T
treez
Guest
Hello,
Q6:A and Q6:B of the following schematic (page 2) show a dual PNP being used for high side LED current sensing. This method depends on the Vbe's being matched. This can't be guaranteed, thus it is an inaccurate method. Do you agree?
The MMDT5401 datasheet just says its a dual PNP transistor, not "matched " transistors. There is no reason to believe that their Vbe's will be matched? BJTs are manufactured with attention being payed only to achieving high hfe, and matching of parameters such as vbe, does not have any interest to the manufacturers?
Surely R40 should be 21K, because the vbe's of those transistors will not be the same if there Ic's are different, and in that cicuit, their Ic's are different?
schematic on page 2:
**broken link removed**
Dual PNP datasheet (MMDT5401)
**broken link removed**
Q6:A and Q6:B of the following schematic (page 2) show a dual PNP being used for high side LED current sensing. This method depends on the Vbe's being matched. This can't be guaranteed, thus it is an inaccurate method. Do you agree?
The MMDT5401 datasheet just says its a dual PNP transistor, not "matched " transistors. There is no reason to believe that their Vbe's will be matched? BJTs are manufactured with attention being payed only to achieving high hfe, and matching of parameters such as vbe, does not have any interest to the manufacturers?
Surely R40 should be 21K, because the vbe's of those transistors will not be the same if there Ic's are different, and in that cicuit, their Ic's are different?
schematic on page 2:
**broken link removed**
Dual PNP datasheet (MMDT5401)
**broken link removed**
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