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Lange couplers, which readily provide broadband, 90-deg. phase splits of input signals, are detailed in design and operation in refs. 12 and 13. The Lange coupler for the current amplifier was designed with four fingers for use from 29 to 37 GHz, exceeding the frequency range (30 to 36 GHz) actually required by the LNA.

The coupler was computer-simulated with the aid of a commercial, three-dimensional (3D) electromagnetic (EM) simulation program. The simulated results of the Lange coupler indicate insertion loss of better than 3.52 dB, with amplitude balance of less than 0.48 dB and phase balance of less than 0.53 deg. from the reference 90-deg. phase difference.

For a pHEMT device, the load reflection coefficient, Γopt, is generally much different than S11*, the conjugate reflection coefficient of S11. This implies that an amplifier that is designed for minimum noise figure would also be mismatched for a maximum power-transfer condition.