Why do radio receivers have cascaded RF amplifiers?
- Why does a receiver have several amplifiers cascaded in it's RF stage? Why not just one amplifier that does all of the amplification? (cf picture below)
- Why does every RF amplifier needs a tuned circuit? Why not use a filter at the antenna and amplify with several amplifiers?
Placing all filtering at antenna signal costs attenuation of wanted signal, as filters not are free of losses. This loss can not be compensated for with amplifiers afterwards as amplifiers add noise, resulting in less sensitive receiver (less signal noise ratio).
To much gain in one single amplifier is complicated to handle due to self oscillation. Precision tuning between 100's of channels, say FM or GSM, would require filters of so far uninvented frequency selectivity.
You do not want strong out of band signals should saturate amplifier while trying to amplify a weak signal so each amplifier is protected with some filtering.
This filter is also a part of impedance matching between each stage for best gain and low noise.
Your proposed design is used in some VLF receivers as filtering can be more important then amplifying when listening at radio frequencies below 100 Hz, for best signal noise ratio.
Second picture shows an very old tube radio without local oscillator. LO is common in modern receivers which also have a big part in why and how to use filtering and amplifying as effective as possible.
http://en.wikipedia.org/wiki/Superheterodyne_receiver
http://en.wikipedia.org/wiki/Direct-conversion_receiver