Design of a Low-Noise Amplifier With Highly Efficient Sub-Harmonic Suppression Based on a Self-Biasing Adaptive Filter for Nonlinear Radar

In nonlinear radar (NLR) systems for detecting concealed electromagnetic devices, effective suppression of the fundamental frequency ($f_0$) and amplification of the second harmonic ($2f_0$) are critical for target discrimination. This letter presents a low-noise amplifier (LNA) architecture with a self-biasing adaptive filter (SBAF) for NLRs. The proposed LNA selectively suppresses $f_0$ and amplifies $2f_0$ through power-aware control without external bias. The SBAF dynamically adjusts the Q-factor of the output resonance circuit depending on the input power level, enabling adaptive suppression of $f_0$. The circuit architecture comprises four core blocks: a BJT-based amplifier stage (BAS), a frequency-selective network (FSN), an adaptive biasing rectifier (ABR), and an active resonance circuit (ARC). A prototype was fabricated and measured at $2f_0=6.2$ GHz, achieving 54.6 dB suppression at $f_0$, 14.6 dB gain at $2f_0$, a 9.8 dB improvement in output third-order intercept point (OIP3), and a noise figure (NF) of 1.4 dB. The proposed architecture demonstrates effectiveness in enhancing nonlinear target detection for NLR applications.