I am using the configuration in picture and the MMBT5087 PNP transistor. I am not using the top base resistor, just the lower one, so there will be 5V on the base. buffer configuration to protect chip when not powered from bus voltage

My question is: Using a 1K resistor will it saturate? And is it ok to apply 5V directly on the base or it might damage the device?

I want to use 1K to keep the base current as low as possible.


  • \$\begingroup\$ When I clicked on your link nothing happened, so I gave up after a few seconds. Also, provide a link to the transistor datasheet, this makes it easier to answer. \$\endgroup\$ Jan 8 '13 at 23:02
  • \$\begingroup\$ Someone edited it, sorry not my fault \$\endgroup\$
    – mFeinstein
    Jan 9 '13 at 3:11

"Saturation" is usually defined as the point where hFE drops to 10. So work out the voltage across the base resistor (without chasing your circuit and datasheet, I'm guessing 5V - Vbe = 4.3V), so 4.3ma base current. So, if your load current Ic < 43mA, the transistor is technically in saturation. If I've guessed wrong, put in the correct numbers and decide for yourself.


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