The structure, working principle and working state of the triode

Category:Brake system foundation - Date:2015-11-09

transistor is an important part of electronic circuit. If you are an electronic technician, you don't need to learn deeply about transistor. As a vehicle maintenance technician, it is necessary to understand the basic painting method, principle and function of the triode, although it does not need to carry out a comprehensive study and practice of the triode. In the future, in the automotive circuit and ECU, the triode can meet the requirements. It can analyze the role and the meaning of the transistor. This station collated the function, structure, principle, drawing and usage of triode, which has important guiding significance for vehicle maintenance.

from diode to transistor: the structure of triode.

when learning diodes, we know that intrinsic semiconductors form P and N semiconductors because they are mixed with impurities; a P type semiconductor joins a N type semiconductor to form a PN junction on the interface, and a PN junction constitutes a diode.

the PN junction consisting of.P semiconductor and N semiconductor is a diode.

the triode is composed of three P or N type semiconductors, and the common PNP triode (i.e., N type semiconductor in the middle) and the NPN triode (i.e., P type semiconductors in the middle);

the triode is composed of three regions (base area, collector area and launching area) and the corresponding three pins (base, collector and emitter), which are divided into PNP triode and NPN triode based on the material of these three regions. First, the NPN triode is described as:

structure and pin diagram of.NPN triode

B (B) represents the base, C (C) represents the collector, e (E) represents the emitter, the PN between the base area and the emission region is called the emitter junction, and the PN junction between the base area and the collector region is called the collector junction.

structure and pin diagram of.PNP triode

two. The working principle and state of the transistor is illustrated by NPN transistor.

the role of the transistor can be described as: using the amplification effect of the transistor, using the small current of the base to control the large current of the collector. Who provides the base current? The adjustable power supply VBB and the adjustable resistance RB in the injection diagram.

it is often mentioned that there are three working states of the triode: cut-off, amplification and saturation. How to determine the working state of triode?

the first question is: how to calculate the base current?

in fact, the most basic principle of ohm is: the current is equal to the voltage divided by the resistance. The base current (iB = VBB0.6) /RB. Here, why to use voltage (VBB0.6)? Because the base B of the triode and the emitter E itself is a PN junction, that is, a diode, and if it is for the silicon tube, this 0.6V is the conduction voltage drop between the PN junction. So the voltage resistance on RB (VBB0.6 only). The base current (iB = VBB0.6) /RB.

Second questions: how to tell if the transistor is cut off?

return to the role of the triode: to control the large current of the collector by using the small current of the base. If the triode does not have a base current, there is no collector current naturally. We say the triode is in cut-off state, so the triode is in cut-off state when the base voltage of the input circuit is less than the dead zone voltage of the PN junction, and the VBB is less than the dead zone voltage of the PN junction.

Third question: if there is a base current, how do we calculate the collector current? Is the transistor enlarged or saturated? This problem has been bothering me for many years, and I once suspected Ohm's law was not here. It's a great mistake to come here now.

First, assuming that VBB is 1.6V, RB is 100 ohm, the voltage of PN junction between the base of the triode and the emitter is reduced to 0.6V, then the base current is (1.6V-0.6V) /100, the base current iB=0.01A; the magnification of the triode is β = 100; then the collector current is equal to the fundamental current multiplied by the magnification; and the collector current and electricity at this time. The pressure has nothing to do with the resistance!

why is it saturated? According to the above mentioned, the collector current is equal to the base current multiplied by the amplification factor, and the collector current has nothing to do with voltage and resistance. But one problem is whether the maximum current of collector current will be large, and whether it will increase exponentially with the increase of base current.

It's impossible! Because the maximum collector current is the voltage / current, when the maximum current is reached, the collector current will not increase even if the base current is increased again. At this point, the triode does not conform to its amplification, and we call this state a saturated conduction state.

so there are three states when working on triode.

(1) cut-off state: if there is no basic current, we can cut off the basic circuit, or make the base voltage lower than the dead zone voltage of the PN junction, without the base current; at this time, the collector and the emitter equivalent to a disconnected switch, known as the cut-off state;

(2) amplification state: with the base current, before the collector current is maximum, the collector current is equal to the base current multiplied by the magnification of the triode, at this point the triode is in the magnification state;

(3) saturation conduction state: after increasing the base current and the collector current reaching the maximum, the collector current is not controlled by the base current. The equivalent switch between the collector and the emitter of the transistor is called the saturated conduction state.

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