Answer 2
The collector of Q3 is connected to the junction of D1 and D2 in order to make sure that the pull-up current source (Q2) is cut off when the output is low. Otherwise, all of the capacity of the pull-down circuit would be used by the current supplied by the pull-up source.

D3 provides the path for the output current to flow to Q3 when the output is low. Without it, the current would be blocked by D2.

D4 and D5 are protection diodes that clamp the output voltage if something back-drives the output pin. The 300-ohm resistor helps limit the current in that case.

Contributor: Dave Tweed

Published May 2003