What is the amplification principle of FGA25N 120?

The amplification principle of FGA25N 120 is as follows:

First, pointer multimeter is used to identify FET.

(1) Distinguish the electrodes of the junction field effect transistor by measuring the resistance.

According to the phenomenon that the forward and reverse resistance values of PN junction of field effect transistor are different, the three electrodes of junction field effect transistor can be distinguished. Specific methods: set the multimeter at R× 1k, select two electrodes, and measure the positive and negative resistance values respectively. When the positive and negative resistance values of the two electrodes are equal and several thousand ohms, the two electrodes are drain D and source S, respectively.

Because the drain and source of the junction field effect transistor are interchangeable, the remaining electrode must be the grid G. You can also touch the black stylus (or the red stylus) of the multimeter to one electrode at will, and the other stylus touches the other two electrodes in turn to measure its resistance. When the resistance values measured twice are almost equal, the electrode contacted by the black stylus is the gate, and the other two electrodes are the drain and the source respectively.

If the resistance values measured twice are very large, it means that the PN junction is reversed, that is, both resistances are reversed, and it can be judged that it is an N-channel field effect transistor, and the black contact pin is connected to the gate; If the resistance measured twice is very small, it means that it is a forward PN junction, that is, a forward resistance, and it is judged as a P-channel field effect transistor, and the black probe is also connected to the gate. If the above situation does not occur, the black and red probes can be replaced and tested according to the above method until the grid is identified.

(2) Judge the quality of FET by measuring the resistance.

The resistance measurement method is to measure the resistance values between the source and the drain, the gate and the source, the gate and the drain, and the gate G 1 and the gate G2 of the FET with a multimeter to judge whether the resistance values indicated in the FET specification are consistent. Specific methods: First, put the multimeter in R× 10 or R× 100, and measure the resistance between the source electrode S and the drain electrode D, which is generally in the range of tens of ohms to thousands of ohms (the manual knows that the resistance values of different types of lamps are different). If the measured resistance value is greater than the normal value, it may be a poor internal contact; If the measured resistance is infinite, it may be that the internal magnetic pole is broken.

Then put the multimeter on R× 10k, and then measure the resistance values between grid G 1 and G2, between grid and source, and between grid and drain. When all resistance values are infinite, the electron tube is normal. If the above resistance measurement value is too small or channel, the tube is bad. It should be noted that if there are two broken grids in the pipe, the element substitution method can be used for detection.

(3) Estimating the amplification ability of the field effect transistor by the method of induction signal input.

Specific methods: Use a multimeter resistor of R× 100, connect the red probe to the source S, and the black probe to the drain D, and add a power supply voltage of 1.5V to the FET. At this time, the resistance value between the drain and the source indicated by the stylus. Then pinch the gate G of the junction field effect transistor by hand, and apply the induced voltage signal of human body to the gate. In this way, due to the amplification of the electron tube, both the drain-source voltage VDS and the drain current Ib will change, that is, the resistance between the drain and the source will change, from which it can be observed that the pointer has a large swing. If the hand-pinched grid needle swings slightly, it shows that the amplification ability of the electron tube is poor; The swing of the hand is large, indicating that the amplification ability of the electron tube is large; If the hand does not move, it means that the pipe is broken.

According to the above method, we measured the junction field effect transistor 3DJ2F with a multimeter with the range of R× 100. First, turn on the G pole of the lamp, and measure the drain-source resistance RDS as 600 Ω. After holding the G pole with your hand, swing your hand to the left, indicating that the resistance RDS is12kΩ, indicating that the electron tube is good and has great amplification ability.

When using this method, there are several points to note:

First, pinch the gate of the field effect transistor by hand, and the multimeter pointer may swing to the right (resistance decreases) or to the left (resistance increases). This is because the AC voltage induced by human body is relatively high, and the working point of different field effect transistors may be different (either working in saturated region or unsaturated region) as measured by resistance file. The test shows that the RDS of most transistors increases, that is, both hands swing to the left; The RDS of a few tubes is lowered, making the hand swing to the right. However, no matter what the swing direction of the watch hand is, as long as the swing range of the watch hand is large, it shows that the electron tube has a greater amplification ability.

Secondly, this method is also applicable to MOS field effect transistors. However, it should be noted that the input resistance of MOS FET is high, and the allowable induced voltage of gate G should not be too high. So don't pinch the gate directly with your hands. You must use it to hold the insulating handle of the screwdriver and touch the grid with a metal bar to prevent the artificially induced charge from being directly applied to the grid, resulting in grid breakdown. Third, there should be a short circuit between G-S poles after each measurement. This is because the G-S junction capacitance will carry a small amount of charge, and the VGS voltage will be established, which may cause the watch hand to be immobile when measuring again and only discharge the short circuit between G-S electrodes.

(4) The unmarked field effect transistor is judged by measuring the resistance.

Firstly, two pins with resistance are found by measuring the resistance, namely the source S and the drain D, and the remaining two pins are the first grid G 1 and the second grid G2. First, write down the resistance value between the source electrode S and the drain electrode D measured by the two probes, then adjust the probes and measure again, and write down the measured resistance value. The electrode connected with the black probe is a drain electrode D; The red stylus is connected to the source S. The S and D poles identified by this method can also be photographed by estimating the amplification principle of FGA25N 120.