The principle of transistor substitution

The principle of transistor substitution

Whether it's dedicated radio maintenance personnel. Or radio amateurs, they will encounter transistor replacement problems at work. If you master the transistor replacement principle, you can often double the maintenance work with half the effort and improve the maintenance efficiency. The displacement principle of transistors can be summarized into three things: the same type, similar characteristics, and similar shape.

    First, the type is the same

    1. The material is the same. That is, germanium s replace germanium s, and silicon s replace silicon s.

    2. Same polarity. That is, NPN type  replaces NPN type , and PNP type  replaces PNP type .

    Second, the characteristics are similar

    The transistor used for replacement should have similar characteristics to the original transistors, and their main parameter values and characteristic curves should be similar. There are nearly 20 main parameters of transistor, and it is difficult and unnecessary to require all of them to be similar. In general, as long as the following main parameters are similar, the substitution requirements can be met.

    1. The collector board has a large DC dissipated power (PCM)

    It is generally required to replace transistors with PCM equal to or larger than the original . However, after calculation or testing, if the actual DC dissipation power of the original transistor in the whole circuit is much less than its PCM, it can be replaced with a transistor with a smaller PCM.

    2. The collector is large and allowable DC current (ICM)

    It is generally required to replace the ICM with transistors equal or larger than the original .

    3. Breakdown voltage

    transistor used for replacement must be able to safely withstand very high operating voltages in the whole machine;

    Source: Power transmission and distribution equipment network

    4. Frequency characteristics

    transistor frequency characteristic parameters, commonly used are the following 2:

    (1) Characteristic frequency ft: It refers to the frequency when the transistor common emitter current amplification factor is made when the test frequency is high enough.

    (2) Cutoff frequency fb:

    When replacing transistor, FT and FB are mainly considered. It is usually required that the transistor used for replacement, the ft and fb, should not be less than the ft, fb corresponding to the original transistor.

    5。 Other parameters

    In addition to the above main parameters, for some special transistors, the following parameters should also be considered when displaced:

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

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

    (3) For the switch , its switching parameters should also be considered when displaced.

    Third, the appearance is similar

    Low-power transistor are generally similar in appearance, as long as the lead wires of each electrode are clearly marked and the sequence of the leads is consistent with the  to be replaced, they can be replaced. The shape of high-power transistors varies greatly, and transistorwith similar shapes and installation dimensions should be selected when replacing them in order to install and maintain normal heat dissipation conditions.