diode circuits analysis

Use the i-v characteristic in Figure 2... Problem 3: Find the Q-point for the diode in Figure 4 using a the ideal diode model and b the constant voltage drop model with Von = 0.6 V.. Fig

Problem 1:

Plot the load line and find the Q-point for the diode circuit in Figure 1 if V = 5 V and R =

10 kΩ Use the i-v characteristic in Figure 2

Figure 1

Figure 2

Solution:

4

5 10 | V 0 I 0.500 | I 0 V 5

4.5 Forward biased - V 0.5 I 0.450

10

V

Ω

1 2 3 4

1 mA

2 mA

Q-point

vD 5

Problem 2:

Find the Q-point for the circuit in Figure 3 using the ideal diode model and constant voltage drop model with Von=0.6V

Figure 3

Solution :

Ideal diode model: ID = 1V/10kΩ = 100 µA; (100 µA, 0 V)

Constant voltage drop model: ID = (1-0.6)V/10kΩ = 40.0 µA; (40.0 µA, 0.6 V)

Problem 3:

Find the Q-point for the diode in Figure 4 using (a) the ideal diode model and (b) the

constant voltage drop model with Von = 0.6 V (c) Discuss the results Which answer do you feel is most correct?

Figure 4

Solution :

Using Thévenin equivalent circuits yields and then combining the sources

0.4 V

+

-+ -V

I 2.2 k Ω

(a) Ideal diode model: The 0.4 V source appears to be forward biasing the diode so

we will assume it is "on" Substituting the ideal diode model for the forward region yields 0.4 0.182

2.2

V

k

Ω This current is greater than zero, which is consistent with the diode being "on" Thus the Q-pt is (0 V, +0.182 mA)

+

-+

-+ -V

I 1 kΩ 1.2 k Ω

2 V 1.6 V

Ideal Diode:

0.4 V

+

-+

CVD:

0.4 V +

-0.6 V

I 2.2 k Ω

+

-on

V

(b) CVD model: The 0.4 V source appears to be forward biasing the diode so we will assume it is "on" Substituting the CVD model with Von = 0.6 V yields 0.4 0.6

90.9 2.2

−

Ω This current is negative which is not consistent with the assumption that the diode is "on" Thus the diode must be off The resulting Q-pt is: (0.4 V, 0 mA)

0.4 V +

-I=0 2.2 k Ω

- V +

(c) The second estimate is more realistic 0.4 V is not sufficient to forward bias the diode into significant conduction For example, let us assume that IS = 10-15 A and assume that the full 0.4 V appears across the diode Then

0.025

D

V

V

⎣ ⎦ , a very small current

Problem 4:

(a) Find I and V in the four circuits in Figure 5 using the ideal diode model (b) Repeat using the constant voltage drop model with Von = 0.7 V

Figure 5

(a)

( ) ( )

( ) D

5 5 ( ) Diode is forward biased: = 5+0= 5 | I= 0.500

20 ( ) Diode is reverse biased: =0 | V=7 20 7 | V 10

3 7 ( ) Diode is forward biased: =3 0=3 | I= 0.500

20 ( ) Diode is reverse

k

k d

− −

Ω

− −

Ω

biased: =0 | V= 5 20I − + kΩ I = −5 | VV = −10 V

(b)

( ) ( )

( )D

5 4.3 ( ) Diode is forward biased: = 5+0.7= 4.3 | I= 0.465

20 ( ) Diode is reverse biased: =0 | V=7 20 7 | V 10

2.3 7 ( ) Diode is forward biased: =3 0.7=2.3 | I= 0.465

20 ( ) Diode

k

k d

− −

Ω

− −

Ω

is reverse biased: =0 | V= 5 20I − + kΩ I = −5 | VV = −10 V

(a) Find I and V in the four circuits in Figure 5 using the ideal diode model if the resistor

values are changed to 100 kΩ (b) Repeat using the constant voltage drop model with

Von = 0.6 V

Solution :

(a)

( )

( )

( ) Diode is forward biased: = 5+0= 5 | I= 100

100 ( ) Diode is reverse biased: =0 A | V=7 100 7 | V 10

( ) Diode is forward biased: =3 0=3 | I= 100

100 ( ) Diode is reverse

k

k d

µ

µ

− −

Ω

− −

Ω

biased: =0 A | V= 5 100I − + kΩ I = −5 | VV = −10 V

(b)

( ) ( )

( ) D

5 4.4 ( ) Diode is forward biased: = 5+0.6= 4.4 | I= 94.0

100 ( ) Diode is reverse biased: =0 | V=7 100 7 | V 10

2.4 7 ( ) Diode is forward biased: =3 0.6=2.4 | I= 94.0

100 ( ) Diod

k

k d

µ

µ

− −

Ω

− −

Ω

e is reverse biased: =0 | V= 5 20I − + kΩ I = −5 | VV = −10 V

Problem 6:

Find the Q-points for the diodes in the circuits in Figure 6 using the ideal diode model

Figure 6

Solution :

Diodes are labeled from left to right

10 0 ( ) on, D off, D on: I 0 | I 1

2.5

D : 1.00 mA, 0 V D : 0 mA, 2 V D : 1.00 mA, 0 V

D

k

−

Ω + Ω

− −

Ω

−

( ) on, D off, D off: I 0 | I 0

10 5

: 0.500 , 0 : 0 , 1.00 : 0 , 1.00

D

D

b D

− −

Ω + Ω + Ω

= − − + = −

( ) ( )

( )

12

: 1.25 , 0 : 1.05 m , 0 : 0.783 m , 0

k

− −

Ω

( ) , , : 0, 0

12 5

30

V 5 12 10000 1.33 0

: 0 , 0.667 : 0 , 1.33 : 567 , 0

D

D

V

k

µ

µ

− −

Ω

Problem 7:

Find the Q-points for the diodes in the circuits in Figure 6 using the constant voltage drop

model with Von = 0.6 V

Solution:

Diodes are labeled from left to right

( )

10 0.6 0.6 ( ) on, D off, D on: I 0 | I 1.00

0.6 5

I 1.00 I 0.760 | V 5 10 0.6 3000 1.40

2.5

D : 1.00 mA, 0.600 V D : 0 mA, 1.40 V D : 0.760 mA, 0.600V

D

k

− − −

Ω + Ω

− − −

Ω

−

( ) on, D off, D off: I 0 | I 0

10 0.6 5

V 5 12000 0.760

D : 0.480 mA, 0.600 V D : 0 A, 0.560 V D : 0 A, 0.760 V

D

D

b D

− − −

Ω + Ω + Ω

= − − + = −

( ) ( )

( )

( ) on, D on, D on

1.4 5

12

D : 1.10 mA, 0.600 V D : 0.900 mA, 0.60

V

k

− −

Ω ( 0 V D : 0.733 mA, 0.600 V) 3 ( )

( ) , , : 0, 0

11.4 5

30

V 5 11.4 10000 0.933 0

: 0 , 0.467 : 0 , 0.933 : 547 , 0

D

D

V

k

µ

µ

− −

Ω