The drive will typically either be holding \$IN\$ close to ground (with output HI) so that \$Q_3\$ is OFF or else by sourcing a current of about \$700\:\mu\text\approx 12\$.
The first thing to understand how the input (labeled \$IN\$ in the schematic) is actually driven. I've added the typical resistor values for TTL here, as well. Simulate this circuit – Schematic created using CircuitLab It's helpful to do so, despite having a 'nice picture.' If for no other reason, it allows others to quickly snap up and use your schematic as a starting point for adding additional notes. Draw out the schematic using the existing schematic editor that you have access to when writing out your question.
