ece321 – electronics ipzarkesh/ece321/lectures/lecture2.pdf · ece321 - lecture 2 university of...

ECE321 - Lecture 2 Slide: 1University of New Mexico

Office: ECE Bldg. 230B

Office hours: Tuesday 2:00-3:00PM or by appointment

E-mail: [email protected]

Payman Zarkesh-Ha

ECE321 – Electronics I

Lecture 2: Diode Circuit Analysis:

Approximation Method

mailto:[email protected]


❑ Brief Review of Logic Gates

❑ Brief Review of Circuits

❑ Circuit Analysis with Diodes

Review of Last Lecture


Today’s Lecture

❑ Diode Circuit Analysis

● Exact solution (review of last lecture)

● Ideal diode approximation

● Realistic diode approximation


Diode in a Circuit: Exact Solution

5 V

1 KΩ

Assume that Vth is about 26mv and Is is 1pA. Find ID and

VD (Q-point).

ID

Answer: Q-point (4.42mA, 0.58V)


Numerical Solutions


Ideal Diode Approximation


Ideal Diode Approximation

If diode is forward-biased, voltage across diode

is zero. If diode is reverse-biased, current

through diode is zero.

vD =0 for iD >0 and iD =0 for vD < 0

Thus diode is assumed to be either on or off.

Analysis is conducted in following steps:

• Guess diode’s region of operation from circuit.

• Analyze circuit using diode model appropriate

for assumed operation region.

• Check results to check consistency with

assumptions.


Ideal Diode Circuit Analysis Process

1) Make initial assumption for diode region of operation (i.e.

forward or reverse)

2) Redraw the circuit with assumed diode model:

● Close if forward

● Open if reverse

3) Analyze the circuit and confirm your assumptions:

● If forward (close) then ID>0

● If reverse (open) then VD<0

4) If assumptions are correct, then done!

5) Otherwise make new assumptions and repeat


Example: Two Diode Circuit Analysis

Use ideal diode approximation to solve the following circuit:


Example: Ideal Diode Approximation

5 V

1 KΩ

Find ID and VD (Q-point).

ID

Exact answer: Q-point (4.42mA, 0.58V)

Ideal diode approximation: Q-point (5.00mA, 0.0 V)


Realistic Diode Approximation


Realistic Diode Circuit Analysis Process

1) Make initial assumption for diode region of operation (i.e.

forward or reverse)

2) Redraw the circuit with assumed diode model:

● Voltage source (VON) if forward

● Current source (IS) if reverse

3) Analyze the circuit and confirm your assumptions:

● If forward then ID>0

● If reverse then VD<0

4) If assumptions are correct, then done!

5) Otherwise make new assumptions and repeat


Example: Two Diode Circuit Analysis

Use realistic diode approximation to solve the following circuit:

Assume VON=0.7V and IS=10nA.


Example: Constant Voltage Diode Model

5 V

1 KΩ

Assume that VD,on=0.7V. Find ID and VD (Q-point).

ID

Exact answer: Q-point (4.42mA, 0.58V)

Constant voltage diode model: Q-point (4.30mA, 0.70 V)


❑ Typically, we use approximation technique to solve diode

circuits

❑ If only IS is given, it is expected to use exact exponential model

❑ If VON is given, then it is expected to use approximation method:

● If IS is given, then use the current source model for reverse

bias

● If IS is not given, then use open circuit for reverse bias

Few Points

ece321 – electronics ipzarkesh/ece321/lectures/lecture2.pdf · ece321 - lecture 2 university of...

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