Question 8-10A3

Fig. 8A6 commonly represents the input stage of a Transistor-Transistor Logic (TTL) NAND gate. In this circuit, Q1 is a multi-emitter transistor whose base is connected to a resistor from Vcc, and its collector drives the base of Q2. Let's analyze option B: "With either A or B low, Q1 is saturated and Q2 is off." If either input A or B (or both) is pulled low, the corresponding base-emitter junction of Q1 becomes forward-biased. This causes current from Vcc and Q1's base resistor to flow out through the low input terminal. This action effectively shunts current away from Q1's collector. With insufficient base current, **Q2 turns off**. Q1's base-emitter junction is forward-biased and conducting, a state often described as being "saturated" in its base-emitter path. Thus, option B is true. Let's quickly look at other options: * **A) With A and B high:** Current flows from Q1's base, through its collector, to the base of Q2, turning Q2 ON. So, Q2 is not off, making A incorrect. * **C) With A and B low:** As explained for B, Q2 is OFF. Thus, C is incorrect. * **D) With either A or B low:** Q1 is conducting from base to emitter (not off), and Q2 is OFF. Thus, D is incorrect.