Transistor materials are germanium materials and silicon materials. The choice of triode needs to understand the main parameters of the triode.
First, the type of transistor and material Beginners must first understand the type and material of the transistor. The types of commonly used transistors are NPN type and PNP type. Because these two types of transistors work (work summary) with different voltage polarity requirements, they cannot be replaced with each other.
Transistor materials are germanium materials and silicon materials. The biggest difference between them is the starting voltage is not the same. The turn-on voltage of the PN junction is about 0.2V, and the turn-on voltage of the PN junction of the silicon tube is 0.6-0.7V. In the amplifying circuit, if the same type of helium tube is used to replace the same type of silicon tube, or the same type of helium tube is replaced with the same type of silicon tube, generally it is possible to perform the necessary voltage on the base bias voltage. The adjustments because their starting voltages are not the same. However, whether the triodes of different materials in the pulse circuit and the switch circuit can be interchanged must be specifically analyzed and cannot be replaced blindly.
Second, the main parameters of the triode selection Triode needs to understand the main parameters of the triode. It is best if you have a transistor feature manual in your hand. Transistor parameters are many, according to practical experience, I think mainly to understand the four extreme parameters of the transistor: ICM, BVCEO, PCM and fT can meet the needs of more than 95% of the use.
1. ICM is the maximum allowable collector current. When the transistor work (work summary) when its collector current exceeds a certain value, its current amplification factor β will decrease. For this reason, the maximum current of the collector when the current amplification factor β of the transistor does not exceed the permissible value is called ICM. Therefore, in use, when the collector current IC exceeds the ICM, it will not damage the transistor, but it will reduce the beta value and affect the work (work summary) performance of the circuit.
2. BVCEO is the collector-emitter reverse breakdown voltage when the transistor base is open-circuited. If the voltage applied between the collector and the emitter exceeds this value during use, it may cause the transistor to generate a large collector current. This phenomenon is called breakdown. Triode breakdown will cause permanent damage or performance degradation.
3. PCM is the maximum allowable dissipation power of the collector. When the triode is in operation (work summary), the collector current generates heat on the collector junction and heats the triode. If the dissipated power is too large, the transistor will burn out. In use, if the transistor is operated longer than the PCM (work summary), the transistor will be damaged. It should be noted that the maximum allowable dissipated power given by the high-power transistor is in the case of a radiator with a certain size. Be sure to pay attention to this in use.
4. Characteristic frequency fT. As the frequency of the work (work summary) increases, the amplification capability of the triode will decrease, and the frequency fT corresponding to β=1 will be referred to as the characteristic frequency of the triode.
Third, the choice of general low-power transistor Small power transistor in the electronic circuit applications. Mainly used as a small signal amplification, control or oscillator. When choosing a transistor, you first need to figure out what the frequency of the electronic circuit is (work summary). For example, the highest frequency of the medium-wave radio oscillator is about 2 MHz; and the highest frequency of the FM radio is about 120 MHz; the highest oscillation frequency of the VHF band in the TV is about 250 MHz; and the highest oscillation frequency of the UHF band is about 1000 MHz. In engineering design, it is generally required that the triode's fT be more than 3 times the actual work (work summary) frequency. Therefore, the characteristic frequency fT of the triode can be selected according to this requirement. Since the fT of silicon high-frequency triodes is generally not lower than 50 MHz, the use of such tubes in audio electronic circuits does not consider the fT parameter.
The choice of low-power transistor BVCEO can be determined based on the power supply voltage of the circuit. Under normal circumstances, as long as the BVCEO of the transistor is greater than the highest voltage of the power supply in the circuit. When the triode load is inductive load, such as transformers, coils and other BVCEO values ​​should be carefully selected, the induced voltage on the inductive load may reach 2 to 8 times the power supply voltage (such as energy-saving lamps in the boosting triode). The BVCEO of a general low-power transistor is no lower than 15V, so this parameter is not taken into consideration in low-voltage circuits without inductance components.
The ICM of general low-power triode is between 30 and 50mA, can generally not be considered for the small-signal circuit. However, it is necessary to carefully calculate the tubes that drive the relay and drive the high-power speakers. Of course, the first thing to know is how many milliamps the relay's pull-in current is, in order to determine the transistor's ICM.
When we estimate the current (ie, collector current) of the transistor's operation (work summary) in the circuit and know the collector-to-emitter voltage of the transistor, we can calculate the collector of the transistor based on P=U×I. Maximum allowed power dissipation PCM.
There are many types of low-power triodes made in China and abroad. Some of their parameters are the same, and some are different. As long as you use the above analysis conditions, in line with the principle of “power generation is small†(ie BVCEO high triode can replace BVCEO low triode; ICM large triode can replace ICM small triode, etc.), you can apply triode well Now.
Fourth, the choice of high-power transistor For high-power transistor, as long as it is not a high-frequency transmission circuit, we do not have to consider the triode characteristic frequency fT. The consideration of the limit parameter of the collector-emitter reverse breakdown voltage BVCEO of the triode is the same as that of the low-power triode. The choice of the maximum allowable collector current ICM is also based on the load condition of the triode. The maximum allowable power dissipation of the collector of the transistor is the key consideration of the high-power transistor. It should be noted that the high-power transistor must have a good heat sink. Even a four- and fifty-watt high-power transistor can only withstand power dissipation of two or three watts without a heat sink. The choice of high-power transistor should also leave sufficient margin. In addition, when selecting a high-power transistor, it is also necessary to consider its installation conditions to decide whether to select a plastic tube or a metal-packaged tube.
If you get a triode and can't find its parameters, you can speculate about its parameters based on its shape. The most common low-power transistor is a plastic tube with a TO-92 encapsulation, and there are also some metal-shell packages. Their PCM is generally between 100 and 500mW, the largest not more than 1W. Their ICM is generally between 50 ~ 500mA, the largest does not exceed 1.5A. Other parameters are not good to judge.
First, the type of transistor and material Beginners must first understand the type and material of the transistor. The types of commonly used transistors are NPN type and PNP type. Because these two types of transistors work (work summary) with different voltage polarity requirements, they cannot be replaced with each other.
Transistor materials are germanium materials and silicon materials. The biggest difference between them is the starting voltage is not the same. The turn-on voltage of the PN junction is about 0.2V, and the turn-on voltage of the PN junction of the silicon tube is 0.6-0.7V. In the amplifying circuit, if the same type of helium tube is used to replace the same type of silicon tube, or the same type of helium tube is replaced with the same type of silicon tube, generally it is possible to perform the necessary voltage on the base bias voltage. The adjustments because their starting voltages are not the same. However, whether the triodes of different materials in the pulse circuit and the switch circuit can be interchanged must be specifically analyzed and cannot be replaced blindly.
Second, the main parameters of the triode selection Triode needs to understand the main parameters of the triode. It is best if you have a transistor feature manual in your hand. Transistor parameters are many, according to practical experience, I think mainly to understand the four extreme parameters of the transistor: ICM, BVCEO, PCM and fT can meet the needs of more than 95% of the use.
1. ICM is the maximum allowable collector current. When the transistor work (work summary) when its collector current exceeds a certain value, its current amplification factor β will decrease. For this reason, the maximum current of the collector when the current amplification factor β of the transistor does not exceed the permissible value is called ICM. Therefore, in use, when the collector current IC exceeds the ICM, it will not damage the transistor, but it will reduce the beta value and affect the work (work summary) performance of the circuit.
2. BVCEO is the collector-emitter reverse breakdown voltage when the transistor base is open-circuited. If the voltage applied between the collector and the emitter exceeds this value during use, it may cause the transistor to generate a large collector current. This phenomenon is called breakdown. Triode breakdown will cause permanent damage or performance degradation.
3. PCM is the maximum allowable dissipation power of the collector. When the triode is in operation (work summary), the collector current generates heat on the collector junction and heats the triode. If the dissipated power is too large, the transistor will burn out. In use, if the transistor is operated longer than the PCM (work summary), the transistor will be damaged. It should be noted that the maximum allowable dissipated power given by the high-power transistor is in the case of a radiator with a certain size. Be sure to pay attention to this in use.
4. Characteristic frequency fT. As the frequency of the work (work summary) increases, the amplification capability of the triode will decrease, and the frequency fT corresponding to β=1 will be referred to as the characteristic frequency of the triode.
Third, the choice of general low-power transistor Small power transistor in the electronic circuit applications. Mainly used as a small signal amplification, control or oscillator. When choosing a transistor, you first need to figure out what the frequency of the electronic circuit is (work summary). For example, the highest frequency of the medium-wave radio oscillator is about 2 MHz; and the highest frequency of the FM radio is about 120 MHz; the highest oscillation frequency of the VHF band in the TV is about 250 MHz; and the highest oscillation frequency of the UHF band is about 1000 MHz. In engineering design, it is generally required that the triode's fT be more than 3 times the actual work (work summary) frequency. Therefore, the characteristic frequency fT of the triode can be selected according to this requirement. Since the fT of silicon high-frequency triodes is generally not lower than 50 MHz, the use of such tubes in audio electronic circuits does not consider the fT parameter.
The choice of low-power transistor BVCEO can be determined based on the power supply voltage of the circuit. Under normal circumstances, as long as the BVCEO of the transistor is greater than the highest voltage of the power supply in the circuit. When the triode load is inductive load, such as transformers, coils and other BVCEO values ​​should be carefully selected, the induced voltage on the inductive load may reach 2 to 8 times the power supply voltage (such as energy-saving lamps in the boosting triode). The BVCEO of a general low-power transistor is no lower than 15V, so this parameter is not taken into consideration in low-voltage circuits without inductance components.
The ICM of general low-power triode is between 30 and 50mA, can generally not be considered for the small-signal circuit. However, it is necessary to carefully calculate the tubes that drive the relay and drive the high-power speakers. Of course, the first thing to know is how many milliamps the relay's pull-in current is, in order to determine the transistor's ICM.
When we estimate the current (ie, collector current) of the transistor's operation (work summary) in the circuit and know the collector-to-emitter voltage of the transistor, we can calculate the collector of the transistor based on P=U×I. Maximum allowed power dissipation PCM.
There are many types of low-power triodes made in China and abroad. Some of their parameters are the same, and some are different. As long as you use the above analysis conditions, in line with the principle of “power generation is small†(ie BVCEO high triode can replace BVCEO low triode; ICM large triode can replace ICM small triode, etc.), you can apply triode well Now.
Fourth, the choice of high-power transistor For high-power transistor, as long as it is not a high-frequency transmission circuit, we do not have to consider the triode characteristic frequency fT. The consideration of the limit parameter of the collector-emitter reverse breakdown voltage BVCEO of the triode is the same as that of the low-power triode. The choice of the maximum allowable collector current ICM is also based on the load condition of the triode. The maximum allowable power dissipation of the collector of the transistor is the key consideration of the high-power transistor. It should be noted that the high-power transistor must have a good heat sink. Even a four- and fifty-watt high-power transistor can only withstand power dissipation of two or three watts without a heat sink. The choice of high-power transistor should also leave sufficient margin. In addition, when selecting a high-power transistor, it is also necessary to consider its installation conditions to decide whether to select a plastic tube or a metal-packaged tube.
If you get a triode and can't find its parameters, you can speculate about its parameters based on its shape. The most common low-power transistor is a plastic tube with a TO-92 encapsulation, and there are also some metal-shell packages. Their PCM is generally between 100 and 500mW, the largest not more than 1W. Their ICM is generally between 50 ~ 500mA, the largest does not exceed 1.5A. Other parameters are not good to judge.
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