One of the most important radio components is the amplifier. RF amplifiers are necessary to intensify radio signals that are not sufficiently strong. They can be used when there is a need to increase the power of a signal and the output power of a transmitter as well as the sensitivity of a receiver.
When thinking of an RF amplifier, it's easy to think simply in terms of output of watts. After all, if somebody only needs to have a more a powerful signal, all it should take is a good, powerful amplifier with a large watt output.
Unfortunately, it's not that easy.
A powerful amplifier does not necessarily mean good sound. Other considerations must come into play: insufficient gain, distortion, noise and interferences, all problems that can affect modulated signals when amplified.
Modulation is the process of changing a signal so that it can transport information. Popular types of modulation are AM and FM.
AM, or amplitude modulation, takes its name from the fact it is the amplitude of the signal that carries the information. FM, or frequency modulation, sees the modulation of the frequency to carry the needed info.
The different characteristics of the two modulation signals reveal they behave differently when transmitted from a transmitter to a receiver and when processed through an amplifier.
RF amplifiers themselves don't come in a single model but they are divided by classes: A, B, AB, C and D are examples of classes that differ for angle, efficiency and power output capability. Different classes also manage modulated signals in different ways.
Amplifiers can also be divided in two larger types: linear and non-linear. Linear amplifiers are a special class of RF amplifiers that output a signal that is proportional to the input. In linear amplifiers the output signal is an accurate reproduction of the input signal. Non-linear amplifiers are more efficient but any changes in the input do not correspond to proportional changes in the output.
In terms of distortions, AM and FM signals passed through an amplifier behave differently. The parameter to consider is the peak power of the signal that is passed through. The more accurately the amplifier is able to process the peak power, the less the distortion. With FM signaling where average power is the same as peak power, distortion is minimal.
Problems can occur with AM signals as the peak power varies and depends on the percentage of modulation. An amplifier with not enough headroom could cut the peaks of the signal and create distortion.
To prevent some of the distortion it's better to use linear amplifiers, The most commonly used linear amplifiers are Class A, B and AB which are normally very inefficient (inefficiency is the relationship between power output an power input with a lower or higher heat dispersion), with the Class A said to be the worst in terms of dispersing power.
FM signals with their constant amplitude tend to require less care than AM signals: It's less prone to be affected by noise, distortion and signal level variations which instead can greatly affect AM transmissions. It is, then, possible to use amplifiers that are non-linear (switching) which have a higher efficiency (i.e., better power conversion efficiency), disperse less heat and cost sensibly less. An example of switching amplifier is the Class D amplifier.
The differences in the way AM and FM signals are modulated determine the way they are affected when transmitted or when they are passed through an amplifier. Knowing how the two signals behave can help predict how to better handle them for maximum performance and prevent costly mistakes when shopping for radio components.
