Question

If \(V_A\) and \(V_B\) are the input voltages (either 5 V or 0 V) and \(V_o\) is the output voltage then the two gates represented in the following circuits (A) and (B) are :
\begin{center} \includegraphics[width=0.5\textwidth]{logic_gates} \end{center}

• A. NAND and NOR Gate
• B. AND and OR Gate
• C. AND and NOT Gate
• D. OR and NOT Gate

Hint:

Diodes with cathodes joined together typically form OR gates (positive logic). A single transistor with input at the base and output at the collector is almost always a NOT gate.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Logic gates can be constructed using electronic components like diodes and transistors. Diodes act as switches for specific voltage levels, while transistors can invert signals in a common-emitter configuration.
Step 2: Detailed Explanation:
Circuit (A): This circuit uses two diodes in parallel connected to a resistor leading to ground. - If \(V_A = 5V\) or \(V_B = 5V\), the corresponding diode becomes forward biased, allowing current to flow. The output \(V_o\) becomes high (\(\approx 5V\)). - The output is low (\(0V\)) only when both inputs are low. This is the logic for an OR gate.
Circuit (B): This is a standard NPN transistor inverter circuit. - If \(V_B = 5V\) (High), the transistor is switched ON (saturated), effectively connecting the output \(V_o\) to ground. Thus, \(V_o = 0V\) (Low). - If \(V_B = 0V\) (Low), the transistor is switched OFF, and \(V_o\) is pulled up to 5V through the collector resistor \(R_C\). - This inversion of logic defines a NOT gate.
Step 3: Final Answer:
Circuit (A) is an OR gate and Circuit (B) is a NOT gate.