LNA

A low noise amplifier is a high-gain electronic amplifier positioned as close as possible to a signal source, usually within a few centimetres or at the antenna itself. Its job is to boost faint incoming signals before they travel down cables or through circuits where they would otherwise be buried under thermal and electronic noise. LNAs are essential in any system receiving weak signals: satellite earth stations, radar receivers, radio telescopes, and cellular base stations all rely on them.

The key metric is noise figure, measured in decibels. A typical LNA has a noise figure between 0.3 and 1.5 dB, meaning it adds very little noise relative to the signal it amplifies. This matters because noise added early in the chain gets amplified along with the signal in every subsequent stage. Placing a low-noise stage first means that even if later stages are noisier, the overall system performance is dominated by that initial amplifier. Gain is typically 15 to 25 dB, enough to make the signal robust but not so high that it saturates downstream equipment.

LNAs are built from low-noise transistors, usually gallium arsenide field-effect transistors (GaAs FETs) or silicon germanium bipolar transistors (SiGe), chosen for their low thermal noise and high gain at microwave frequencies. Design involves careful impedance matching between the source, the transistor, and the load, using capacitors and inductors to tune out reflection losses. Operating point (bias current and voltage) must be set precisely: too little bias and the transistor becomes noisy; too much and power consumption and heat become problems.

Where LNAs live in a receiver chain

In a typical receiving system, the LNA sits immediately after the antenna or filter. Its output connects to a mixer or second amplifier stage. Because the LNA operates at the original signal frequency (not yet down-converted), it must handle the full bandwidth of interest, from VHF to millimetre-wave frequencies depending on the application. At higher frequencies, above about 10 GHz, noise performance matters even more because thermal noise becomes a larger proportion of the signal power budget.

Common problems include instability, where positive feedback causes oscillation; intermodulation distortion, where strong nearby signals create unwanted products; and thermal drift, where temperature changes alter gain and noise figure. Cooling, shielding, and careful circuit layout are standard remedies. LNAs are often integrated into receiver front-end modules alongside filters and switches, reducing cable loss and simplifying system assembly.