PNP load switching circuit that uses opposing PNP transistors
Q3 turns Q4 into a constant-current source, with the current limited to approximately 0.65V divided by the value of R2. If Q4 tries to draw more current than this, then Q3 "steals" its base current.

In this application, that constant current produces the desired voltage drop across R7 &mdash; about 2.2V. This voltage, when added to the VCE(SAT) of Q1 (about 200-300 mV) becomes the desired 2.5V output.

You end up with four output states:

 R1 input    R3 input    output
 --------    --------    ------
    low         low       5.0V     Q1 cut off, Q4 turned on
    low        high       2.5V     Q1 saturated, Q4 in current limiting
   high        high       0.0V     Q1 saturated, Q4 cut off
   high         low       hi-Z     Q1 cut off, Q4 cut off