Analog Circuits Questions and Answers – Characteristic Equation of Diode-1

This set of Analog Circuits Multiple Choice Questions & Answers (MCQs) focuses on “Characteristic Equation of Diode 1”.

1. Calculate the forward bias current of a Si diode when forward bias voltage of 0.4V is applied, the reverse saturation current is 1.17×10^-9A and the thermal voltage is 25.2mV.
a) 9.156mA
b) 8.23mA
c) 1.256mA
d) 5.689mA
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2. Calculate the thermal voltage when the temperature is 25°C.
a) 0V
b) 0V
c) 0.026V
d) 0.25V
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3. Calculate the reverse saturation current of a diode if the current at 0.2V forward bias is 0.1mA at a temperature of 25°C and the ideality factor is 1.5.
a) 5.5x 10^-9 A
b) 5.5x 10^-8 A
c) 5.5x 10^-7 A
d) 5.6x 10^-10 A
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Answer: c
Explanation: Equation for diode current
I=I₀×(e^(V/ηV_T) – 1) where I₀ = reverse saturation current
η = ideality factor
V_T = thermal voltage
V = applied voltage
Here, I = 0.1mA, η = 1.5, V = 0.2V, V_T = T_K/11600
Therefore, V_T at T = 25+273=298 is 298/11600 = 0.0256V.
Therefore, reverse saturation current
I_O=0.00055mA = 5.5×10^-7A.

4. Find the applied voltage on a forward biased diode if the current is 1mA and reverse saturation current is 10^-10. Temperature is 25°C and takes ideality factor as 1.5.
a) 0.658V
b) 0.726V
c) 0.526V
d) 0.618V
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Answer: d
Explanation: Equation for diode current
I=I₀×(e^(V/ηV_T)-1) where I₀ = reverse saturation current
η = ideality factor
V_T = thermal voltage
V = applied voltage
V_T at T = 25+273=298 is 298/11600 = 0.0256V, η = 1.5, I = 1mA, I₀ = 10^-10A

5. Find the temperature at which a diode current is 2mA for a diode which has reverse saturation current of 10^-9 A. The ideality factor is 1.4 and the applied voltage is 0.6V forward bias.
a) 69.65°C
b) 52.26°C
c) 25.23°C
d) 70.23°C
View Answer

Answer: a
Explanation: Equation for diode current
I=I₀×(e^(V/ηV_T)-1) where I₀ = reverse saturation current
η = ideality factor
V_T = thermal voltage
V = applied voltage
I₀ = 10^-9A, η = 1.4, V = 0.6V, I = 2mA

We know thermal voltage V_T = T_K/11600. Therefore, T_K = V_Tx11600 = 0x11600 = 342.65K = 69.65°C.

6. Consider a silicon diode with η=1.2. Find the change in voltage if the current changes from 0.1mA to 10mA.
a) 0.154V
b) 0.143V
c) 0.123V
d) 0.165V
View Answer

Answer: b
Explanation: Equation for diode current
I=I₀×(e^(V/ηV_T)-1) where I₀ = reverse saturation current
η = ideality factor
V_T = thermal voltage
V = applied voltage
η = 1.2, I2 = 10mA, I1 = 0.1mA and take V_T = 0.026V

7. If current of a diode changes from 1mA to 10mA what will be the change in voltage across the diode. The ideality factor of diode is 1.2.
a) 0.718V
b) 7.18V
c) 0.0718V
d) 0.00728V
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Answer: c
Explanation: η = 1.2, I2 = 10mA, I1 = 1mA and take V_T = 0.026V
.

8. What will be the ratio of final current to initial current of a diode if the voltage of a diode changes from 0.7V to 872.5mV. Take ideality factor as 1.5.
a) 90.26
b) 52.36
c) 80.23
d) 83.35
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Answer: d
Explanation: η = 1.5, ΔV = 0.8725V and take V_T = 0.026V

9. What will be the current I in the circuit diagram below. Take terminal voltage of diode as 0.7V and I₀as 10^-12A.

a) 2.4mA
b) 0.9mA
c) 1mA
d) 4mA
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10. Find current I through the circuit using characteristic equation of diode. The terminal voltage of each diode is 0.6V. Reverse saturation current is 10^-12A.

a) 0.84mA
b) 1.84mA
c) 2.35mA
d) 3.01mA
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