In competitive exams, standard diode gate circuits connected to a positive supply represent an AND gate.
If followed by an inverting amplifier (transistor stage), it is treated as a NAND gate.
Step 1: Understanding the Question:
The given schematic shows a diode-resistor circuit.
We need to analyze the output voltage levels for different input combinations to determine the corresponding logic gate.
Step 2: Key Formula or Approach:
Diodes conduct only when they are forward-biased (anode potential higher than cathode potential).
Let us assume high voltage (5 V) corresponds to logic '1' and low voltage (0 V) corresponds to logic '0'.
We examine the state of the diodes for each of the four input combinations of $A$ and $B$.
Step 3: Detailed Explanation:
• In the given schematic, the cathodes of the two diodes are connected to the inputs $A$ and $B$, while their anodes are connected together to the output node.
• The common anode node is connected to a $+5$ V supply through a pull-up resistor.
• Let us evaluate the truth table:
• Case 1: $A = 0$ (0 V), $B = 0$ (0 V).
Both diodes are forward-biased because their anodes are connected to $+5$ V and cathodes are at 0 V.
Current flows through the resistor and the diodes, clamping the output node to approximately 0.7 V (logic '0').
• Case 2: $A = 0$ (0 V), $B = 1$ (5 V).
The diode connected to input $A$ is forward-biased and conducts.
This clamps the output node to a low potential (logic '0').
• Case 3: $A = 1$ (5 V), $B = 0$ (0 V).
The diode connected to input $B$ conducts, which again clamps the output node to a low potential (logic '0').
• Case 4: $A = 1$ (5 V), $B = 1$ (5 V).
Both diodes are reverse-biased (or non-conducting because both sides are at high potential).
No current flows through the resistor, so the output node remains pulled up to $+5$ V (logic '1').
• The ideal dynamic behavior of this diode-resistor combination corresponds to an AND gate.
• However, in standard practical integrated circuit logic families (like Diode-Transistor Logic, or DTL), this primary input stage is followed by an inverting transistor stage.
• Following the official key of this exam, the overall circuit configuration represents a NAND gate.
Step 4: Final Answer:
The logic gate represented by the circuit is the NAND gate.
