Low noise amplifier
A special type of electronic amplifier is called a low-noise amplifier or low-noise amplifier ( LNA ). The circuitry is characterized by a particularly low noise level in order to amplify extremely weak signals without converting the entire frequency band into lower frequencies. The noise figure is a special characteristic of an LNA .
In satellite communication , the signal received by a parabolic antenna first reaches an LNA, and after amplification is then either fed directly into a cable or fed to a next stage, which then converts the frequency to better convert the signal via a cable to mix down the frequencies to be transmitted , e.g. B. a Low Noise Block Converter (LNB). The signal is fed directly into the next transmission unit - in the case of the LNA this is the connected connection cable, in the case of the LNB it is the conversion block in front of the signal forwarding. Therefore an LNA - in contrast to an LNB - is also suitable for connecting several parabolic mirrors to form an array. The phase position of each antenna must be adjusted separately by moving it forwards or backwards (towards the transmitter) millimeter by millimeter. Due to the relatively large line loss, the cables must be kept extremely short. Therefore, LNAs are only used in exceptional cases, e.g. B. in GPS antennas. Instead, the signal is converted to lower frequencies in blocks directly in the device and fed into the cable with significantly less loss - this combination of LNA and block converter is an LNB.
An LNA can also be found in the reception path of a cell phone . There he is the first member of the analog signal processing chain and is in the analog front end integrated . Due to the integration, no adaptation to line impedance is necessary on the output side , since the next component - usually a mixer for frequency conversion to the baseband - follows directly and can easily be implemented with a high input resistance . The gain of an LNA is highly frequency-dependent, so that the LNA has a frequency-selective effect due to its design. The transfer function of the voltage gain has a bandpass character. On the input side, an LNA must be adapted to the field wave resistance of the antenna.
literature
CD Motchenbacher and JA Connelly: Low-Noise Electronic System Design . 1st edition. Wiley-Interscience, 1993, ISBN 0-471-57742-1 .