Basic knowledge and working principle of triode

Basic knowledge and working principle of triode

The semiconductor triode is also called a crystal triode. It can be said that it is the most important device in the electronic circuit. Its main function is current amplification and switching. As the name implies, the triode has three electrodes. The diode is composed of a PN structure, while the triode is composed of two PN structures. The shared electrode becomes the base of the triode (indicated by the letter b). The other two electrodes become the collector (indicated by the letter c) and the emitter (indicated by the letter e). Due to different combinations, one is an NPN type triode and the other is a PNP type triode.

There are many types of transistors, and different models have different uses. Transistors are mostly plastic or metal packages. The appearance of common transistors is shown in the figure. The large ones are very large, and the small ones are very small. There are two types of transistor circuit symbols: the electrode with an arrow is the emitter. The arrow pointing outward is the NPN transistor, and the arrow pointing inward is the PNP transistor. In fact, the direction indicated by the arrow is the direction of the current.

The commonly used triodes in electronic production are 90×× series, including low-frequency low-power silicon tube 9013 (NPN), 9012 (PNP), low-noise tube 9014 (NPN), high-frequency low-power tube 9018 (NPN), etc. Their models are generally marked on the plastic shell, and they all look the same, all in TO-92 standard package. In old electronic products, you can also see 3DG6 (low-frequency low-power silicon tube), 3AX31 (low-frequency low-power germanium tube), etc., and their models are also printed on the metal shell. There is a set of naming rules for transistors produced in China, and electronic enthusiasts are better off understanding them:

the 3 in the first part indicates a triode. The second part indicates the material and structure of the device, A: PNP type germanium material B: NPN type germanium material C: PNP type silicon material D: NPN type silicon material The third part indicates: photoelectric tube K: switch tube X: low-frequency low-power tube G: high-frequency low-power tube D: low-frequency high-power tube A: high-frequency high-power tube. In addition, the 3DJ type is a field effect transistor, and the BT-prefixed type indicates a special semiconductor component.

The most basic function of a transistor is amplification. It can convert a weak electrical signal into a signal of a certain strength. Of course, this conversion still follows the law of conservation of energy. It just converts the energy of the power supply into the energy of the signal. An important parameter of a transistor is the current amplification factor β.

When a tiny current is added to the base of the transistor, a current β times the injected current can be obtained at the collector, which is the collector current. The collector current changes with the base current, and a small change in the base current can cause a large change in the collector current. This is the amplification effect of the transistor.

Transistors can also be used as electronic switches, and can be used in conjunction with other components to form oscillators.

In addition to being used as amplifiers and switching elements, semiconductor transistors can also be made into some two-terminal or three-terminal devices that can be used independently.

1. Expand flow.

By combining a low-power thyristor with a high-power transistor, a high-power thyristor can be obtained. Its maximum output current is determined by the characteristics of the high-power transistor, see Figure 1. Figure 2 is a capacitor capacity expansion circuit. The current amplification effect of the transistor is used to expand the capacitance several times. This equivalent capacitor can work in a floating state like a general capacitor, and is suitable for use as a timing capacitor in a long delay circuit. Although the voltage-stabilizing circuit composed of a voltage-stabilizing diode has the advantages of simplicity, few components, and economical and convenient production, the voltage-stabilizing diode has a stable current of only tens of milliamperes, which determines that it can only be used in occasions where the load current is not too large. Figure 3 can greatly expand the stable current and dynamic resistance range of the original voltage-stabilizing diode, and the stability performance can be greatly improved.

2. Substitution.

The two transistors in Figure 4 can be directly connected in series to replace the bidirectional trigger diode in the dimming desk lamp; the transistor in Figure 5 can replace a voltage regulator of about 8V. The transistor in Figure 6 can replace a voltage regulator of about 30V. In the above applications, the base of the transistor is not used.

3. Simulation.

The circuit made of transistors can also simulate other components. High-power variable resistors are expensive and hard to find. The circuit in Figure 7 can be used as a simulation. By adjusting the resistance of the 510 resistor, the impedance between the C and E electrodes of the transistor can be adjusted. This impedance change can replace the variable resistor. Figure 8 is a voltage regulator simulated by a transistor. Its voltage regulation principle is: when the input voltage applied to both ends of A and B rises, because the voltage drop of the B and E junctions of the transistor remains basically unchanged, the voltage drop across R2 rises, the current passing through R2 rises, the forward bias of the emitter junction of the transistor is enhanced, and its conductivity is also enhanced. The equivalent resistance between the C and E electrodes decreases, and the voltage drop is reduced, thereby reducing the input voltage at the AB end. The voltage regulation value of this simulated voltage regulator can be adjusted by adjusting R2, which is equivalent to

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