The application of triode in circuit

The application of triode in circuit
1. What is a triode
Semiconductor triode is also called "transistor" or "transistor".
Two PN junctions that can influence each other are prepared on a single crystal of semiconductor germanium or silicon to form a PNP (or NPN) structure. The middle N area (or P area) is called the base area, and the areas on both sides are called the emitter area and the collector area. Each of these three parts has an electrode lead called the base B, the emitter E and the collector C. A semiconductor electronic device that functions as amplifying, oscillating, or switching.
When the above two diodes are connected back-to-back, they are only used as an indication, otherwise, how can the amplifier function of the triode come about? In other words, the triode is not the same as the two diodes connected back to back!
2. the analysis of the role of the triode
1. Switch function
All transistors can act as switches to control the opening and closing of a circuit. When the triode is working in the on-off state, it must switch between the saturation and cut-off regions, and not in the amplification region.
Analysis: From the equivalent diagram of the NPN transistor, it can be seen that there is a diode between the BEs. Taking the silicon diode conduction voltage of 0.7V as an example, as long as VB>0.7V, the BE can be turned on.
That is, when the MCU outputs a high level, the transistor is in a saturated state. At this time, the transmitter junction and the collector junction are both positively biased. At this time, the voltage between CE is very small, which is lower than the conduction voltage of the PN junction (the silicon tube is at 0.5 Below volts, the standard is 0.2 volts), the CE is quite "short-circuited", that is, it is in an "open" state.
When the MCU outputs a low level, the triode is in the off state, and the transmitting junction and the collector junction are both reverse biased. At this time, the current between the CE poles is very small (the silicon tube is basically less than that), which is equivalent to "disconnected", showing "Off state".

Features of triode switch circuit:
① The switching speed is extremely fast, much faster than mechanical switches;
② There is no mechanical contact and no electric spark is generated;
③ The control of the switch is sensitive, and the requirements for the control signal are low;
④ When the switch is turned on, the voltage drop of the switch is larger than that of the mechanical switch, and the leakage current of the switch when it is off is larger than that of the mechanical switch;
⑤ Not suitable for direct control of high voltage and strong current
2. Amplification
The function of the amplifying circuit is to use the current control function of the triode or the voltage control function of the field effect tube to amplify the weak electric signal (referred to as the signal, which refers to the changing voltage, current, power) to the required value without distortion, and realize the The energy of the DC power supply is partially converted into an output signal that changes according to the law of the input signal and has a larger energy.
The essence of the amplifying circuit is an energy conversion device that uses smaller energy to control larger energy conversion.
Principles of amplifier circuit composition:
① There must be a DC power supply, and the power supply should be set to ensure that the triode or field effect tube is working in a linear amplification state;
② The arrangement of components must ensure the transmission of the signal, that is, ensure that the signal can be input from the input end of the amplifying circuit, and output from the output end after being amplified by the amplifying circuit;
③ The selection of component parameters should ensure that the signal can be amplified without distortion and meet the performance index requirements of the amplifying circuit. To enable the triode to amplify the signal normally, a forward voltage should be applied to the transmitting junction, and a reverse voltage should be applied to the collecting junction. Here only take the common emitter amplifier circuit of NPN triode as an example to illustrate the basic principle of the triode amplifier circuit. In the actual amplifier circuit, in addition to the common emitter connection, there are also common collector and common base connections.