Qualitative judgment of the quality of field-effect transistors and transistors

Issuing time:2023-08-18 16:02

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

First, use a multimeter R × 10k Ω gear (with built-in 15V battery), connect the negative lead (black) to the gate (G), and the positive lead (red) to the source (S). Charge between the gate and source electrode, and at this time, the multimeter pointer has slight deflection. Switch to a multimeter R again × At 1 Ω, connect the negative lead to the drain (D) and the positive lead to the source (S). If the multimeter indicates a few ohms, it indicates that the field-effect transistor is good.

2、 Judging the electrodes of junction field-effect transistors

Set the multimeter to R × At level 100, the red pen is connected to any pin, while the black pen is connected to another pin, causing the third pin to hang in the air. If a slight oscillation of the watch needle is found, it proves that the third pin is the gate. To achieve a more obvious observation effect, one can also use the human body to approach or touch the suspended foot with their fingers. As long as the watch needle undergoes significant deflection, it indicates that the suspended foot is the gate electrode, and the other two feet are the source electrode and drain electrode, 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 cut off or conduct. If the human induced voltage is directly applied to the gate, the above phenomenon will be more obvious due to the strong input interference signal. If the watch needle deviates significantly to the left, it means that the tube tends to cut off, the drain source resistance RDS increases, and the drain source current IDS decreases. On the contrary, if the watch needle deviates significantly to the right, it indicates that the tube tends to conduct, with RDS ↓ and IDS ↑. But the direction in which the watch needle deflects depends on the polarity of the induced voltage (forward or reverse voltage) and the working point of the tube.

Precautions:

(1) Experiments have shown that when both hands are insulated from the D and S electrodes and only touch the gate electrode, the watch needle generally deflects to the left. However, if both hands touch the D and S electrodes separately and touch the gate electrode with your fingers, it is possible to observe the watch needle deflecting to the right. 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 lick the gate with the tip of your tongue, as described above.

3、 Pin discrimination of crystal transistor

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 flat tubes and germanium alloy tubes, each of which has two types: PNP and NPN.

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

1. Find the base electrode

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

Set the multimeter to R × 100 or R × On 1K gear. When the red pen contacts a certain pin, connect the black pen to the other two pins separately, so as to obtain three sets of readings (two times per group). When one set of secondary measurements has a low resistance value of several hundred ohms, the pin contacted by the red pen is the base, and the transistor is of PNP type; If a set of high resistance values with secondary values ranging from tens to hundreds of kiloohms are measured using the above method, the pin contacted by the red probe is the base, and the transistor type is NPN.

2. Distinguish between emitter and collector

Due to the different doping concentrations in the two P or 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 distinguishing the tube type and base b, one of the following methods can be used to distinguish the collector and emitter.

(1) Set the multimeter to R × On 1st gear. Pinch the base electrode with the other pin by hand (be careful not to directly touch the electrodes). To make the measurement obvious, wet your fingers and connect the red probe to the pin that is pinched together with the base electrode. Connect the black probe to the other pin, and observe the amplitude of the multimeter pointer swinging to the right. Then switch the two pins and repeat the above measurement steps. Compare the amplitude of the needle swinging to the right in two measurements, and find the one with the larger swing amplitude. For PNP type transistor, connect the black lead to the pin pinched together with the base electrode, repeat the above experiment, and find the one with the larger swing of the watch needle. At this time, the black lead is connected to the collector electrode, and the red lead is connected to the emitter electrode.

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, making it have amplification ability. When a hand pinches its base and collector, it is equivalent to adding a forward bias current to the transistor through the resistance of the hand, making it conductive. At this time, the swing amplitude of the watch needle to the right reflects its amplification ability, so it can correctly distinguish the starting emitter and collector.

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