AMPLIFICATION: GENERAL INFORMATION
matv amplifiers are classified into various categories such as:
these components are designed to amplify the incoming signal within a specified band of frequencies while introducing as little noise as possible.
preamplifiers are generally used in conjunction with antennas to amplify relatively weak off-air signals.
some models may incorporate one amplifier covering the entire band while others may have two or three, each tuned to a smaller portion of the frequency band.
the noise figure is the most important preamplifier specification and determines how much noise is added to the matv system while the very weak input signal is being amplified to an acceptable signal to noise ratio (snr).
a minimum value that is expected in a good quality unit is 2 db, measured from the input to the output of the device (higher than the nf of the first transistor, indicated by some manufacturers).
a preamplifier must have an adequate gain so that the signal level is increased to overcome the downlead losses.
the bandwidth specification provides information on which are the channels that a preamplifier covers and also indicates how the device is constructed.
if the amplifier has several input frequency bands it may mean that a bandpass filter was used to eliminate the intermediary portions among those different bands.
on the other hand, if two amplifier stages were used, the harmonics from the vhf band would not be a problem.
amplifiers can suffer from a phenomenon called compression. this is a point where the input signal is so high that it saturates the amplifier so that the output cannot be higher.
single channel amplifiers can also be affected by a type of distorsion known as intermodulation .
this occurs when the picture and the sound carriers in the tv signal, interfere with each other and beging interacting or beating together.
another phenomenon produced by amplifiers is a distorsion due to cross modulation .
the greater the bandwidth of a preamplifier, the greater the possibility of cross modulation.
the maximum output capability of a broadband amplifier is indicated at a certain cross modulation level when the device is carrying a specified number of channels.
the standard most used all over the world is din 450008.
distribution amplifiers differ from off-air amplifiers because they are designed to deliver high output levels, provided that the input signals have an adequate snr.
while distribution amplifiers can either have fixed or adjustable gains, they usually have a 20 db adjustment range.
they also may have either a built-in or a plug-in slope control to compensate for losses at high frequencies.
manufacturers specify the maximum output level so that the gain can be reduced if the input level is too high, to avoid signal compression.