Field Effect Transistor (MOS) Notes

(1) In order to use the field effect transistor safely, the dissipated power of the tube, the maximum drain-source voltage, the maximum gate-source voltage and the maximum current cannot be exceeded in the design of the circuit. limit value.

(2) When all types of FETs are used, they must be connected to the circuit according to the required bias, and the polarity of the FET bias must be observed. For example, the junction field effect transistor is a PN junction between the gate, source and drain, and the gate of the N-channel transistor cannot be positively biased; the gate of the P-channel transistor cannot be negatively biased, and so on.

(3) Due to the extremely high input impedance of MOS field effect transistors, the lead pins must be short-circuited during transportation and storage, and metal shielding should be used to prevent external induced potential gate breakdown.

(4) In order to prevent the induction breakdown of the FET grid, it is required that all test instruments, workbenches, soldering irons, and circuits must be well grounded; When soldering, solder the source first; before connecting to the circuit, all lead ends of the tube are kept short-circuited to each other, and the short-circuit material is removed after welding; when the tube is removed from the component rack, it should be in an appropriate way Ensure that the human body is grounded such as using a grounding ring; the above safety measures must be paid attention to when using FETs.

(5) When installing the field effect tube, pay attention to the installation position to avoid being close to the heating element as much as possible; in order to prevent the vibration of the tube fitting, it is necessary to fasten the tube shell; When the pin lead is bent, it should be carried out at a point 5 mm larger than the size of the root to prevent the pin from being bent and cause air leakage.

(6) When using VMOS tube, a suitable heat sink must be added. Taking VNF306 as an example, the maximum power can only reach 30W after the tube is installed with a radiator of 140×140×4 (mm).

(7) After multiple tubes are connected in parallel, due to the corresponding increase in inter-electrode capacitance and distributed capacitance, the high-frequency characteristics of the amplifier are deteriorated, and it is easy to cause high-frequency parasitic oscillation of the amplifier through feedback. . For this reason, there are generally no more than 4 parallel composite tubes, and an anti-parasitic oscillation resistor is connected in series on the base or grid of each tube.

(8) The gate-source voltage of the junction field effect transistor cannot be reversed, and can be saved in an open state, while the insulated gate field effect transistor is not in use, due to its The input resistance of the tube is very high, and each electrode must be short-circuited to avoid damage to the tube due to the action of the external electric field.

(9) When soldering, the shell of the soldering iron must be equipped with an external ground wire to prevent damage to the tube due to the electrification of the soldering iron. For a small amount of soldering, you can also heat the soldering iron and unplug it or cut off the power supply before soldering. Especially when welding insulated gate field effect transistors, it should be welded in the order of source-drain-gate, and the welding should be powered off.