Testing of Field Effect Transistors
Testing of Field Effect Transistors:
Pin identification of 1 junction field-effect transistor:
The gate of a field-effect transistor is equivalent to the base of the transistor, and the source and drain correspond to the emitter and collector of the transistor, respectively. Place the multimeter in the R × 1k range and use two probes to measure the forward and reverse resistance between each two pins. When the forward and reverse resistances between two pins are equal, both in the order of K Ω, these two pins are the drain D and source S (interchangeable), and the remaining pin is the gate G. For a junction field-effect transistor with four pins, the other pole is the shield (grounded during use).
2 fixed gate
Touch one electrode of the tube with the black probe of the multimeter, and touch the other two electrodes with the red probe. If the resistance values measured twice are both very small, it indicates that they are both forward resistors. This transistor belongs to the N-channel field-effect transistor, and the black probe is also connected to the gate.
The manufacturing process determines that the source and drain of the field-effect transistor are symmetrical, interchangeable, and do not affect the normal operation of the circuit, so there is no need to distinguish them. The resistance between the source and drain is about several thousand ohms.
Note that this method cannot be used to determine the gate of an insulated gate field-effect transistor. Because the input resistance of this type of tube is extremely high and the inter electrode capacitance between the gate and source is very small, a small amount of charge can form a high voltage on the inter electrode capacitance during measurement, which can easily damage the tube.
3. Estimate the amplification capability of field-effect transistors
Set the multimeter to R × 100 mode, connect the red probe to the source S and the black probe to the drain D, which is equivalent to applying a power supply voltage of 1.5V to the field-effect transistor. At this point, the needle indicates the resistance value between the D-S poles. Then pinch the grid G with your fingers and apply the induced voltage of the human body as the input signal to the grid. Due to the amplification effect of the tube, both UDS and ID will change, which is equivalent to a change in the resistance between the D-S poles, and a significant swing of the meter needle can be observed. If the pointer swings very little when pinching the grid, it indicates that the amplification ability of the tube is weak; If the needle does not move, it indicates that the tube has been damaged.
Due to the high 50Hz AC voltage induced by the human body, the working point of different field-effect transistors may be different when measured in resistance mode. Therefore, when pinching the grid by hand, the pointer may swing to the right or left. A few tubes have reduced RDS, causing the pointer to swing to the right, while most tubes have increased RDS, causing the pointer to swing to the left. Regardless of the direction of the pointer's swing, as long as there is a noticeable swing, it indicates that the tube has the ability to amplify.
This method is also applicable for measuring MOS transistors. In order to protect the MOSFET, it is necessary to hold the insulating handle of the screwdriver by hand and touch the gate with a metal rod to prevent the direct application of induced charges to the gate by the human body, which may damage the transistor.
After each measurement of the MOS transistor, a small amount of charge will be charged on the G-S junction capacitor, establishing a voltage UGS. When measuring again, the probe may not move. At this time, short circuit the G-S electrode.
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