What are the characteristics and precautions of Field-effect transistor?

Field-effect transistor is one of the basic modules of electronic products, and it is also the most frequently manufactured component. Field-effect transistor (fet) is the dominant component in digital and Linear integrated circuit, and it is also a common power device. Due to its high input impedance (up to l09) to lO15 Ω), low noise, wide dynamic range, and small temperature coefficient, it is a voltage control component widely used. What should be paid attention to in the characteristics and application of Field-effect transistor? If you are interested in the content that will be covered in this article, why not continue reading.

The FET has the following characteristics:

(1) Field-effect transistor is a voltage controller that adjusts ID through UGS;

(2) The input current of Field-effect transistor is very small, so its input resistance is very large.

(3) Adopting most charge carriers for conductivity, with good temperature stability;

(4) The voltage amplification coefficient of the amplification circuit composed of it is lower than that of the amplification circuit composed of triodes;

(5) Field-effect transistor has strong radiation protection ability;

(6) The noise is low because there is no Shot noise caused by the chaotic spread of moving minority carriers.

Precautions for field effect tube:

(1) In order to safely apply Field-effect transistor, the specified values of pipeline dissipation power, large leakage voltage, large grid voltage, maximum current and other parameters shall not be exceeded in the circuit design.

(2) When using various types of Field-effect transistor, the circuit shall be connected in strict accordance with the required bias, and the bias polarity of Field-effect transistor shall be observed. If there is a PN junction between the source leakage of junction type field effect pipe network, the N-channel pipe network cannot increase the positive deviation pressure; The P-ditch pipeline network cannot increase negative deviation pressure, etc.

(3) Due to the high input impedance, MOS Field-effect transistor must be short circuited at the lead pin during transportation and storage, and must be packed with metal shield to avoid external induced potential penetrating the grid. Special attention shall be paid to MOS Field-effect transistor, which cannot be put into a plastic box. It is better to store it in a metal box, but attention shall also be paid to pipeline moisture-proof.

(4) In order to avoid inductive penetration of the FET grid, it is specified that all test equipment, operating platforms, ferrochrome and lines must be well grounded; When welding pins, first weld the source electrode; Before connecting the circuit, all wire ends of the pipeline should be kept short circuited to each other, and the short circuiting material should be removed after welding; When removing the pipeline from the component rack, it should be ensured that the body is grounded in an appropriate form, such as a grounding ring; Naturally, if advanced gas thermoelectric ferrochrome can be used, it is relatively convenient to weld the on-site effect tube to ensure safety; When the power is not turned off, it is not allowed to insert or unplug the circuit. When using the above safety measures, pay attention to Field-effect transistor.

(5) When using the field effect tube, pay attention to the installation position and try to avoid being close to the heating element; To prevent pipeline vibration, the pipe shell must be tightened; When bending, the pin wire should exceed the root specification by 5mm to avoid bending the pin and air leakage.

(6) When using VMOS tubes, appropriate heat sinks must be added. Taking VNF306 as an example, pipeline installation 140 × one hundred and forty × After a 4 (mm) radiator, 30W can achieve high power.

(7) After multiple transistors are connected in parallel, due to the corresponding increase in inter pole capacitance and distributed capacitance, the high-frequency characteristics of the amplifier become worse, which can easily cause high-frequency parasitic oscillation of the amplifier based on feedback. Therefore, generally no more than 4 composite tubes are connected, and parasitic oscillation resistors are connected to the base or gate of each tube.

(8) The gate source voltage of JFET cannot be connected in reverse, and can be stored under the lead condition. However, because its input resistance is very high, insulated gate Field-effect transistor is not used, so all power levels must be short circuited to prevent damage to the pipeline caused by external electric field effects.

(9) When welding, the electric chromium iron shell must be equipped with an external grounding wire to prevent damage to the pipeline due to the electrification of the electric chromium iron. For a small amount of welding, remove the plug or turn off the power after the electric chromium iron is burned. In particular, when welding insulated grid Field-effect transistor, the grid shall be welded in the order of source - drain - and power off welding.

(10) When using a 25W soldering iron for welding, it is necessary to be fast. If using a 45W soldering iron to weld with a 75W soldering iron, use tweezers to clamp the root of the pin to help remove heat. The JFET can use the meter resistance gear to qualitatively check the pipeline quality (check the resistance value between the forward and reverse resistance of the PN junction and the leakage source). The insulated grid Field-effect transistor cannot be checked with a multimeter, but must use a detector, and remove the short circuit of each electrical level after connecting the detector. When taking out, it should be short circuited before taking out, with the focus on preventing the grid from hanging.

When used in places with high input impedance, moisture-proof measures must be taken to prevent the input resistance of Field-effect transistor from being reduced due to temperature. If four wire Field-effect transistor is used, its substrate wire shall be grounded. Ceramic encapsulated sesame tubes have photosensitive properties and should pay attention to shading applications. For power Field-effect transistor, there should be a good heat removal standard. As power Field-effect transistor is used in high load environment, enough radiators must be designed to ensure that the shell temperature does not exceed the rated value, so that the equipment can work safely and reliably for a long time.

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