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| SEC. 12-1 AMPLIFIER TERMS
The classes of operation are A, B, and C.
The types of coupling are capacitive,
transformer, and direct. Frequency terms
include audio, RF, narrowband, and
wideband. Some types of audio amplifiers
are preamps and power amplifiers. SEC. 12-2 TWO LOAD LINES Every amplifier has a dc load line and an ac load line. To get maximum peak-topeak output, the Q point should be in the center of the ac load line. SEC. 12-3 CLASS A OPERATION The power gain equals the ac output power divided by the ac input power. The power rating of a transistor must be greater than the quiescent power dissipation. The efficiency of an amplifier stage equals the ac output power divided by the dc input power, times 100 percent. The maximum efficiency of class A with a collector and load resistor is 25%. If the load resistor is the collector resistor or uses a transformer, the maximum efficiency increases to 50 percent. SEC. 12-4 CLASS B OPERATION Most class B amplifiers use a push-pull connection of two transistors. While one transistor conducts, the other is cut off, and vice versa. Each transistor amplifies one-half of the ac cycle. The maximum efficiency of class B is 78.5 percent. SEC. 12-5 CLASS B PUSH-PULL EMITTER FOLLOWER Class B is more efficient than class A. In a class B push-pull emitter follower, complementary npn and pnp transistors are used. The npn transistor conducts on one half-cycle, and the pnp transistor on the other. SEC. 12-6 BIASING CLASS B/AB AMPLIFIERS To avoid crossover distortion, the transistors of a class B push-pull emitter follower have a small quiescent current. This is referred to as a class AB. With voltage divider bias, the Q point is unstable and may result in thermal runaway. Diode bias is preferred because it can produce a stable Q point over a large temperature range. SEC. 12-7 CLASS B/AB DRIVER Rather than capacitive couple the signal into the output stage, we can use a direct-coupled driver stage. The collector current out of the driver sets up the quiescent current through the complementary diodes. SEC. 12-8 CLASS C OPERATION Most class C amplifiers are tuned RF amplifiers. The input signal is negatively clamped, which produces narrow pulses of collector current. The tank circuit is tuned to the fundamental frequency, so that all higher harmonics are filtered out. SEC. 12-9 CLASS C FORMULAS The bandwidth of a class C amplifier is inversely proportional to the Q of the circuit. The ac collector resistance includes the parallel equivalent resistance of the inductor and the load resistance. SEC. 12-10 TRANSISTOR POWER RATING The power rating of a transistor decreases as the temperature increases. The data sheet of a transistor either lists a derating factor or shows a graph of the power rating versus temperature. Heat sinks can remove the heat more rapidly, producing a higher power rating. |