I am trying to select a pre-biased transistor, and I noticed some of them have a what is listed as an infinite resistance base pull-down resistor.

I do not understand how or why an infinite resistance base pull-down resistor works, and if this has an advantage or disadvantage over lets say a normal 10K pull-down resistor.

Thank you for the help.

Screenshot from a datasheet:

enter image description here


1 Answer 1


This just means that r2 is not installed. Actual infinite resistance is difficult to achieve ;-)

As to why do this instead of 10K? Perhaps there are applications where base current should be restricted but it is not desired to divide the input voltage in half?

  • \$\begingroup\$ Would this transistor be a good choice for an arduino if you say R2 is effectively not installed, can I assume the base is still held low even if something goes wrong with the microcontroller? \$\endgroup\$
    – klcjr89
    Feb 26, 2017 at 15:31
  • \$\begingroup\$ @aviatorken89, BJTs generally don't have the same issue as MOSFETs with the base/gate drifting around if its in a high impedance state. Because BJTs need non-zero base current to turn "on". \$\endgroup\$
    – The Photon
    Feb 26, 2017 at 15:58
  • \$\begingroup\$ Hi Peter, as to why: I imagine this is part of a family of these transistors with R1 and R2 values each of 10K, 4K7, no-fit and so on. I've used 4K7/4K7 ones. \$\endgroup\$
    – TonyM
    Feb 26, 2017 at 16:10

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