The feedback technology for gain flatness and stability of UWB LNA

In this paper, based on small signal models of hetero-junction bipolar transistors (HBTs) with different feedback circuits, the expressions for S parameters of the device are derived to analyze the influence of different feedback technologies on amplifier performance, especially on the stability and the gain flatness of Ultra-Wideband Low Noise Amplifier(UWB LNA). When a serial inductor or a parallel capacitor is added into the feedback circuits to induce reactive feedback, the amplitude of S₂₁ increases as the frequency increases, the HBT´s gain inherent decreasing at high frequency is compensated, a good stability performance can also be reached simultaneously. Based on reactive feedback techniques, we design a 3GHz-10GHz UWB LNA. The simulation results show that throughout the whole frequency band, the amplifier remains unconditionally stable, has a high transmission gain, good gain flatness and low noise figure.