Transistor Types and Materials: First, we need to understand the types and materials of transistors. There are two common types: NPN and PNP. Because these two types of transistors have different voltage polarity requirements, they cannot be interchanged.
Transistor materials include germanium and silicon. The biggest difference between them is their starting voltage. The conduction voltage of the PN junction of a germanium transistor is approximately 0.2V, while that of a silicon transistor is 0.6~0.7V. In amplifier circuits, a germanium transistor of the same type can replace a silicon transistor of the same type, and vice versa. However, due to the different starting voltages, the base bias voltage must be adjusted. It is essential to analyze whether different materials are suitable for pulse circuits and switching circuits; blind replacement is not advisable.
Main Transistor Parameters: Selecting a transistor requires understanding its main parameters. It is best to have a transistor characteristic manual. Transistors have many parameters. Based on practical experience, I believe that ICM, BVCEO, PCMICM, BVCEO, PCM, and FT can meet more than 95% of usage requirements. 1. ICM is the maximum allowable collector current. When a transistor is operating, the current amplification factor β decreases when the collector current exceeds a certain value. Therefore, the maximum collector current, assuming the change in the current amplification factor β does not exceed the allowable value, is called ICM. Thus, when the collector current IC exceeds ICM, it will not damage the transistor, but it will reduce the β value, affecting the circuit's performance.
2. BVCEO is the reverse breakdown voltage when the collector-emitter junction is open at the base of the transistor. If the voltage between the collector and emitter exceeds this value, the transistor may generate a large collector current, which is called breakdown. Breakdown will cause permanent damage or performance degradation.
3. PCM is the maximum allowable power dissipation in the collector. When a transistor is operating, the collector current generates heat at the collector junction, heating the transistor. If the power dissipation is too high, the transistor will burn out. If the transistor operates above PCM for an extended period, it will be damaged. It is important to note that when designing heatsinks for high-power transistors, the maximum allowable power dissipation given by the transistor is a parameter. This must be considered during use.
4. Characteristic Frequency ft. As the operating frequency increases, the amplification capability of the transistor decreases. The frequency ft corresponding to β=1 is called the characteristic frequency of the transistor.
Generally, low-power transistors are chosen: Low-power transistors are widely used in electronic circuits. They are mainly used for small-signal amplification, control, or oscillators. When selecting a transistor, the operating frequency of the electronic circuit must first be determined. For example, the highest frequency of an oscillator in a medium-wave radio is approximately 2MHz; the highest oscillation frequency in an FM radio is approximately 120MHz; the highest oscillation frequency in the VHF band is approximately 250MHz; and the highest oscillation frequency in the UHF band is approximately 1000MHz. Engineering designs generally require the actual operating frequency of the transistor's ft to be at least three times that frequency. Therefore, the characteristic frequency ft of the transistor can be selected based on this requirement. The ft parameter cannot be considered in audio electronic circuits because the ft of high-frequency silicon transistors is generally not lower than 50MHz.
Select the BVCEO of a small-power transistor based on the circuit's power supply voltage. Generally, the transistor's BVCEO should be greater than the highest voltage of the power supply in the circuit. When the transistor's load is inductive, such as a transformer or coil, the selection of the BVCEO value should be cautious, as the induced voltage on an inductive load can reach 2 to 8 times the power supply voltage (e.g., the boost transistor in an energy-saving lamp). Generally, the BVCEO of a small-power transistor should not be lower than 15V, therefore this parameter does not need to be considered in low-voltage circuits without inductive components.
Generally, the ICM of a small-power transistor is between 30 and 50 mA, and small-signal circuits are generally not considered. However, careful calculations are needed for circuits driving relays and high-power speakers. Of course, the relay's pull-in current in milliamps must first be known to determine the transistor's ICM. When estimating the transistor's operating current (i.e., collector current) and the voltage between the collector and emitter in the circuit, we can calculate the maximum allowable power dissipation PCM of the transistor's collector using P=U×I.
There are many models of low-power transistors produced both domestically and internationally, some with the same parameters and others with different ones. Based on the usage conditions analyzed above, high-BVCEO transistors can replace low-BVCEO transistors; large-ICM transistors can replace small-ICM transistors. This allows for flexible application of transistors.
Selecting high-power transistors: For high-power transistors, unless it's a high-frequency transmitting circuit, the transistor's characteristic frequency (ft) does not need to be considered. The limiting parameters of BVCEO are the same as those of low-power transistors. The selection of the maximum allowable collector current (ICM) is mainly calculated based on the transistor's load. The maximum allowable collector power dissipation (PCM) is a key issue for high-power transistors. It is important to note that high-power transistors must have a good heatsink. Even a 40-50 watt high-power transistor can only dissipate 2-3 watts of power without a heatsink. Sufficient margin should be provided when selecting high-power transistors. Furthermore, when selecting high-power transistors, their installation conditions should be considered to determine whether to use plastic-sealed or metal-sealed transistors.
If you obtain a transistor but cannot find its parameters, you can infer them based on its shape. Currently, the most common low-power transistors are TO-92 packaged plastic-sealed tubes, while some are packaged in metal casings. PCM (Power Capacitor) voltage is generally between 100 and 500mW, with a maximum of no more than 1W. ICM (Inlet Capacitor) voltage is generally between 50 and 500mA, with a maximum of no more than 1.5A. Other parameters are difficult to determine.
