# If Va and Vb are the input voltages

Question:

If $\mathrm{V}_{\mathrm{A}}$ and $\mathrm{V}_{\mathrm{B}}$ are the input voltages (either $5 \mathrm{~V}$ or $0 \mathrm{~V}$ ) and $\mathrm{V}_{\mathrm{o}}$ is the output voltage then the two gates represented in the following circuit (A) and (B) are:-

1. AND and OR Gate

2. OR and NOT Gate

3. NAND and NOR Gate

4. AND and NOT Gate

Correct Option: , 2

Solution:

If $\mathrm{A}=\mathrm{B}=0$, there is no potential anywhere here $\mathrm{V}_{0}=0$

If $A=1, B=0$, Diode $D_{1}$ is forward biased, here $\mathrm{V}_{0}=5 \mathrm{~V}$

If $A=0, B=1$, Diode $D_{2}$ is forward biased hence $\mathrm{V}_{0}=5 \mathrm{~V}$

If $\mathrm{A}=1, \mathrm{~B}=1$, Both diodes are forward biased hence $\mathrm{V}_{0}=5 \mathrm{~V}$

Truth table for $\mathrm{I}^{\mathrm{st}}$

$\therefore$ Given circuit is OR gate

For $\mathrm{II}^{\mathrm{nd}}$ circuit

$\mathrm{V}_{\mathrm{B}}=5 \mathrm{~V}, \quad \mathrm{~A}=1$

$\mathrm{V}_{\mathrm{B}}=0 \mathrm{~V}, \quad \mathrm{~A}=0$

When $\mathrm{A}=0, \mathrm{E}-\mathrm{B}$ junction is unbiased there is no current through it

$\therefore \mathrm{V}_{0}=1$

When $\mathrm{A}=1, \mathrm{E}-\mathrm{B}$ junction is forward biased

$\mathrm{V}_{0}=0$

$\therefore$ Hence this circuit is not gate.