Four points of detailed description of field effect tube application

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Four points of detailed description of field effect tube application [Copy link]

　　We often come into contact with crystal triodes and are familiar with their use. Relatively speaking, we are a little unfamiliar with crystal field effect tubes. However, due to the unique advantages of field effect tubes, such as high input impedance, low noise, and good thermal stability, they are also common in our use. We know that there are many types of field effect transistors, which are divided into junction field effect transistors and insulated gate field effect transistors according to their structures; insulated gate field effect transistors are also called metal oxide conductor field effect transistors, or MOS field effect transistors for short.

　　1. How to prevent the breakdown of insulated gate field effect transistors

　　Since the input impedance of the insulated gate field effect tube is very high, this is originally its advantage, but it brings new problems in use. Due to the high input impedance, once a charged object approaches the gate, the charge induced in the gate is difficult to discharge through this resistor. The accumulation of charge causes the voltage to rise, especially when the inter-electrode capacitance is relatively small. A small amount of charge will produce a higher voltage, so that the tube has been broken down or the index has dropped before it is used or during welding. Especially for MOS tubes, the insulation layer is very thin and it is more prone to breakdown and damage. In order to avoid such accidents, the key is to avoid the gate from being suspended, that is, a DC path must be maintained between the gate and the source. Usually, a resistor (within 100K) is connected between the gate and the source to prevent the accumulated charge from being too much, or a voltage regulator is connected to prevent the voltage from exceeding a certain value. When storing, the three electrodes should be short-circuited and placed in a shielded metal box; when soldering the tube to the circuit or removing it, the electrodes should also be short-circuited first; the soldering iron instruments used during installation and testing should have a good grounding, and it is best to unplug the power supply of the soldering iron before soldering.

　　2. How to determine the electrode of the junction field effect transistor

　　Set the multimeter to RX1K, touch the assumed gate G pin with the black test lead, and then touch the other two pins with the red test lead. If the resistance is relatively small (about 5~10 ohms), swap the red and black test leads and measure again. If the resistance is large (infinite), it means that they are all reverse resistance (PN junction reverse), which belongs to N-channel tube, and the tube touched by the black test lead is the gate G, and it means that the original assumption is correct. The resistance measured again is very small, indicating that it is a forward resistance, which belongs to the P-channel field effect tube, and the black test lead is also touching the gate G. If the above situation does not occur, you can swap the red and black test leads and test according to the above method until the gate is determined. Generally, the source and drain of the junction effect tube are symmetrical during manufacturing. Therefore, after the gate G is determined, it is not necessary to judge the source S and drain D, because these two poles can be used interchangeably, so there is no need to judge. The resistance between the source and the drain is about several thousand ohms.

　　3. Estimation of field effect tube amplification capability

　　The RX100 gear of the multimeter can be used to estimate the amplification capacity of the field effect tube. The specific test is as follows: the red test pen is connected to the source S, and the black test pen is connected to the drain D. This is equivalent to adding a 1.5 volt power supply voltage to the field effect tube. At this time, the needle indicates the resistance value between the DS electrodes. Then pinch the gate G with your fingers and add the induced voltage of the human body as the input signal to the gate. Due to the amplification of the field effect tube, Uds and Id will change, which is equivalent to the change of the resistance between the DS electrodes. It can be observed that the needle has a large swing. If the needle swings very little when the gate is pinched, it means that the amplification capacity of the field effect tube is weak. If the needle does not move, it means that the field effect tube is damaged.

　　Note that the Rds of most field effect tubes increases, and the needle swings to the left. The Rds of a few field effect tubes decrease, and the needle swings to the right. But no matter what the direction of the needle swings, as long as it can swing obviously, it means that the tube has the ability to amplify. However, since the input resistance of the MOS tube is higher, the induced voltage allowed by the gate should not be too high, so you cannot pinch the gate directly with your hands. You must hold the insulated handle of the screwdriver with your hand and touch the gate with a metal rod to prevent the induced charge from being directly added to the gate, causing the gate of the MOS tube to break down.

　　4. Example (summarizing the role of MOS gate resistance in analog circuits)

　　1. It is the pressure division effect

　　2. The pull-down resistor is to discharge the gate charge as quickly as possible to cut off the MOS tube as soon as possible

　　3. Prevent surge overvoltage on the gate (the voltage regulator connected in parallel on the gate also prevents overvoltage)

　　4. The gate resistor of the full bridge also has the same mechanism, which is to discharge the gate charge as soon as possible and turn off the MOS tube as soon as possible. Avoid the gate being suspended, because the suspended gate MOS tube will be turned on, causing the full bridge to short-circuit.

　　5. The resistor between the driver tube and the gate serves to isolate and prevent parasitic oscillation

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This is mentioned in general books.