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数据表中的导通阈值和关断阈值分别被定义为“输入电压(导通)”(VIN(ON))和“输入电压(关断)”(VIN(OFF))。
我们来考虑一下NPN BRT导通的情况。
当NPN BRT关断时,其内部晶体管Q不工作。因此,施加至B端子的输入电压(VI)被R1和R2分压,然后被施加至Q的基极(b)。随着VI的增加,基极-发射极(be)电压(Vbe)超过阈值(典型值:0.7V),Q导通。输入电压(导通)是指为提供5mA或更大的集电极电流所需的电压,但概念相同。
因此,电阻比(R1/R2)较小的BRT在较低的输入电压(VIN(ON))时达到阈值电压。但是,如果电阻比相等,R1值较大的BRT的IB较低。
例如,将提供5mA集电极电流所需的输入电压设为VI1。然后,无论内置电阻如何,内置晶体管的基极电流ib和b-e电压Vbe分别在ib1和Vbe1条件下保持恒定。使用这些变量,Vin可用下列等式表示:
Vin1 = R1 * Ib1 + ( 1 + R1 / R2 ) * Vbe
在特定的阻值比下,仅右侧的第一项“R1*Ib”有变化。因此,可认为Vin1随着R1的增加而增加。(详见图3)
同理,电阻比较低的BRT的输入电压(关断)也较低。但是,相较于电阻比和输入电压(导通)之间的关系,电阻比和输入电压(关断)之间关系的不那么明显,因为输入电压(关断)的测试条件低至0.1mA,因此往往会受器件变化的影响。
尽管PNP BRT的数字符号与NPN BRT的数字符号相反,但以上讨论也适用于PNP BRT。
图2的IC – VI(ON)曲线表明,在特定的输入电压下,集电极电流骤变。集电极电流也随着温度升高而减小。确保您留有足够的设计余量。
特性 | 符号 | 测试条件 | 最小值 | 典型值 | 最大值 | 单位 | |
---|---|---|---|---|---|---|---|
输入电压(导通) |
RN1401 | VI(ON) | VCE=0.2V,IC=5mA |
1.1 | — | 2.0 | V |