Multimeter - Selection of Semiconductor Transistors
The selection of transistors must first comply with the requirements of equipment and circuits, and secondly, comply with the principle of economy. Depending on the application, the following factors should generally be considered: operating frequency, collector current, dissipated power, current amplification factor, reverse breakdown voltage, stability and saturation voltage drop, etc. These factors have a mutually restrictive relationship. During the election and management, we should grasp the main contradictions and take into account the secondary factors.
The characteristic frequency fT of low-frequency tubes is generally below 2.5MHz, while the fT of high-frequency tubes ranges from tens of MHz to hundreds of MHz or even higher. When selecting control, fT should be 3 to 10 times the operating frequency. In principle, high-frequency tubes can replace low-frequency tubes, but the power of high-frequency tubes is generally relatively small and the dynamic range is narrow. Pay attention to the power conditions when replacing.
Generally, we hope that β should be larger, but bigger is not always better. If β is too high, it will easily cause self-excited oscillation, not to mention that generally tubes with high β are unstable and are greatly affected by temperature. Usually, the β value is between 40 and 100, but for tubes with low noise and high β value (such as 1815, 9011~9015, etc.), the temperature stability is still good when the β value reaches hundreds. In addition, for the entire circuit, β should be selected from the coordination of all levels. For example, if the front stage uses a high beta tube, the rear stage can use a low beta tube; conversely, if the front stage uses a low beta tube, the rear stage can use a high beta tube.
The collector-emitter reverse breakdown voltage UCEO should be chosen to be greater than the supply voltage. The smaller the penetration current, the better the stability against temperature. The stability of ordinary silicon tubes is much better than that of germanium tubes, but the saturation voltage drop of ordinary silicon tubes is larger than that of germanium tubes, which will affect the performance of the circuit in some circuits. It should be selected according to the specific conditions of the circuit and the consumption of the transistor. When dissipating power, a certain margin should be left according to the requirements of different circuits.
For transistors used in circuits such as high-frequency amplification, intermediate-frequency amplification, and oscillators, transistors with high characteristic frequency fT and small inter-electrode capacitance should be selected to ensure high power gain and stability at high frequencies.
