The principle of transistor substitution

The principle of transistor substitution

Whether it's professional radio maintenance personnel. Even amateur radio enthusiasts will encounter transistor swapping problems in their work. If one masters the principle of transistor substitution, it can often make maintenance work more efficient and improve maintenance efficiency. The principle of transistor substitution can be summarized into three aspects:

Same type, similar characteristics, and similar appearance

1、 Same type

1. Same material

Replace germanium diode with germanium diode, replace silicon diode with silicon diode

2. Same polarity

Replace NPN diode with NPN diode, and replace PNP diode with PNP diode.

2、 Similar characteristics

The transistor used for replacement should have similar characteristics to the original transistor, and their main parameter values and characteristic curves should not differ much. The main parameters of transistors are nearly 20, and it is not only difficult but also unnecessary to require all these parameters to be similar. Generally speaking, as long as the following main parameters are similar, they can meet the permutation requirements.

1. Maximum DC dissipated power of collector plate (Pcm)

It is generally required to replace the transistor with a Pcm that is equal to or larger than the original transistor. But after calculation or testing, if the actual DC dissipated power of the original transistor in the entire circuit is much smaller than its Pcm, then Pcm can be used to replace it with a smaller transistor

2. Maximum allowable DC current of collector (Icm)

Generally, it is required to replace with a transistor with an Icm equal to or larger than the original transistor

3. Breakdown voltage

The transistor used for replacement must be able to safely withstand the highest operating voltage in the entire machine

4. Frequency characteristics

There are two commonly used frequency characteristic parameters for transistors:

(1) Characteristic frequency fT: It refers to the frequency at which the common emitter current of the transistor is amplified by a factor when the test frequency is high enough

(2) Cut off frequency fb: When replacing transistors, Ft and fb are mainly considered. Normally, the Ft and fb of the transistor used for replacement should not be less than the corresponding Ft and Fb of the original transistor

5. Other parameters

In addition to the main parameters mentioned above, for some special transistors, the following parameters should also be considered during replacement:

(1) For low-noise transistors, transistors with lower or equal noise coefficients should be used for replacement

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

(3) For switch diodes, their switching parameters should also be considered when replacing them

3、 Similar in appearance

Small power transistors generally have similar appearances, as long as the lead wires of each electrode are clearly marked and arranged in the same order as the transistor to be replaced, they can be replaced. The appearance differences of high-power transistors are significant. When replacing them, transistors with similar appearance and installation size should be selected to facilitate installation and maintain normal heat dissipation conditions