signal gain bandwidth (BW3dB) from 20.8 to 41.6 GHz covering the multiple 5G bands with only 39.6-mW quiescent power consumption. The measured Psat is 16.1-dBm at 30 GHz with over 50% Psat output power 1dB bandwidth (BW1\dB) from 23 to 38.5-GHz. Also, this PA reports a 35% peak power added efficiency (PAEMAX) at 25 GHz and a 13-dBm peak OP1\dB at 41 GHz. In the 64-QAM OFDM modulated signal measurement, this power amplifier obtains an output power of 7.2/6.9-dBm and a modulated PAE of 8.5%/8.8% at 23 and 41-GHz respectively when keeping the EVM below -25 dBc.