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Transistor replacement principle

Transistor replacement principles can be summarized into three types: the same type, similar characteristics and similar appearance.

One—same type

1. The materials are the same, that is, germanium tubes replace germanium tubes, and silicon tubes replace silicon tubes.

2. The polarity is the same, that is, the NPN type tube replaces the NPN type tube, and the PNP type tube replaces the PNP type tube.

Transistor

Two—similar characteristics

The characteristics of the transistor used for replacement should be similar to those of the original transistor, and the main parameter values and characteristic curves should be similar.

1. The maximum DC power loss of the collector plate (PCM)

The PCM of the replacement transistor is usually required to be equal to or greater than the original transistor. However, in the actual test, if the actual DC power loss of the original transistor in the entire circuit is much smaller than its PCM, a transistor with a smaller PCM can be used instead.

2. The maximum allowable DC current of the collector (icm)

The icm of the replacement transistor is usually required to be equal to or greater than the original transistor.

3. Breakdown voltage

The transistor used for replacement must be able to withstand the maximum operating voltage of the entire machine.

4. Frequency characteristics

The frequency characteristic parameters of the transistor are as follows:

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

(2) Cut-off frequency fb: When replacing the transistor, the main considerations are ft and fb. Usually the transistors that need to be replaced should not be smaller than the corresponding ft and fb of the original transistor.

5. Other parameters

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

(1) For low-noise transistors, a transistor with a smaller or equal noise figure should be used instead.

(2) For transistors with automatic gain control performance, use transistors with the same automatic gain control characteristics when replacing them.

(3) For the switch tube, the relevant switch parameters should be considered when replacing the switch tube.

Three—similar appearance

The small power transistors are similar in shape and can be replaced as long as the lead of each electrode is clearly marked and the order of the leads is the same as the order of the tube to be replaced. The appearance of high-power transistors is quite different. In order to install well and maintain normal heat dissipation conditions, a transistor with a similar appearance and the same size should be selected for replacement.

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