Simple assessment of the quality of field-effect transistors

1. Discrimination of junction field-effect transistor electrodes

According to the different positive and negative resistance values of the PN junction in junction field-effect transistors A multimeter can be used to distinguish the three electrodes of a junction field-effect transistor. Set the multimeter to the R x 1k position, connect the negative probe of the multimeter to any electrode, and sequentially touch the other probe to the other two electrodes to measure their resistance values. If the resistance values measured twice are approximately equal, the electrode in contact with the negative probe is the gate, and the other two electrodes are the drain and source, which can be interchanged in principle. If the resistance values measured twice are both very high, it can be determined that it is an N-channel junction field-effect transistor; If the resistance values measured twice are both very small, it can be determined as a P-channel junction field-effect transistor.

2. Estimate the amplification capability of field-effect transistors

Use a multimeter for measurement, set the multimeter to R x 100, connect the positive probe to the source S, and connect the negative probe to the drain D At this point, it is equivalent to adding a 1 A power supply voltage of 5V When the multimeter indicates the resistance value between the drain and source, hold the gate of the field-effect transistor by hand The induced voltage of the human body is applied to the gate, and due to the amplification effect of the diode, the drain current ID will increase Change the resistance between the drain and source electrodes The hands of a multimeter will swing. The greater the amplitude of the pointer swing, the stronger the amplification ability of the diode; The smaller the amplitude of the pointer swing, the poorer the amplification ability of the diode; If the pointer does not swing, it indicates that the diode may be damaged.

3. Determine the quality of field-effect transistors

The quality of field-effect transistors can be easily judged using a multimeter The method is to use a multimeter to measure the resistance value between the electrodes of the field-effect transistor, and then compare it with the resistance value indicated in the manual. The specific measurement method is to place the multimeter in the Rx10 or R x 100 range, and first measure the resistance between the source S and drain D, which is generally in the range of tens of ohms to several thousand ohms (different types of field-effect transistors have different resistance values). If the measured resistance value is infinite, it may be an internal pole break; If the measured resistance value is very small, it may be due to pipe breakdown and damage; If the measured resistance value is greater than the specified value, it may be due to poor internal contact of the diode. Then place the multimeter in the R x 10k gear Measure the resistance values between the gate and source, gate and drain, and the gates G1 and G2 of the double gate transistor. When all the measured resistances are infinite, it indicates that the transistor is good; If the measured resistance is too small or is a circuit, it indicates that the diode is faulty.