Transistor Types and Materials: First, we need to understand the types and materials of transistors. There are two common types of transistors: 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 base bias voltage. The conduction voltage of the PN junction in a germanium transistor is approximately 0.2V, while that in a silicon transistor is 0.6~0.7V. In amplifier circuits, a germanium transistor of the same type can be used to replace a silicon transistor of the same type, and vice versa. However, due to the different base bias voltages, the base bias voltage must be adjusted. It is essential to analyze whether different materials are suitable for the pulse circuit and switching circuit; 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 over 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, when the change in the transistor's 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 of the transistor will cause permanent damage or performance degradation.
3. PCM is the maximum allowable power dissipation in the collector. When the 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 at frequencies above the PCM level for an extended period, it will be damaged. It is important to note that when using a heatsink for a high-power transistor, the maximum allowable power dissipation specified is a parameter. This must be considered during use.
4. Characteristic Frequency ft. As the operating frequency increases, the transistor's amplification capability decreases. The frequency ft corresponding to β=1 is called the transistor's characteristic frequency.
Generally, low-power transistors are preferred: 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 Fourier Transmission Frequency (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 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 is not lower than 15V, so 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 50mA, and it is generally not considered in small-signal circuits. However, careful calculations are needed for circuits driving relays and high-power speakers. Of course, the pull-in current in milliamps of the relay must first be known to determine the transistor's ICM.
When estimating the operating current (collector current) and the voltage between the collector and emitter of a transistor in a circuit, we can calculate the maximum allowable collector power dissipation (PCM) using P=U×I.
Many models of low-power transistors are produced domestically and internationally, some with identical parameters and others with different parameters. Based on the usage conditions analyzed above, high-BVCEO transistors can replace low-BVCEO transistors; large-ICM transistors can replace small-ICM transistors. Transistors can be used freely.
Selecting high-power transistors: For high-power transistors, unless it's a high-frequency emission circuit, the transistor's characteristic frequency (ft) does not need to be considered. The limiting parameters of BVCEO are the same as for 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 high-power transistor of 40-50 watts can only dissipate 2-3 watts of power without a heatsink. Sufficient power margin is essential when choosing a high-power transistor. Furthermore, the installation conditions should be considered when selecting a high-power transistor to determine whether it should be a plastic-sealed or metal-sealed tube.
If you have a transistor but cannot find its parameters, you can infer them from its shape. Currently, the most common low-power transistors are in TO-92 packaged plastic-sealed tubes; some are in metal-cased packaging. PCM (Power Capacitor) is generally between 100-500mW, with a maximum of 1W. ICM (Inlet Capacitor) is generally between 50-500mA, with a maximum of 1.5A. Other parameters are difficult to determine.
