Warm hints: The word in this article is about 986 and reading time is about 6 minutes. Guidance: We often come into contact with crystal three-stage transistors, and are familiar with their use. They are relatively unfamiliar to crystal field effect transistors. However, because of their unique advantages, such as high input impedance, low noise and good thermal stability, they are also common in our use. We know that there are many kinds of field effect transistors, which can be divided into junction field effect transistors and insulated gate field effect transistors according to their different structures. Insulated gate field effect transistors are also called metal oxide conductor field effect transistors, or MOS field effect transistors for short.
How to Prevent Breakdown of Insulated Gate FET
Because the input impedance of insulated gate FET is very high, this is its advantage, but it brings new problems in use. Because of the high input impedance, once the charged object is close to the gate, it is difficult to discharge the charges induced by the gate through the resistance. The accumulation of charges results in an increase in the voltage, especially when the capacitance between the electrodes is small, a small amount of charges will produce a higher voltage, so that the tube is not used. Or when welding, it has already broken down or the index drops down. Especially for MOS tube, its insulating layer is very thin and it is easier to break down and damage. In order to avoid such an accident, the key is to avoid the suspension of the grid, that is, the DC path must be maintained between the two poles of the grid source. Usually, a resistor (less than 100K) is connected between the two poles of the gate source to keep the accumulated charge from excessive, or a voltage regulator is connected to keep the voltage from exceeding a certain value. The three electrodes should be short-circuited and placed in the shielded metal box during storage; the electrodes should be short-circuited first when the pipe is welded to the circuit or taken off; the soldering iron instruments used in installation and testing should be well grounded, and it is better to unplug the power supply of the soldering iron before welding.
How to Judge the Electrode of JFET
Place the multimeter in RX1K block, contact the assumed gate G pin with a black pen, and then contact the other two pins with a red pen. If the resistance values are relatively small (about 5-10 Euros), then exchange the red and black pens and measure them again. If the resistance values are large (infinite), it means that they are all reverse resistance (PN junction reverse), belonging to the N channel tube, and the black meter. The tube contacted by the pen is gate G, and the original assumption is correct. The resistance values measured again are very small, indicating that the forward resistance belongs to the P-channel field effect transistor, and the black pen is also in contact with the gate G. If this does not happen, the red and black pen can be changed and tested according to the above method until the gate is judged. Generally, the source and drain poles of junction effect transistors are symmetrical in manufacture, so when the gate G is determined, it is not necessary to judge the source S and drain D, because the two poles can be used interchangeably, so it is not necessary to distinguish them. The resistance between source and drain is about several thousand ohms.
Estimation of amplification capability of field effect transistors
The amplification capability of FETs can be estimated by using the RX100 gear of the multimeter. Specific tests are as follows: red pen connects source S, black pen connects drain D, which is equivalent to adding 1.5 volt power supply voltage to FET, when the pin indicates the resistance between D-S poles. Then pinch the grid G with your finger and add the human body’s induced voltage to the grid as an input signal. Because of the amplification effect of FET, both Uds and Id will change, which is equivalent to the change of D-S interelectrode resistance. It can be observed that the needle oscillates considerably. If the hand-pinched grid gauge needle oscillates very little, it means that the amplification ability of FET is weak, and if the gauge needle does not move, it means that FET has been damaged.
Notice that the Rds of most FETs increase, the needle swings to the left, the Rds of a few FETs decrease, and the needle swings to the right. However, no matter the direction of the needle swing, as long as the needle can swing clearly, it shows that the tube has the ability to magnify. However, due to the higher input resistance of the MOS transistor and the acceptable inductive voltage of the gate, it is impossible to pinch the gate directly by hand. The insulating handle of the screwdriver must be grasped by hand and the gate must be touched by a metal rod in order to prevent the inductive charge from being directly added to the gate and causing the gate breakdown of the MOS transistor.
Examples (Summarizing the role of MOS gate resistance in analog circuits)
- Partial pressure
- Pull-down resistance is to discharge gate charge as soon as possible and cut off MOS transistor as soon as possible.
- Prevent grid surge and overvoltage (grid parallel regulators also prevent overvoltage)
- The full bridge gate resistance is the same mechanism. The gate charge can be released as soon as possible and the MOS transistor can be cut off as soon as possible. To avoid gate suspension, the suspended gate MOS transistor will turn on, resulting in short circuit of the whole bridge.
- The resistance between the drive tube and the gate acts as isolation and prevents parasitic oscillation. 报错
- What is Field Effect Transistor
- Application of Field Effect Transistor in Switching Circuit
- Working Principle of Power Mosfet
- Basic Knowledge of Operational Amplifiers
- What is Organic Field Effect Transistor and its Application
- Working Principle of Organic Field Effect Transistor