What are the basic usages of triodes for analog circuit knowledge?

      Transistors are the most common basic knowledge of analog circuits. Transistors are widely used semiconductor components in circuit design. 

      The triode is divided into two types: NPN and PNP. The triode is composed of two PN junctions. The NPN triode shares one P region (also called the base region) for the two PN junctions. In the construction process, the base region is made very thin, generally several micrometers to several tens of micrometers, and the two PN junctions are closely combined by the base region, interacting and correlating each other, so that the characteristics of the triode are not at all two separate PN junctions. The superposition of characteristics, the relationship between the base current, the collector current and the emitter current after the external voltage is applied to the triode, forms a current amplification effect. 

      The amplification factor of the triode is related to its physical structure. When selecting the triode, the amplification factor β is one of the important technical indicators that must be considered. The current direction of the triode and the current relationship between the three poles are as follows, IC=βIB, IE=IB+IC 

      The triode has three working intervals of saturation, amplification and cutoff. Some common usages are described below; 

      (1) Drive amplification, general IO port drive current of single-chip microcomputer, DSP, ARM, CPLD/FPGA and other CPU is relatively weak, can not directly drive the load, the most common is to use the triode for current drive capability amplification, as shown in the following figure; 

      The schematic principle drives the LED lamp, which can be converted to other loads according to actual needs, such as relays, motors, etc.; +5V power supply can also be changed, such as 12V/24V, etc., but the selected transistor must be able to withstand this voltage. 

      (2)When the electronic switch is used, when the electronic switch is used, the saturation region of the triode is generally used. By controlling the base to make the triode in a saturated or cut-off region, the triode is turned on or off, and functions as a switch. 

      (3) The logic is inverted. As shown in the figure below, it is very convenient to use the NPN transistor for inversion. When the input is high, the output is low. When the input is low, the output is high (5V). 

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