A novel quantitative design method is proposed herein for a 120-GHz 8-stage broadband amplifier using the stagger tuning technique. The pole frequencies of the unit amplifiers are determined by the parasitic capacitors and the size of the matching inductors, and resonance peaks are formed at the pole frequency. We proposed the stagger tuning technique at two different frequencies, where an inductance of four stages is chosen to match the pole frequency at a lower frequency (f_L) and an inductance of the other four stages is chosen for the upper frequency (f_H). With this technique, the maximum bandwidth can be achieved with the given amplifier gain. Moreover, it was shown that the amplifier designed with stagger tuning at two different frequencies yielded the lowest noise figure in the target band because the gain was concentrated in the target band. A 120-GHz 8-stage amplifier is designed with the proposed technique and fabricated using 40-nm CMOS technology. The measured peak gain of the amplifier is 20.6 dB, and the 3-dB bandwidth is 40.6 GHz while consuming 45-mW of dc power.