We present a -push BJT clamping harmonic generator and an optimal operation condition is derived to maximize the desired harmonic component. Under the hard switching condition, harmonic currents are calculated from time domain waveform generated from a simplified BJT model. Output waveform and the harmonic currents match well with Harmonic Balance (HB) simulation. For verification, measurements from a 380 GHz quadrupler with the on-chip patch antenna implemented in 0.13 SiGe BiCMOS are presented. Optimal biasing point for maximum EIRP corresponds well with the calculated value.