A highly-linear I-channel receiver prototype is presented for a 3 to 6 GHz broadband radio system with a 200 MHz baseband bandwidth and verified to operate under congested spectrum environments. A direct conversion receiver developed from this prototype is suitable for a cognitive radio, fifth-generation (5G) receiver, and other wireless systems with a total (in-band signal plus blocker) power above −6.0 dBm. The broadband receiver consists of a low-noise transconductance amplifier, a passive mixer, a wideband transimpedance amplifier and a power-efficient minimally-invasive baseband filter. The low- noise transconductance amplifier with high linearity employs a cross-coupled structure and resistive degeneration to achieve low noise and high linearity simultaneously.