The MOS tube is a metal-oxide-semiconductor field effect transistor, or metal-insulator-semiconductor. The source and drain of the MOS tube can be reversed, and they are all N-type regions formed in the P-type backgate. In most cases, these two regions are the same, even if the two ends are reversed, it will not affect the performance of the device. Such devices are considered symmetrical. The bipolar transistor amplifies the small change of the current at the input end and outputs a large current change at the output end. The gain of a bipolar transistor is defined as the ratio of output to input current. Another type of transistor, called a FET, converts changes in input voltage into changes in output current.
Use a pointer-type multimeter to discriminate the MOS tube, as follows.
(1) Distinguish the electrode of the junction field effect tube by the method of measuring resistance
According to the phenomenon that the forward and reverse resistance values of the PN junction of the FET are different, the three electrodes of the junction FET can be identified.
Specific method: Put the multimeter on the R×1k gear, choose two electrodes, and measure the positive and negative resistance values respectively. When the forward and reverse resistance values of two electrodes are equal and are several thousand ohms, the two electrodes are the drain electrode D and the source electrode S, respectively. Because for the junction field effect transistor, the drain and source electrodes are interchangeable, and the remaining electrode must be the gate G.
You can also touch the black test lead (red test lead) of the multimeter to one electrode at will, and the other test lead to contact the other two electrodes in turn to measure the resistance value. When the resistance values measured twice are approximately equal, the electrode contacted by the black test lead is the gate electrode, and the other two electrodes are the drain electrode and the source electrode respectively. If the resistance values measured twice are large, it means that it is the reverse of the PN junction, that is, all reverse resistances. It can be determined that it is an N-channel field effect transistor, and the black test lead is connected to the gate; The resistance values of all are very small, indicating that it is a forward PN junction, that is, a forward resistance, which is determined to be a P-channel field effect transistor, and the black test lead is also connected to the gate. If the above situation does not occur, you can replace the black and red test leads and test according to the above method until the grid is identified.
(2) Use the resistance measuring method to judge the quality of the FET
The resistance measurement method to detect the MOS tube is to use a multimeter to measure the resistance value between the source and drain, gate and source, gate and drain, gate G1 and gate G2 of the FET, as indicated in the FET manual. Whether the resistance value is consistent to judge the quality of the tube.
Specific method: first put the multimeter in the R×10 or R×100 gear, measure the resistance between the source S and the drain D, usually in the range of tens of ohms to several thousand ohms (it is known in the manual that various models If the measured resistance value is greater than the normal value, it may be due to poor internal contact; if the measured resistance value is infinite, it may be an internal pole break. Then put the multimeter in the R×10k gear, and then measure the resistance values between the gates G1 and G2, between the gate and the source, and between the gate and the drain. When the measured resistance values are all infinite, then It means that the tube is normal; if the above resistance values are too small or are channels, it means that the tube is bad. It should be noted that if the two grids are disconnected in the tube, the component replacement method can be used for detection.
(3) Use the induction signal input method to estimate the amplification ability of the FET
Specific method: use the R×100 gear of the resistance of the multimeter, the red test lead is connected to the source S, the black test lead is connected to the drain D, and the power supply voltage of 1.5V is applied to the field effect tube. At this time, the resistance between the drain and source indicated by the needle value. Then pinch the gate G of the junction field effect transistor by hand, and add the induced voltage signal of the human body to the gate. In this way, due to the amplification effect of the tube, both the drain-source voltage VDS and the drain current Ib will change, that is, the resistance between the drain-source electrodes will change, and it can be observed that the needle has a relatively large swing. If the hand pinching the grid needle swings less, it means that the tube has poor amplifying ability; the needle swings more, indicating that the tube has a large amplifying ability; if the needle does not move, it means that the tube is bad.
According to the above method, we use the R×100 file of the multimeter to measure the junction field effect transistor 3DJ2F. Open the G pole of the tube first, and measure the drain-source resistance RDS to be 600Ω. After pinching the G pole with your hand, the needle swings to the left, the indicated resistance RDS is 12kΩ, and the swing of the needle is larger, indicating that the tube is good , and has a larger magnification capacity.
(4) Use the resistance measuring method to identify the FET without mark
First, to detect the MOS tube by measuring the resistance, two pins with resistance values must be found, that is, the source S and the drain D, and the remaining two pins are the first gate G1 and the second gate G2. Write down the resistance value between the source S and the drain D measured by the two test pens first, and measure the resistance value by switching the test pens again, and write down the measured resistance value. The connected electrode is the drain D; the red test lead is connected to the source S.
The S and D poles identified by this method can also be verified by estimating the amplification ability of the tube, that is, the black test pen with large amplification ability is connected to the D pole; the red test pen is grounded to the 8-level pole. The results of both methods should be the same. When the positions of drain D and source S are determined, install the circuit according to the corresponding positions of D and S. Generally, G1 and G2 will also be aligned in sequence, which determines the positions of the two gates G1 and G2. Thus, the order of D, S, G1, and G2 pins is determined.
(5) Judging the size of the transconductance by measuring the change of the reverse resistance value
When measuring the transconductance performance of the VMOSN channel enhancement mode FET, the red test lead can be connected to the source S and the black test lead to the drain D, which is equivalent to adding a reverse voltage between the source and the drain. At this time, the grid is open, and the reverse resistance value of the tube is very unstable. Select the ohm gear of the multimeter in the high resistance gear of R×10kΩ, and the voltage in the meter is higher at this time. When the grid G is touched by hand, it will be found that the reverse resistance value of the tube changes significantly. The greater the change, the higher the transconductance value of the tube; if the transconductance of the tested tube is small, use this method to measure , the reverse resistance does not change much.
(6) The pin is used to measure the quality of the MOS tube
① Determination of grid G: Use the multimeter R&TImes; 100 gear to measure the forward and reverse resistance between any two feet. If the resistance measured in one of them is hundreds of Ω, then the two feet are D and S, and the third foot is G .
②Drain D, source S and type determination: use the multimeter R&TImes; 10kΩ to measure the forward and reverse resistances of D and S, the forward resistance is about 0.2&TImes; 10kΩ, and the reverse resistance (5-∞) X100kΩ. When measuring the reverse resistance, the red test lead does not move. After the black test lead is separated from the pin, touch it with G, and then go back and connect the original pin. There are two situations:
a. If the reading changes from the original larger value to 0, the red test lead is connected to S, and the black test lead is D. It is effective to contact G with a black test lead, so that the forward and reverse resistance values between the MOS tubes D and S are both 0Ω, which can also prove that the tube is an N-channel.
b. If the reading is still a large value, the black test lead does not move, use the red test lead to touch G, and immediately return to the original pin after touching it. If the reading is 0Ω at this time, the black test lead is connected to the S pole, and the red test lead is D pole, it is effective to contact the G pole with a red test lead, the MOS tube is a P channel.
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