选择R6的阻值时要确保在负载电流的最高允许值的条件下，Q2的基极-发射极电压大约低于0.5V。
When choosing a value for R6, ensure that Q2's base-emitter voltage is less than approximately 0.5V at the maximum permissible value of the load current.
由于Q2的基极-发射极的电压为0 V而截止，因此没有负载电流流过r6。
Consequently, no load current flows through R6 because Q2, whose base-emitter voltage is now 0v, has turned off.
特点：基极-发射极间内置电阻，高电压。
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