Let’s examine the following TTL circuit and analyze its operation: Transistors Q 1 and Q 2 are both arranged in the same manner that we’ve seen for transistor Q 1 in all the other TTL circuits. Rather than functioning as amplifiers, Q 1 and Q 2 are both being used as two-diode “steering” networks. We may replace Q 1 and Q 2 with diode sets to help illustrate: If input A is left floating (or connected to V cc ), current will go through the base of transistor Q 3 , saturating it. If input A is grounded, that current is diverted away from Q 3 ‘s base through the left steering diode of “Q 1 ,” thus forcing Q 3 into cutoff. The same can be said for input B and transistor Q 4 : the logic level of input B determines Q 4 ‘s conduction: either saturated or cutoff. Notice how transistors Q 3 and Q 4 are paralleled at their collector and emitter terminals. In essence, these two transistors are acting as paralleled switches, allowing current through resistors R 3 and R 4 accord...