Diode As Circuit Elements

Section 3.1-3.3

Key Concepts

• Diode models– Exponential model

• Derivation of n

– Ideal model– Constant-voltage model

Choosing a Diode Model

Use the ideal model to develop a quick, rough understanding of a circuit.

If the ideal model is not adequate, uses the constant voltage model, which issufficient for most cases.

Occasionally, we will use the exponential model

IS=Reverse Saturation=leakage current

Obtain an Exponential Diode Model Empirically

Ideal Model of a Diode

(exponential model)(ideal model)

An ideal diode will turn on even for the slightest forward bias voltage.(VD≥0)An ideal diode will turn off even for the slightest reverse bias voltage.(VD<0)

Behavior of Ideal Diode

Ideal diode:Vanode>Vcathode: Diode is onVanode<Vcathode: Diode is offAn ideal current experieincing Vanode=Vcathode, carries no current

I/V Characteristics

An Open—can’t get a current to flow.

A short--can’t get aV to develop across a diode. A diode

Vanode>Vcathode: Diode is onVanode<Vcathode: Diode is offAn ideal current experieincing Vanode=Vcathode, carries no currentIn practice, consider a slightly positive or negative voltage to determine the response of a diode.

Example 1: An OR Gate Realized By Diodes

Analysis of an OR Gate

Observations:1.If D1 is on, VA=VOUT and VOUT=“1”2.If D2 is on, VB=VOUT and VOUT=“1”.3.VOUT is 0 if and only if D1 and D2 are “0”

This is an OR gate.

Logic 1=3 VLogic 0=0V

Example of an OR Gate

VA=3 VVB=0 VVOUT=2.424 V≈3V

In Class Exercise

VA

VB

In Class Exercise

VA

VB

If VA=“0”, node 1 is “0”.If VB=“0”, node 1 is “0”.If both VA and VB are “1” or 3V, no current can flow through R1, node 1 is 3 V. So we have an AND gate.

Assume node 2 is 3V.

Simulation Example

VA=3VVB=0VVOUT=0.575 V

Simulation Example

VA=3VVB=3VVOUT=3 V

In-Class Exercise

What is logic implemented by A, B and LED?

Solution

0 V

0 V

Basic Gates

Decimal to Binary Converter

Big idea: If you deprive an LED with current,it will turn off.

If you provide an LED withcurrent, it will turn on.

If VD is less than VD, On, the diode behaves like an open circuit.The diode will behave like an open circuit for VD=VD,on

Constant Voltage Model

Analysis of an OR Gate

VA=3V, VB=0VObservations:D1 is ON. VD1=0.7VSo VOUT=2.3 VD2 is OFF

Logic 1=3 VLogic 0=0V

Example of an OR Gate

VA=3 VVB=0 VVOUT=2.424 V≈3V (ideal diode model) ≈2.3 V (constant voltage model)

Cascade Two Stages of AND gates

Cascade an AND with an OR

Limitations of Diode Logic Circuit

Vout=1.23 V

Level Shift

Vout of 606 mV

Vout can depend on input voltage combination!