三层三端半导体器件称为晶体管的一个常用用途是信号放大和开关。 晶体管是最重要的电子元件之一，用途广泛，包括嵌入式系统、数字电路和控制系统。
晶体管广泛应用于各种应用领域，包括开关电路、放大器电路、电源电路、数字逻辑电路、电压调节器、振荡器电路等，因此它们可以在数字和模拟世界中找到。
本文主要讨论晶体管的开关功能，并简要介绍晶体管作为开关。
浅谈BJT
双极结晶体管（BJT）和场效应晶体管是两种主要的晶体管（FET）。 双极结晶体管，有时也称为BJT，是一种具有三层、三端子和两结点的半导体器件。 它由两个PN结背对背连接，中间层连接组成。
当我们使用”晶体管”这个词时，我们经常指的是BJT。 它是一个电流控制的设备，这意味着输入电流决定了输出多少电流。 “双极”一词是指双极结晶体管（BJT）中的电流由正负电荷载流子（即电子和空穴）携带的事实。
晶体管作为开关的实例
如前所述，晶体管可以用作开关。 发光二极管使用晶体管进行开关，如下图所示。
当基极的开关打开时，晶体管处于截止状态，因为没有任何电流通过基极。 由于晶体管充当开路，LED熄灭。
当开关关闭时，基极电流开始流过晶体管，最终达到饱和并开启LED。
为了控制流经基座和LED的电流，电阻器被定位。
通过调整基极电流路径中的电阻, LED的强度也可以改变.
操作继电器的晶体管
晶体管也可以用来调节继电器的功能。 使用紧凑型晶体管电路配置，继电器的线圈可以被激活，控制连接到它的外部负载。
要了解晶体管如何为继电器线圈供电，请考虑以下电路。 晶体管被施加在基极上的输入驱动到饱和状态，从而进一步导致电路短路。 结果，继电器触点接通，继电器线圈接通电源。
在感应负载中，特别是在电动机和电感器被切换时，线圈中仍然存在高电位。
四. 结论
使用晶体管作为开关的彻底指南。 了解双极结晶体管的基本原理、工作范围、NPN和PNP晶体管如何作为开关，以及开关晶体管的实际用途。
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