Link-budget anchor: 6.5 dB gain at 8 GHz fT
The BFU590QX is an NPN RF transistor with a transition frequency of 8 GHz and a small-signal gain of 6.5 dB. In a cascaded receiver front-end, that gain stage improves the noise figure of the following mixer by lowering the overall cascaded NF — the Friis formula applies, so every dB of gain before the mixer reduces the noise contribution of later stages by that same dB. Rated for a 12 V collector-emitter breakdown and 200 mA maximum collector current, the device can deliver roughly 300 mW linear output power in a class-A stage before hitting the 1 dB compression point. The 12 V rail must stay below breakdown with margin for antenna mismatch reflections that can double the voltage at the collector node.
Package and thermal path: SOT-89-3
Housed in a SOT-89-3 (TO-243AA) surface-mount package, the large collector tab on the back provides a low-inductance ground path and conducts heat to the PCB copper. The DC current gain is a minimum of 60 at 80 mA collector current and 8 V Vce — base-drive losses are negligible in the bias network, and the bias resistor values are set by the desired quiescent current rather than by beta spread across the production lot.