The Radiotelescope Pre-amplifier

Cosmic radio signals are generally very weak.  To measure them we have to amplify them by factors of millions of times. The electronic components in our radiotelescope produce random electrical noise which also gets amplified by this huge amount. It is easy to see that the noise from the early parts of the chain of amplifiers gets multiplied by more than the later stages. If we are not careful, this noise can totally hide the weak noise we are trying to measure from the cosmic radio source. The role of the preamplifier is to boost this incoming signal from the antenna many times while adding as little noise as possible. The preamplifier is often called an LNA or low noise amplifier. Special transistors are used in this stage to accomplish this. Professional observatories also use cooling of the amplifiers to very low temperatures, just a few degrees above absolute zero to minimize the amount of noise contributed by the components.  You will sometimes hear amateur radio astronomers and other people interested in receiving weak signals refer to the noise figure of an amplifier.  We won't get into the specifics of what this means but a noise figure of less than .5 decibels (dB) is considered very good.

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