1、 Qualitative assessment of the quality of field-effect transistors

First, use the multimeter R × 10k Ω (with a built-in 15V battery), connect the negative probe (black) to the gate (G), and the positive probe (red) to the source (S). Charging between the gate and source causes a slight deviation in the multimeter pointer. Switch to the R × 1 Ω range of the multimeter again, connect the negative probe to the drain (D) and the positive probe to the source (S). If the multimeter reading is a few ohms, it indicates that the field-effect transistor is good.

2、 Determine the electrodes of junction field-effect transistors

Set the multimeter to the R × 100 position, connect the red probe to any pin, and connect the black probe to the other pin, leaving the third pin hanging. If there is a slight oscillation of the pointer, it proves that the third pin is the grid. To achieve a more obvious observation effect, one can also use the human body to approach or touch the suspended foot with fingers. As long as the pointer is significantly deflected, it indicates that the suspended foot is the gate, and the other two feet are the source and drain, respectively.

Reason for judgment: The input resistance of JFET is greater than 100M Ω, and the transconductance is very high. When the gate is open, the spatial electromagnetic field can easily induce a voltage signal on the gate, causing the tube to tend to turn off or turn on. If the human induced voltage is directly applied to the gate, the above phenomenon will be more pronounced due to strong input interference signals. If the pointer deviates significantly to the left, it means that the tube tends to cut off, the resistance RDS between the drain and source increases, and the current IDS between the drain and source decreases. On the contrary, if the needle deviates significantly to the right, it indicates that the tube is trending towards conduction, RDS ↓, IDS ↑. But the direction in which the pointer is deflected depends on the polarity of the induced voltage (forward or reverse voltage) and the operating point of the tube.

Notes:

(1) Experiments have shown that when both hands are insulated from the D and S poles and only touch the grid, the hands of the meter generally deflect to the left. However, if both hands are in contact with the D and S poles respectively and the grid is touched with fingers, it is possible to observe a rightward deviation of the pointer. The reason is that several parts of the human body and resistors bias the field-effect transistor, causing it to enter the saturation zone.

(2) You can also use the tip of your tongue to lick the grid electrode, with the same phenomenon as above.

3、 Identification of transistor pins

A transistor is composed of a core (two PN junctions), three electrodes, and a shell. The three electrodes are called collector C, emitter E, and base B. Currently, there are two common types of transistors: silicon planar transistors and germanium alloy transistors, each of which has PNP and NPN types.

Here is a simple method to measure the three pins of a transistor using a multimeter.

1. Find the base

For PNP transistors, the C and E poles are respectively the positive poles of the two PN junctions inside, and the B pole is their common negative pole. However, for NPN transistors, the opposite is true: the C and E poles are respectively the negative poles of the two PN junctions, and the B pole is their common positive pole. Based on the small forward resistance and large reverse resistance of the PN junction, it is easy to determine the type of base and transistor. The specific method is as follows:

Set the multimeter to the R × 100 or R × 1K position. The red pen contacts a certain pin, and the black pen is connected to the other two pins separately. This way, three sets of readings (twice per set) can be obtained. When one of the sets has a low resistance of several hundred ohms in the second measurement, the pin contacted by the red pen is the base, and the transistor type is PNP; If a set of high resistance values ranging from tens to hundreds of kiloohms are measured using the above method, then the pin contacted by the red probe is the base, and the transistor type is NPN.

Distinguish between emitter and collector electrodes

Due to the different doping concentrations in the two P regions or two N regions during the production of a transistor, if the emitter and collector are used correctly, the transistor has strong amplification ability. Conversely, if the emitter and collector are used interchangeably, the amplification ability is very weak, which can distinguish the emitter and collector of the transistor.

After identifying the transistor type and base B, one of the following methods can be used to distinguish between the collector and emitter.

(1) Set the multimeter to the R × 1 gear. Pinch the base and the other pin together by hand (be careful not to let the electrodes directly touch each other). To make the measurement phenomenon obvious, moisten your fingers and connect the red probe to the pin pinched together with the base and the black probe to the other pin. Pay attention to the amplitude of the multimeter pointer swinging to the right. Then swap the two pins and repeat the above measurement steps. Compare the amplitude of the pointer swinging to the right in two measurements and find the one with the larger swing amplitude. For PNP type transistors, connect the black probe to the pin that is pinched together with the base, repeat the above experiment, and find the one with the largest swing amplitude of the probe. At this time, connect the black probe to the collector and the red probe to the emitter.

The principle of this electrode discrimination method is to use the battery inside the multimeter to apply voltage to the collector and emitter of the transistor, giving it amplification capability. When the base and collector are pinched by hand, it is equivalent to applying a forward bias current to the transistor through the resistance of the hand, making it conductive. At this time, the amplitude of the pointer swinging to the right reflects its amplification ability, so the emitter and collector can be correctly distinguished.

(2) Set the multimeter to the R × 1 position, connect the two probes of the multimeter to the other two pins of the tube, lick the base with your tongue, check the needle indication, and then swap the probes. Repeat the above steps to find the one with the largest swing. For PNP type transistors, the red probe is connected to the collector and the black probe is connected to the emitter; For NPN transistors, the black probe is connected to the collector and the red probe is connected to the emitter.