Low-noise Amplifier (LNA)

At a satellite communications transponder, the very weak RF uplink signal is first amplified by a low-noise amplifier (LNA) to preserve signal quality. Early LNAs employed devices such as tunnel-diode amplifiers, but modern satellite transponders predominantly use solid-state technologies. C- and Ku-band LNAs are typically based on gallium-arsenide field-effect transistors (GaAs FETs) or high-electron-mobility transistors (HEMTs), while Ka-band LNAs commonly use advanced GaAs or indium-phosphide (InP) HEMT devices rather than parametric amplifiers.

To limit the noise power presented to the LNA, the received signal is band-limited by a preselection filter, often implemented as a waveguide or RF band-pass filter located ahead of, or immediately following, the antenna feed. The noise contribution at the LNA input depends on the system noise temperature and the effective noise bandwidth. The overall gain of the transponder receiving chain, including the LNA and subsequent stages, is typically on the order of 60–70 dB.

A low-noise block (LNB) downconverter integrates an LNA with additional receiver functions, most notably frequency down-conversion and filtering, into a single compact unit mounted close to the antenna feed. The LNB amplifies the received satellite signal and converts it from the RF downlink frequency to a lower IF suitable for transmission over coaxial cable to indoor receiver equipment. Modern LNBs typically include a local oscillator, mixer, filters, and sometimes polarization and band-switching circuitry.

LNAs are commonly used as standalone devices in large Earth stations and gateway terminals, while LNBs are widely used in small terminals such as VSATs and direct-to-home receivers to reduce feeder losses, cost, and installation complexity.