Problem Sheet
Diode Circuits
1. Calculate the diode current in the circuit below assuming that the diode
a) is an ideal diode,
Answer: 1 mA.
b) has a forward voltage of 0.7 V (constant voltage model).
Answer: 0.93 mA.
c) can be represented by a piecewise linear model with V0 = 0.65 V and Rf = 20 Ω.
Answer: 0.93 mA.
2. Calculate the diode current in the circuit below for V = 12 V and R = 4 kΩ, assuming the diode
VD
+ −
I
V R
a) is ideal,
Answer: 3 mA.
b) has a forward voltage of 0.7 V (constant voltage model),
Answer: 2.83 mA.
c) can be represented by a piecewise linear model with V0 = 0.65 V and Rf = 20 Ω.
Answer: 2.82 mA.
3. Calculate the diode current in the circuit below for V = 100 V and R = 4 kΩ, assuming the diode
VD
+ −
I
V R
a) is ideal,
Answer: 25 mA.
b) has a forward voltage of 0.7 V (constant voltage model),
Answer: 24.8 mA.
c) can be represented by a piecewise linear model with V0 = 0.65 V and Rf = 20 Ω.
Answer: 24.7 mA.
4. Calculate the diode current in the circuit below for V = 3 V and R = 4 kΩ, assuming the diode
VD
+ −
I
V R
a) is ideal,
Answer: 0.75 mA.
b) has a forward voltage of 0.7 V (constant voltage model),
Answer: 0.58 mA.
c) can be represented by a piecewise linear model with V0 = 0.65 V and Rf = 20 Ω.
Answer: 0.58 mA.
5. What are your conclusions on the answers to questions 2-4?
6. A silicon diode has a current of 1 mA at a forward voltage of 0.7 V at room temperature (300 K).
The ideality factor of the diode is η = 2. The diode is used in the circuit below with V = 6 V and
R = 100 Ω.
a) Find the diode current and the voltage using the precise method (using the diode
equation).
Answer: 50.96 mA, 0.905 V.
VD
+ −
I
V R
b) The voltage V is decreased to 3 V. Calculate the value of resistor R to keep the current
at the same value in (a).
Answer: 41 Ω.
Repeat parts (a) and (b) assuming η = 1.
Answer: a) 55.46 mA, 0.454 V, b) 46 Ω.
7. A silicon diode with the IV characteristics shown below is used in the circuit below with V = 5 V
and R = 1 kΩ.
a) Use the graphical method to find the current in and the voltage across the diode.
Answer: Approximately 4.4 mA, 0.62 V.
b) What is the diode current and voltage if R is changed to 2 kΩ?
Answer: Approximately 2.2 mA, 0.58 V.
c) What is the diode current and voltage if R is changed to 5 kΩ?
Answer: Approximately 0.9 mA, 0.55 V.
d) With an applied voltage of 1.5, calculate R to have the diode current of 2 mA.
Answer: Approximately 450 Ω.
VD
+ −
+
I
V R VR
8. A silicon diode has a forward voltage of 0.6 V at a current of 5 mA. The forward voltage is
increased by nearly 0.025 V every time the current is doubled. The diode is used in the circuit
below with V = 6 V.
a) Design the circuit (calculate the value of R) to have a current of 20 mA in the diode.
Answer: 270 Ω.
b) Calculate the diode current when the value of the resistor is 540 Ω.
VD
+ −
I
V R
Answer: 9.95 mA.
9. A silicon diode with 𝐼0 = 1 pA and 𝜂 = 1 is biased as in the figure below.
a) By calculating the diode current at 5 different diode voltages in the range of 0.4-0.6 V,
plot the diode IV characteristics.
b) Draw the load line and estimate the current in the circuit graphically.
Answer: 4.7 mA.
10. In the circuit below, the diodes are assumed to have a constant forward voltage of 0.6 V.
Calculate the current in each diode and the voltage at point A.
Answer: 12 mA, 8.4 mA, 0 mA, 3.6 mA, 2.4 V.
