A new feedback method for power amplifier with unilateralization and improved output return loss

Given a device with a conventionally optimized P_{1 dB} point, this paper proposes a new theory for feedback amplifiers to optimize gain and improves S₂₂ (ideally resulting in S₂₂=0). The design procedure only requires the small-signal S-parameters and the measured load-pull data of the transistor. To verify this theory, two 800-mW 2-GHz GaAs MESFET amplifiers with and without the lossless feedback were implemented and evaluated. The feedback amplifier achieved significant improvement in linear gain (6 dB), reverse isolation (12 dB), and output return loss (5 dB) with the same output P₁dB for both types of amplifiers.