Diode

Chap. 4 Analysis of Nonlinear Circuits

Diode: Nonlinear Element Analytical Solutions

Graphical Solutions Piecewise Linear Analysis

Diode: Nonlinear resistor

+ v -

i

(a) Diode circuit symbol (b) characteristic equation

) 1

( 



^TH

D

V v S

e I

i

V V

I_S ~ 10^12, _TH  0.025

(c) i-v characteristic 0.7

i

v

Circuit Example: Analytical Method

 Node method

– Solve the equation by

• Trial and error, or

• Numerical method

E +

R

iD



 vD











 

0 0

2 0

D D D

D for v

v for i Kv

Another Example

 Step 1: Simplify the circuit using Thevenin’s theorem

 Step 2: Solve for the nonlinear element

 Step 3: Solve for the other elements

V +

R1 i^D

R2 Io R3

Graphical Method

 Load line:

Example 4.10 Diode Rectifier

This is a non-linear circuit with a zenor diode. The i-v characteristic of the zenor diode is shown below. Find the minimum R to make the output voltage equal to 5V.

v -

R_L +

10V

1k

A B

D i (a)

i +

-

v v

i V_Z=-5V

(b)

Exercise

Piecewise Linear Analysis

 (1) Idea Diode Model

Circuit containing ideal diode

Determine the conduction state

Example

















10 ,

10 ,

5

, 11 ,

12

3 2

1

R R

R

V V

V

V

_{S B}

Revisit: Halfwave Rectifier

Offset Diode Model

V i

Von = 0.6V

How will the Rectifier output change with this model?

Exercise

D1 D2

10k 10V

-10V 5k

+ vo -

What would happen if we switch two resistor values?

Piecewise linear diode model

T D

D

d

V

I v

i

r 



 

1

Example

1

10 

v_s +

Incremental Analysis

 Motivational Example

How to solve this problem?

 Insight

 Solution: biasing

Solution

Incremental Method: small signal method

 Operate at some DC offset: bias point

 Superimpose small signal on top of DC offset

 Solve the small signal circuit

response to small signal is (approximately) linear

) ,

( V

_D

I

_D

D D

D

D D

D D

D

i I

i

v V

v v

f i













 ( ),

In our example

 Operating point

 Small signal

Small Signal Circuit

Example

 Draw the small signal circuit

E +

R

iD



 vD

V V

V mA K

v for

v for i Kv

D

D D D

D

1 / 1

0 0

0

2 2















 

Conclusion

 Nonlinear element: Diode is an example

 Solution methods

– Analytical method – Graphical method

– Piecewise linear method

 Incremental analysis: small signal analysis