Analog Circuits Questions and Answers – Characteristic Equation of Diode-2

This set of Analog Circuits Questions and Answers for Freshers focuses on “Characteristic Equation of Diode 2”.

1. Find the current I if both diodes are identical. Voltage V = 0.8V and let the reverse saturation current be 10^-9A.

a) 4.8mA
b) 3.2mA
c) 2.5mA
d) 7mA
View Answer

Answer: a
Explanation: Since both diodes are identical V_D = 0.8/2 = 0.4V
Equation for diode current

where I₀ = reverse saturation current
η = ideality factor
V_T = thermal voltage
V = applied voltage
Since in this question ideality factor is not mentioned it can be taken as one. Take V_T as 0.026 which is the standard value.
Hence current through one diode is 10^-9x(e^0.4/0.026) = 4.8mA.

2. Find voltage VOUT if the reverse saturation current of the diode is 1.1×10^-8A, the cut-in voltage of diode is 0.6V and assume the temperature as 25^oC.

a) 0.235V
b) 0.3148V
c) 0.456V
d) 0.126V
View Answer

Answer: b
Explanation: Equation for diode current

where I₀ = reverse saturation current
η = ideality factor
V_T = thermal voltage
V = applied voltage
Since in this question ideality factor is not mentioned it can be taken as one. Take V_T as its standard value 0.026V.
Voltage V_D = ηV_Tln(I/I_O + 1) = 1 x 0.0257ln (2 x 10^-3/(1.1 x 10^-9))+1) = 0.0257ln(1,818,182) = 0.3704 V.

3. The current Ix in the circuit is 1mA then find the voltage across diode D1. The resistance R1 is 1KΩ. Assume the reverse saturation current is 10^-9A. Voltage across resistor in this condition was 0.4V. Take V_T of diode as 0.026V.

a) 2.3mA
b) 3.2mA
c) 5.2mA
d) 4.6mA
View Answer

Answer: c
Explanation: Since voltage drop across diode is 0.4V current through resistor is 0.4/1k = 0.4mA
Current through diode

where I₀ = reverse saturation current
η = ideality factor
V_T = thermal voltage
V = applied voltage
Since ideality factor is not given take it as one.
Current through diode I = 10^-9 x (e^0.4/0.026) = 4.8mA
Total current = 4.8mA+0.4mA = 5.2mA.

4. Find the current Ix if the voltage across the diode is 0.5V. The reverse saturation current of diode 10^-11A, the cut-in voltage of diode is 0.6V. Assume that the temperature at which diode operates is 25°C. The resistance R1=2KΩ.

a) 3.97mA
b) 4.51mA
c) 2.45mA
d) 3.05mA
View Answer

Answer: d
Explanation: Since voltage drop across diode is 0.5V current through resistor is 0.5/2k = 0.25mA
Current through diode

where I₀ = reverse saturation current
η = ideality factor
V_T = thermal voltage
V = applied voltage
Since ideality factor is not given take it as one.
Current through diode I = 10^-11 x (e^0.5/0.0257) = 2.8mA
Total current = 2.8mA + 0.25mA = 3.05mA.

5. If the current I is 2mA then find the temperature at which diode operates. The cut-in voltage of diode is 0.6V. The reverse saturation current of diode is 10^-9A. Resistance R is 1.3KΩ.

a) 45.85°C
b) 50.47°C
c) 60.26°C
d) 56.89°C
View Answer

Answer: a
Explanation: Equation for diode current

where I₀ = reverse saturation current
η = ideality factor
V_T = thermal voltage
V = applied voltage
Since in this question ideality factor is not mentioned it can be taken as one.
V_D = 3-(2mAx1.3k) = 0.4V

V_T = T/11600
Thus Temperature, T = 11600×0.0275 = 319 Kelvin = 45.85°C.

6. If the temperature increases 10°C, the ratio of final reverse saturation current to initial reverse saturation current is _______
a) 1
b) 2
c) 1.5
d) 3
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7. The reverse saturation current of a diode at 25°C is 1.5 x 10^-9A. What will be the reverse current at temperature 30°C?
a) 3 x 10^-9A
b) 2 x 10^-9A
c) 2.12 x 10^-9A
d) 1.5 x 10^-9A
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8. How much times reverse saturation current will increase if temperature increases 15^oC?
a) 2.52
b) 4.62
c) 4.12
d) 2.82
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9. The input voltage V1 of the circuit the circuit is 2V and resistor has a resistance of 1KΩ. The cut-in voltage of the silicon diode is 0.7V and the reverse saturation current is 10^-8A. The temperature at which diode operates is 30°C. The voltage across resistor when diode starts conducting is _________________

a) 0.7V
b) 1.3V
c) 0.306V
d) 1.7V
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10. If V1 is 5V and resistance R₁ is 5KΩ and the cut-in voltage of the diode is 0.7V, what will be the voltage V_out across the diode? Take reverse saturation current as 10^-8A and operating temperature as 25°C.
a) 0V
b) -4.5V
c) -5V
d) -3.2V
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