Schottky TTL Squaring Circuit / Active Pull-Down

The circuit is a BJT logic inverter with selectable base pulldown. When the switch (Sw) is slant (\), the base is passively pulled down by resistor Rppd. This is the usual method used in discrete transistor implementation of BJT logic inverter and it is also used in Standard TTL and other logic/interface ICs. When the switch (Sw) is vertical (|), the base is pulled down by Qsqr and associated components. The active pulldown with Qsqr as the main component is called a squaring circuit, it makes the transition between logic states steeper.