الترانزستورات - التغذية المرتدة الداخلية - الصمامات ذات المهبط المسخن .. الإلكترونيات

Transistors - Internal Feedback -
Heated Cathode Diodes .. Electronics

Transistors

Transistors usually consist of three sectors, the middle sector of which is called the base, which is a semiconductor crystal of the positive or negative type, and on its sides are two crystals of a semiconductor material that differ in their type from the crystal of the middle sector, they are called the collector and the emitter • The M-type transistor contains a middle crystal of a negative semiconductor material, and on its sides are two crystals of a positive semiconductor material (105).

105 - M-type transistor.
106 - Flat transistor.

As for the M-type transistor, it contains a middle crystal of a semiconductor material and on its sides are two crystals of a negative semiconductor material. The M-type transistor is made in the following way: Indium strips are melted and then spread inside the base to form the M-type regions. Then all are placed inside a sealed cover from which the connecting wires for each sector come out. The disadvantage of this type of transistors is its low efficiency at high frequencies. It is preferable to the adhesive transistor or the flat transistor, which consists of a germanium base that is a very thin slice that does not exceed a fraction of a millimeter in thickness (106) with two thin layers of germanium attached to its sides. This type of transistor is characterized by high efficiency and effectiveness in conduction.

107 - Large current flow.

108 - Small current flow.

The emitter of the transistor is connected to the positive pole of the battery, while the collector is connected to its negative terminal (107). If the base voltage is more negative than the emitter, the diode consisting of the emitter and the base plays the role of a forward biased rectifier. In this case, the holes move from the emitter to the base easily, and then the collector pulls them because of their attraction to the negative charges present in it. With this method of connection, the current intensity passing through the transistor is very large.

If the negative bias between the emitter and the base decreases, the holes that move from the emitter to the collector become few, and with them the current intensity passing through the transistor (108) decreases. Therefore, we find in the MCM type transistors that the electrons carrying negative charges are the "minority" and the holes carrying positive charges are the "majority". It is known that the charge of the electron is equal to the charge of the hole. Therefore, the sum of the collector and base currents is almost equal to the current coming out of the emitter. We can say that almost all the emitter current passes through the collector. The basic differences between MCM transistors and SMCS transistors (109) are summarized as follows: The base of the SMCS transistors, i.e. its middle number sector, is a negatively conductive crystal and the majority of the current-carrying charges in it are positively conductive holes resulting from the lack of electrons. The emitter is connected to the positive pole of the battery. In the SMAS transistors, the base or middle sector is a positively conductive crystal and the current flow is caused by negatively charged electrons. In this transistor, the emitter is connected to the negative pole of the battery.

It is always noted in the symbolic drawings of the transistor that the arrow points to the emitter and that its direction is consistent with the direction of movement of the holes carrying positive charges in it. This is consistent with the conventional direction of the flow of electric current from positive to negative as is followed and agreed upon in many books. As for the direction of the flow of electrons, it is from negative to positive. It is always noted that the intensity of the current flowing through the transistor increases if the correct connection direction is reversed, i.e. the polarity of the battery that supplies it is reversed. If the resistors connected to the transistor are not sufficient to limit the intensity of this current, this leads to its immediate damage.

Internal Feedback

It is known that the two paths of the transistor are very close to each other, and that this, in addition to some other effects, leads to the presence of unwanted feedback between the collector and the base. This unwanted feedback assumes the presence of a fictitious capacitor connected between the collector and the base (110). It is worth noting that this effect is worthless at low frequencies, but at high frequencies it leads to many problems that require taking the necessary precautions to prevent vibrations or any other side effects.

109 - Comparison between the MCM transistor and the SMCS transistor.
110 - Shielded transistor.

Transistors used in high frequencies are sometimes provided with a fourth terminal or a protective sheath between the base and the collector (110), in order to reduce the value of unwanted feedback. In addition, transistors used in receivers (111) are characterized by being very small in size and being covered on the outside with an insulating sheath or a metal sheath to protect them. Transistors used in amplifying audio frequencies and radio frequencies are also very small.

As for power transistors (112), used in radio frequency circuits or amplifiers, they are larger in size and can be glued or fixed on a metal plate or any surface that dissipates excess heat. Some transistors are made in the form of similar or matching pairs placed in one sheath. Others are designated for computer “logic” and other special uses.

111 - Small transistors for amplification and reception.
112 - Power transistors and dual transistors.