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  Radio signals are all around us, constantly being transmitted from one place to another. They contain information such as music, the news, communication--just about anything. Before this information ever reaches our radios, however, it needs the power to reach them. That power comes from RF amplifiers.
  
  

RF Amplifiers

• RF amplifiers increase the strength of a radio signal for broadcast purposes. These amplifiers come in a variety of types, or classes. The difference between them is primarily in the level of voltage applied to the power amplifier control grid. Most RF amplifiers still use the increasingly antiquated technology of vacuum tubes.
  
  

Class A

• A low output, low efficiency class of amplifier. The vacuum tube plate current is never cut off, producing only 30 percent energy efficiency. Class A and B amplifiers operate by allowing a small amount of grid current to flow as necessary.
  
  

Class B

• A half-wave output amplifier with about 65 percent efficiency. This amplifier uses an increased control grid bias so the plate current will be just at cut-off. The applied signal's positive portion flows immediately, while the negative never flows, no matter how far the grid may go into negative. To drive the grid positively, sufficient voltage is required and two tubes may be used in a push-pull fashion.
  
  

Class AB

• Class AB amplifiers are a compromise between A and B operations. This compromise allows for a selectable bias point between the range of 360 and 180 degrees of the plate flow during the RF cycle. The changeable bias point provides a selection range that offers low-distortion, low-efficiency output on one end of the spectrum, and higher distortion, higher efficiency output on the other.
  
  

Class C

• A similar amplifier to AB types, with 90 percent efficiency. The control grid is biased far past cut-off, with the plate current only flowing with high excitation. The current can reach saturation, but the waveform of the signal produced can be terribly distorted in class B or C operation.
  
  

Class D

• Also known as a switching, or digital, amplifier. The class D amplifier uses transistors that are either completely turned on or off. When the transistors are conducting, there is almost no voltage across the transistor. When there is significant voltage across the transistor, there is no current flowing through the transistor. With current that switches on and off, class D amplifiers are highly efficient.