Question

If $V_A$ and $V_B$ are the input voltages (either $5V$ or $0V$ ) and $V_o$ is the output voltage then the two gates represented in the following circuit (A) and (B) are:-

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

Answer 1

Answer: (B)

$V_A=5V\Rightarrow A=1$
$V_A=0V\Rightarrow A=0$
$V_B=5V\Rightarrow B=1$
$V_B=0V\Rightarrow B-0$
If $A=B=0$, there is no potential anywhere here $V_0=0$
If $A=1,B=0$, Diode $D_1$ is forward biased, here $V_0=5V$
If $A=0,B=1$, Diode $D_2$ is forward biased hence $V_0=5V$
If $A=1,B=1$, Both diodes are forward biased hence $V_0=5V$
Truth table for $I^{st}$

A	B	Output
0	0	0
0	1	1
1	0	1
1	1	1

$\therefore$ Given circuit is OR gate
For $I{I}^{\text{nd }}$ circuit
$V_B=5V,A=1$
$V_B=0V,A=0$
When $A=0,E-B$ junction is unbiased there is no current through it
$\therefore V_0=1$
When $A=1,E-B$ junction is forward biased
$V_0=0$
$\therefore$ Hence this circuit is not gate.