Capitulo 2 ( Electronic fundamentals )

Capitulo 2 ( Electronic fundamentals )

1. Semiconductor theory

   • Materials that combine some of the electrical characteristics of conductors with those of insulators are known as semiconductors. Common types of semiconductor material are silicon, germanium, selenium and gallium. In the pure state, these materials may have relatively few free electrons to permit the fl ow of electric current. However, it is possible to add foreign atoms (called impurity atoms) to the semiconductor material in order to modify the properties of the semiconductor and allow it to conduct electricity.
     • Temperature effects
       • The properties of most semiconductor devices are profoundly affected by temperature. In order to maintain performance within specifi cation, care must be taken to ensure that temperature variations are minimized.
       • Thermistors provide us with a means of sensing temperature changes. The resistance of an NTC thermistor falls with temperature whilst that of a PTC thermistor increases with temperature.

2. Diodes

   • When a junction is formed between N-type and P-type semiconductor materials, the resulting device is called a diode . This component offers an extremely low resistance to current fl ow in one direction and an extremely high resistance to current fl ow in the other.
     • Diode characteristics
       • Key point The forward voltage for a germanium diode is approximately 0.2V whilst that for a silicon diode is approximately 0.6V.
     • Zener diodes
       • Zener diodes begin to conduct heavily when the applied voltage reaches a particular threshold value (known as the Zener voltage). Zener diodes can thus be used to maintain a constant voltage which is often used as a voltage reference.
     • Silicon-controlled rectifi ers
       • Silicon-controlled rectifi ers (SCR) are diodes that can be triggered into conduction by applying a small current to their gate input. SCRs are able to control large voltages and currents from a relatively small (low-current, low-voltage) signal.
     • Light-emitting diodes
       • Light-emitting diodes produce light when a small current is applied to them. They are generally smaller and more reliable than conventional fi lament lamps and can be used to form larger and more complex displays.
     • Rectifiers
       • The frequency of the ripple voltage present on the DC output of a half-wave power supply will be the same as the frequency of the AC input. The frequency of the ripple voltage present on the DC output of a full-wave power supply will be double that of the AC input.
         • The forward voltage for a germanium diode is approximately 0.2V whilst that for a silicon diode is approximately 0.6V.

3. Transistors

   • Typical applications for transistors in aircraft electrical and electronic systems are controlling generator fi eld current, driving lights and warning displays, amplifying signals from sensors and transducers and for use as amplifying devices in cabin interphone and aircraft radio and navigation aids.
     • Bias and current fl ow
       • The emitter current of a transistor is the sum of its base and collector currents.
     • Transistor characteristics
       • The characteristics of a bipolar junction transistor are usually presented in the form of a set of graphs relating voltage and current present at the transistors terminals.
     • Transistor operating confi gurations
       • Three basic circuit arrangements are used for transistor amplifi ers and these are based on the three circuit confi gurations that we met earlier (i.e. they depend upon which one of the three transistor connections is made common to both the input and the output).
     • Current gain
       • The common emitter current gain of a transistor is the ratio of collector current to base current and is typically in the range 40 to 200.

4. Integrated circuits

   • Considerable cost savings can be made by manufacturing all of the components required for a particular circuit function on one small slice of semiconductor material (usually silicon). The resulting integrated circuit may contain as few as 10 or more than 100,000 active devices (transistors and diodes). With the exception of a few specialized applications (such as amplifi cation at high power levels) integrated circuits have largely rendered conventional circuits (i.e. those based on discrete components ) obsolete.
     • Integrated circuits contain large numbers of individual components fabricated on a single slice of silicon. Integrated circuits are often classifi ed as either digital (logic) or linear (analogue).
     • Operational amplifi ers are linear integrated circuits that can be used as versatile ‘gain blocks ’ within a wide variety of linear circuits.