🔍 Key Concepts

• Diode logic vs. diode–transistor logic (DTL) – look at whether a transistor is included in the circuit and what it does to the output. • How a NAND truth table compares to a NOR truth table (which inputs give a LOW or HIGH output). • How diodes wired from the inputs toward the junction behave when inputs are HIGH or LOW (which states cause current to flow and pull the junction low).

💭 Think About

• For circuit 3, trace what happens at the transistor base and the output when both A and B are HIGH, when one is HIGH and the other LOW, and when both are LOW. • Ask yourself: are the diodes being used to create an AND condition at the transistor base, or an OR condition? Then notice that the transistor and load resistor invert that result. • Compare the behavior you find to the truth tables of NAND and NOR gates to see which one matches.

✅ Before You Answer

• Verify whether a transistor is present and actually switching the output (this points to diode–transistor logic rather than pure diode logic). • Check which input combinations make the output LOW vs HIGH, and match that pattern to either NAND or NOR logic. • Confirm that the diodes are arranged so that any LOW input or any HIGH input will clamp the node in a particular direction; this tells you whether the diode network is implementing AND or OR before inversion.