Question

The figure shows a circuit containing two diodes \(D_1\) and \(D_2\) with threshold voltages \(V_{TH}\) of 0.7 V and 0.3 V, respectively. Considering the simplified diode model, which assumes diode I–V characteristics as shown in the plot on the right, the current through the resistor \(R\) is ......... µA. 

Hint:

In circuits with diodes of different threshold voltages, identify the conducting diode by checking the polarity and voltage magnitude. Use simplified diode model \(V = V_{TH}\) when conducting.

Answer 1

Correct Answer: 97

Step 1: Analyze the diode configuration.
The two diodes are connected in opposite directions across the voltage source and resistor. One diode conducts when the applied voltage exceeds its threshold, while the other remains reverse-biased.

Step 2: Determine the effective voltage across the resistor.
When \(D_1\) (0.7 V) is forward-biased, \(D_2\) (0.3 V) will be reverse-biased since their orientation is opposite. Hence, total voltage drop across both diodes is approximately: \[ V_{D1} + V_{D2} = 0.7 + 0.3 = 1.0 \, \text{V} \] Effective voltage across the resistor: \[ V_R = 10 - 1.0 = 9.0 \, \text{V} \]

Step 3: Calculate current through the resistor.
\[ I = \frac{V_R}{R} = \frac{9.0}{100 \times 10^3} = 90 \, \mu\text{A} \] However, since both diodes don't conduct simultaneously (only one forward conducts), the actual current corresponds to one conduction path: \[ I = \frac{(10 - 0.7 - 0.3)}{100 \times 10^3} = \frac{9}{100000} = 90 \, \mu\text{A} \]

Step 4: Conclusion.
Thus, the current through the resistor is \(90 \, \mu\text{A}\).