Question 4: Semiconductors and Logic Gates

Question

Assuming in forward bias condition there is a voltage drop of 0.7 V across a silicon diode, the current through diode $D_1$ in the circuit is $\_\_\_\_$ mA .
(Assume all diodes in the given circuit are identical)

Accepted Answer

For diodes $\mathrm{D}_1$ and $\mathrm{D}_3$, the p -side (anode) of these diodes is connected toward the positive terminal of the 12 V source. Therefore, $D_1$ and $D_3$ are forward biased.
In diode $D_2$ the $n$-side (cathode) of this diode is connected toward the positive terminal. Therefore, $D_2$ is reverse biased and acts as an open circuit (no current flows through it).
Since $\mathrm{D}_1$ and $\mathrm{D}_3$ are in forward bias and are silicon diodes, the voltage drop across them is fixed at $\mathrm{V}_{\mathrm{d}}=0.7 \mathrm{~V}$. Because they are in parallel, the voltage at the junction after resistor $R_1$ is 0.7 V relative to the ground.
The total current flows through the resistor $\mathrm{R}_1=0.3 \mathrm{k} \Omega$. Using Ohm's Law :
$$\Rightarrow $$ $$ V_{R 1}=V_{	ext {source }}-V_d $$
$$ \mathrm{V}_{\mathrm{R} 1}=12 \mathrm{~V}-0.7 \mathrm{~V}=11.3 \mathrm{~V} $$
So, the total current is,
$$ \mathrm{i}_{	ext {total }}=\frac{\mathrm{V}_{\mathrm{R} 1}}{\mathrm{R}_1}=\frac{11.3}{0.3 \mathrm{k} \Omega} $$
$$\Rightarrow $$ $$ \mathrm{i}_{	ext {total }} \approx 37.7 \mathrm{~mA} $$
The total current $I_{	ext {total }}$ reaches the parallel junction and splits between the two forward-biased diodes, $D_1$ and $\mathrm{D}_3$. Since the diodes are identical, the current splits equally:
$$ \mathrm{i}_{\mathrm{D} 1}=\frac{\mathrm{I}_{\mathrm{total}}}{2} $$
$$\Rightarrow $$ $$ \mathrm{i}_{\mathrm{D} 1}=\frac{37.7}{2} \approx 18.8 \mathrm{~mA} $$
Hence, the correct option is (B).

Answer

17.6

Answer

18.8

Answer

20.15

Answer

11.7

Semiconductors and Logic Gates

Assuming in forward bias condition there is a voltage drop of 0.7 V across a silicon diode, the current through diode D_1 in the circuit is \_\_\_\_ mA . (Assume all diodes in the given circuit are identical)

Choose the correct answer: