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如数据表中所示,串联偏置电阻(R1)的值与数据表中的典型值相差±30%。未单独指定基极-发射极电阻(R2)的值,但指定了电阻比(R1/R2)。R1/R2与典型值相比有±10%的变化。
偏置电阻的值取决于温度,如图2所示。电阻以大约0.2%/°C的速率降低。
关于电阻R1:
R1将施加至BRT的B端子的电压转换为电流。双极晶体管是一种电流驱动式器件。当双极晶体管被电压驱动时,集电极电流相对于电压的变化率变大,使得难以控制集电极电流。BRT中的R1使控制集电极电流相对容易一些。当BRT导通时,内部晶体管将在hFE (=IC/Ib)处于10-20范围内的饱和区内工作,具体取决于输入电压。因此,大约几毫安的相对大电流(IB)流经R1。由于R1的允许功耗为1/8W,因此R1值较高的BRT的最大输入电压(VI)由R1值确定。
(详见常见问题(FAQ)“偏置电阻内置晶体管(BRT)的基极可施加的最大电压是多少”。)
关于电阻比(R1/R2)
输入电压(导通)规格(VI(ON))取决于R1/R2。由于基极电流(IB)不会在晶体管导通之前立即流动,因此施加至B端子(VI)的电压会被R1和R2分压。将内部晶体管的导通阈值电压设为Vbe。然后,
Vbe=R2 /(R1+R2)* VI(ON)
如果其包含相同的晶体管,则BRT的Vbe不受R1和R2值的影响。
VI(ON)=Vbe*(R1+R2)/ R2=Vbe*( 1+R1/R2)
因此,VI(ON)取决于电阻比(R1/R2)。