The principle of replacement of transistors

In maintenance, design and experiment or trial production, the replacement of transistors is often encountered. If the principle of replacement (substitution) of transistors is mastered, the work will be effective at the beginning. Its replacement (substitution) principles can be divided into three types: the same type, similar characteristics, and similar appearance.

One, the same type

1. The materials are the same. That is, the germanium tube is replaced by a germanium tube, and the silicon tube is replaced by a silicon tube.

2. The polarity is the same. That is, NPN type pipe is replaced by NPN type pipe, and PNP type pipe is replaced by PNP type pipe.

3. The actual model is the same, but the labeling method is different, such as: D1555 is the same as 2SD1555; R1201 is the same as GR1201; 3DG9014 is the same as 9014; the patch tube uses the code to represent the original model, etc. However, it does not rule out that the parameters of the same model vary greatly due to different manufacturers.

Second, the characteristics are similar

The transistors used for replacement (substitution) should have similar characteristics to the original transistors, and their main parameter values ​​and characteristic curves should be similar to or better than the original transistors. For different circuits, there should be some emphasis. Generally speaking, as long as the following main parameters are similar, the replacement (substitution) requirements can be met.

1. Collector maximum DC power dissipation (Pcm)

It is generally required to replace (substitute) a transistor with a Pcm equal to or larger than the original tube. If the actual DC power dissipation of the original transistor in the whole circuit is much smaller than its Pcm, it can also be replaced (substituted) with a transistor with a smaller Pcm.

2. Maximum allowable DC current of collector (Icm)

It is generally required to replace (substitute) a transistor with Icm equal to or larger than the original tube.

In fact, different manufacturers have different regulations on Icm, and sometimes they are very different. We should pay attention to the test conditions given by the manufacturers. The most common are the following:

(1) Determine Icm according to the maximum current value allowed by the collector lead. This value may be very large, for example, a transistor with Pcn=200mW, its Icm may exceed 1A.

(2) Determine Icm according to Pcm, that is, Pcm=Icm×Uce to determine Icm. The Pcm value under this regulation is smaller than that of ordinary transistors and larger than that of switch tubes. For example, the Pcm of ordinary transistor 2SC2209 and switch tube 2SC2214 are both 10W, but their Icm values ​​are 1.5A and 4A respectively.

(3) Determine Icm according to the limit value of the allowable variation of transistor parameters (saturation voltage drop, current amplification factor, etc.). For example, the Icm of the 3DD103A transistor is determined when its beta value drops to 1/3 of the measured value (Icm=3A).

3. Breakdown voltage

The transistor used for replacement (substitution) must be able to safely withstand the highest working voltage in the whole machine. The breakdown voltage parameters of transistors mainly include the following five:

(1) BVcbo: collector-base breakdown voltage. It refers to the voltage drop between the collector and the base when the emitter is open and the collector current Ic is a specified value (this voltage drop is called the corresponding breakdown voltage, the same below).

(2) BVceo: collector-emitter breakdown voltage. It refers to the collector-emitter voltage drop when the base is open and the collector current Ic is a specified value.

CDBVces: base-emitter short circuit, collector-emitter breakdown voltage.

⑷BVcer: base-emitter series resistance, collector-emitter voltage drop.

⑸BVebo: open collector, emitter-base breakdown voltage.

In transistor replacement (substitution), BVcbo and BVceo are mainly considered, and BVebo should also be considered for switching transistors. Generally speaking, BVcbo>BVceo of the same transistor. It is generally required that the above three breakdown voltages of the transistors used for replacement (substitution) should not be less than the three breakdown voltages corresponding to the original transistors.

4. Frequency characteristics

Transistor frequency characteristic parameters, commonly used are the following four:

⑴Characteristic frequency fT: It refers to the frequency when the test frequency is high enough to make the transistor common-emitter current amplification factor β=1.

(2) β cut-off frequency fβ: In the common emitter circuit, when the output terminal is short-circuited, the current amplification factor β value drops to the frequency at which the low frequency (1kHz) β value is 70.7% (3dB).

⑶α cut-off frequency fα: In the common base circuit, when the output terminal is short-circuited, the current amplification factor α value drops to the frequency when the low frequency (1kHz) β value is 70.7% (3dB).

⑷The highest oscillation frequency fmax: the operating frequency when the power gain of the transistor is 1.

When replacing (substituting) transistors, fT and fβ are mainly considered. It is generally required that the fT and fβ of the transistor used for replacement should not be less than the corresponding fT and fβ of the original transistor. Semiconductor tubes are divided into high-frequency tubes and low-frequency tubes. The transistor ft is lower than 3M for low-frequency tubes, and the FET is lower than 303MH. On the contrary, it is a high-frequency tube.

5. Other parameters

In addition to the above main parameters, for some special transistors, the following parameters should also be considered when replacing (substituting):

(1) For low-noise transistors, transistors with smaller or equal noise figures should be used when replacing (substituting).

(2) For transistors with automatic gain control performance, transistors with the same automatic gain control characteristics should be used when replacing (substituting).

(3) For the switch tube, the switch parameters should also be considered when replacing (substituting), whether it has a built-in resistance.

3. Similar in appearance

Low-power transistors are generally similar in appearance, and can be replaced as long as the lead-out pins of each electrode are clearly marked and the lead-out lines are arranged in the same order as the tube to be replaced.

The appearance of high-power transistors varies greatly. When replacing (substituting), transistors with similar appearance and the same installation size should be selected to install and maintain normal heat dissipation conditions. If it is not available, you can also use a plastic-sealed tube instead of an iron-sealed tube.