D$_{1}$ D$_{4}$ D$_{5}$ D$_{6}$ D$_{3}$ D$_{2}$ Rotor (field) Stator + − + 1 2 3 1 2 3 − Output waveform Load resistor D$_{1}$ D$_{4}$ D$_{3}$ D$_{2}$ + − + 0 − Output waveform Transformer AC Input 11-21 Figure 11-45.  The bridge-type four-diode full wave rectifier circuit  is most commonly used to rectify single-phase AC into DC.  Figure 11-46.  A six-diode, three-phase AC rectifier.  Amplifiers Current flows from it through diode (D$_{1}$), then through the  load resistor, and through diode (D$_{2}$) on its way back to the  top of the secondary coil. When the AC reverses its cycle,  the polarity of the secondary coil changes. Current flows  from the top of the coil through diode (D$_{3}$), then through the  load resistor, and through diode (D$_{4}$) on its way back to the  bottom of the secondary coil. The output waveform reflects  the higher voltage achieved by rectifying the full AC cycle  through the entire length of the secondary coil. Use and rectification of three-phase AC is also possible  on aircraft with a specific benefit. The output DC is very  smooth and does not drop to zero. A six-diode circuit is built  to rectify the typical three-phase AC produced by an aircraft  alternator.  [Figure 11-46] Each stator coil corresponds to a phase of AC and becomes  negative for 120° of rotation of the rotor. When stator 1 or  the first phase is negative, current flows from it through  diode (D$_{1}$), then through the load resistor and through diode  (D$_{2}$) on its way back to the third phase coil. Next, the second  phase coil becomes negative and current flows through diode  (D$_{3}$). It continues to flow through the load resistor and diode  (D$_{4}$) on its way back to the first phase coil. Finally, the third  stage coil becomes negative causing current to flow through  diode (D$_{5}$), then the load resistor and diode (D$_{6}$) on its way  back to the second phase coil. The output waveform of this  three-phase rectifier depicts the DC produced. It is a relatively  steady, non-pulsing flow equivalent to just the tops of the  individual curves. The phase overlap prevents voltage from  falling to zero producing smooth DC from AC.  An amplifier is a circuit that changes the amplitude of an  electric signal. This is done through the use of transistors.  As mentioned, a transistor that is forward biased at the  base-emitter junction and reversed biased at the collector- base junction is turned on. It can conduct current from the  collector to the emitter. Because a small signal at the base  can cause a large current to flow from collector to emitter, a  transistor in itself can be said to be an amplifier. However, a  transistor properly wired into a circuit with resistors, power  sources, and other electronic components, such as capacitors,  can precisely control more than signal amplitude. Phase and  impedance can also be manipulated. Since the typical bipolar junction transistor requires a based  circuit and a collector-emitter circuit, there should be four  terminals, two for each circuit. However, the transistor only  has three terminals (i.e., the base, the collector, and the  emitter). Therefore, one of the terminals must be common to  both transistor circuits. The selection of the common terminal  affects the output of the amplifier. Since the typical bipolar junction transistor requires a base  circuit and a collector-emitter circuit, there should be four  terminals—two for each circuit. However, the transistor only  has three terminals: the base, the collector, and the emitter.  Therefore, one of the terminals must be common to both  transistor circuits. The selection of the common terminal  affects the output of the amplifier.