Question

In the following circuits p-n junction diodes $D_1,D_2$ and $D_3$ are ideal. For the following potential of A and B, the correct increasing order of resistance between A and B will be
i. $-10V,-5V$
ii. $-5V,-10V$
iii. $-4V,-12V$

• A. (i) < (ii) < (iii)
• B. (iii) < (ii) < (i)
• C. (iii ) = (ii) < (i)
• D. (i) = (iii) < (ii)

Answer 1

The correct option is C (iii ) = (ii) < (i)

For case- (i): ($-10V,-5V$)
Here, terminal $A<$ terminal $B$

so, $D_1=$ Reverse bias
$D_2=$ Forward bias
$D_3=$ Reverse bias
hence total resistance of the circuit will be
$R_{eq}=\frac{R}{2}+R+\frac{R}{2}=\frac{3R}{2}$

For case-(ii): ($-5V,-10V$)
Here, terminal $A>$ terminal $B$
so the current will flow from terminal $A$ to $B.$
where, $D_1=$ Forward bias
$D_2=$ Reverse bias
$D_3=$ Forward bias

hence, $R_{eq}=\frac{R}{4}+\frac{R}{2}+\frac{R}{4}=R_{eq}=R$

For case-(iii): ($-4V,-12V$)
Here, terminal $A>$ terminbal $B$
so the current will flow from terminal $A$ to $B.$

where, $D_1=$ Forward bias
$D_2=$ Reverse bias
$D_3=$ Forward bias

hence, $R_{eq}=\frac{R}{4}+\frac{R}{2}+\frac{R}{4}\Rightarrow R_{eq}=R$

So, from above calculation $\text{(iii) = (ii) < (i)}$
Hence, option C is correct.