About the working state of triode - how to learn and improve maintenance skills

　　There have been many articles discussing the three states of transistors in detail, but they always focus on theoretical analysis.
　　
　　From the perspective of maintenance, the author will discuss the characteristics and judgment methods of the three states of transistors, and also introduce a method for learning the application and maintenance of electronic appliances. I hope it will be inspiring and helpful to beginners.
　　
　　1. Characteristics and judgment methods of the three states (taking NPN silicon tube as an example, please refer to the attached figure)

　　1. Cut-off state: When the be junction is reverse biased, zero biased, or shallow forward biased (referring to forward biased, the forward voltage drop is less than the threshold voltage), Ib=0. Ic=Ice≈0, the transistor is cut off, at this time the Rce internal resistance is infinite, and Vce is approximately equal to the power supply voltage.
　　
　　2. On-amplification state: When be is forward biased, bc is reverse biased; and Vbe is greater than the threshold voltage, it is the on-amplification state, at this time Vc>Vb>Ve. And Vbe=0.7V. After conduction, Rce decreases, the deeper the conduction, the smaller the Rce internal resistance, and the smaller Vce. In the amplification state, Ib can control Ic. That is, it has the function of current amplification (essentially, it controls the large with the small).
　　

　　3. Saturation state: When the conduction deepens to the point where Vce decreases to less than Vbe, it is called "saturation". At this time, be and bc are both forward biased, and Vbe is about 0.7V, Vces (saturation voltage drop) drops to the minimum, about 0.1~0.3V, Rce is minimum, Ic is maximum, Ib can no longer control k, that is, the transistor loses its current amplification ability.
　　
　　The three states of the transistor are very important. Regardless of whether it itself or the peripheral components fail, its working state will always change, or transfer from one state to another. We can completely analyze and determine which specific component is damaged by measuring the voltage of its three electrodes and comparing them with the normal state value (see the reasoning below). Beginners should pay special attention to this.
　

　　2. Learning methods

　　1. The basics of components are very important. The whole machine is composed of several unit circuits with certain functions, and the unit circuits are composed of several components. Therefore, it is necessary to master the characteristics, parameters (especially the limit parameters), applications, detection and replacement of various components. Among them, the triode and its three working states are the most complex and the most important. For example, the failure of the line tube (including the power switch tube) is more difficult for beginners. We can analyze it from its limit parameters:
　　
　　it must be that its working environment has changed beyond a certain limit parameter (Icm, BVceo and Pcm) that it can withstand, that is, it is nothing more than overcurrent, overvoltage or excessive power consumption. Then find out the reasons for overcurrent (such as short circuit between turns of the line output transformer or deflection coil), overvoltage (such as +B increase, reverse capacitance reduction, low line frequency, etc.), excessive power consumption (such as insufficient excitation), etc., and then use the simplest method to check and eliminate the faults one by one. Due to space limitations, I will not go into details here. Readers can draw inferences from one example and apply it to other similar cases.
　　
　　2. Unit circuit is the key. This is the most important thing to learn about the whole machine. You must and should know:
　　
　　(1) Function and performance requirements: Each unit circuit has different functions and performance requirements. After it is damaged or fails, it will always fail to complete its work task or fail to reach the original performance indicators. In this way, we can know what kind of faults this circuit may cause during maintenance, and we can also roughly judge which unit circuit the fault is in through the fault phenomenon. For example, we know that the function of the image amplifier is to amplify the image signal, and its performance requirements are gain, amplitude-frequency characteristics, AGC control, stability, etc. Then, when there are faults such as low image sensitivity, normal weak signal and strong signal distortion, and sound interference with the image, we will immediately think that there may be a fault in the image amplifier.
　　
　　(2) Typical circuit and working principle: On the basis of memorizing the typical circuit diagram, we must understand the working principle, including the role of each component in the circuit. For example, for an ordinary triode amplifier, the basic function is to amplify the signal without distortion. In order to solve the problem of no distortion, a reasonable and stable static operating point must be set. Therefore, the role of the bias resistor is to ensure that the input signal is present or not, and the strength changes. All of them make the transistors in the normal amplification state and cannot enter the cutoff or saturation state. The capacitor is used for signal coupling, bypass (eliminating AC negative feedback and improving circuit gain), vibration prevention, frequency compensation, etc.
　　
　　(3) Simulation hypothesis practice method: After being familiar with, understanding and mastering the above content, you can conduct simulation tests, that is, compare the circuit diagram and first imagine: what kind of fault may occur after any component is damaged (of course, the corresponding fault phenomenon is different when the degree of damage is different.
　　
　　At this time, what changes will occur in the voltage and current or the circuit resistance value in the circuit (such as the transistor)? What method can be used to find out which component caused it, etc., and then verify it in practice, find out the deficiencies and summarize experience.
　　
　　For example, in the amplifier circuit in the attached figure, it can be analyzed and inferred that: Rb1 and Rb2 determine Vb and Vbe; Vbe determines Ib; Ib determines Ic; Ic determines VRc, Vc, Ve and Vce, etc.
　　
　　When Rb1 is open, there is no voltage on the base of the tube, the tube is cut off, Ic is 0, and Vce is approximately equal to Vcc. Then it can be deduced that when Vb=0 is measured, Rbl must be open. For example, under normal circumstances, Vb=3V, Ve=2.3V, Vc=8V, and the tube is in the amplification state. When the actual measurement is Vb=1.5V, Ve=0.8V, and Vc=3V, it can be inferred that Vc is low because Ic is large. Ic is large because Ib is large, and Ib is large because Vbe is large. The reason for Vbe to be large can only be that Vb increases or Ve decreases, but at this time Vb is low, which means that it can only be that Ve decreases, causing Vbe to increase, the tube conduction to deepen, Vc decreases. Therefore, we know that the real cause of the fault can only be Ce leakage. Why not check Rb1, Rb2, Rc, and Re? Because when Rb1 increases, although Vb can be reduced, the conduction of the tube will be weakened, and Vc will increase, so it will not be Rb1; when Rc increases, Vc will of course decrease, but it will not make Vb and Ve also low. So it will not be Rc; when Rbl decreases, Vb will be low, but Vc will increase. So it will not be Rbl;; when Re decreases, the above situation will also occur, but the resistance generally will not decrease, so there is no need to check Rbl and Re. The reason for not checking the tube is that no matter it breaks down, leaks or opens, the voltage will not change.
　　
　　3. Memorize the block diagram You must memorize the block diagram and the location of each unit circuit, the connection between each unit circuit and the mutual influence. First analyze the DC power supply, then analyze the signal flow, and finally analyze various functional operations and controls. Because the DC operating point is a means, the processing and transmission of AC signals is the purpose.
　　
　　(1) Clarify the power supply method of each unit circuit, because for each circuit to work properly, it must first have a suitable working voltage and current (including stable voltage, sufficient power, small ripple internal resistance, and necessary protection).
　　
　　(2) Clarify the signal flow (including the characteristics of each level of signal, waveform, frequency, coupling method, etc.) and the mutual influence between unit circuits. (3
　　
　　) Be familiar with various functional operations and controls. Such as analog control, signal switching, state conversion, etc. in television.