ECE 4991 Electrical and Electronic ECE 4991 Electrical and Electronic CircuitsCircuits

Chapter 2Chapter 2

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Hitchhikers Guide to the Hitchhikers Guide to the GalaxyGalaxy

• Chapter 2 - The basic concepts and practice at analyzing simple electric circuits with sources and resistors

• Chapter 3 – More harder networks to analyze and the notion of equivalent circuits

• Chapter 4 – Capacitors and inductors added to the mix

• Chapter 5 – Analyzing transient situations in complex passive networks

• Chapter 8 – New subject – the wonders of operational amplifiers as system elements

• Chapter 9 – Introduction to semiconductors – the basics and diodes – more network analysis

• Chapter 10 – Bipolar junction transistors and how they work – now you can build your own op amp

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What’s Important in What’s Important in Chapter 2Chapter 2

1. Definitions

2. KCL & applications

3. KVL & applications

4. Electric power / power ratings

5. Ohm’s Law

6. Equivalent resistance computation

7. Voltage Dividers / Current Dividers

8. Ideal v real voltage sources

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1. Definitions1. Definitions

• Voltage

• Current

• Charge

• Power

• Energy

• Network

• Branch

• Node

• Loop

• Mesh

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2. KCL & Applications2. KCL & Applications• First, note that for current to flow,

there must be a closed circuit

• Kirchhoff’s Current Law (KCL)

in = 0

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2. Applying KCL2. Applying KCL• Find nodes

• Label currents

• Write node equations

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2 ½ . Current Speed2 ½ . Current Speed

• Metals are seas of electrons – each atom gives up an electron which can move and be part of the current

• i = dq/dt = charge density/length of wire * carrier velocity

• Therefore carrier velocity = v = i / charge density

• Charge density = e * ~3x1023/m3 = ~5x104 Coul/m3

• For a 1 amp current, v = ~ 2x10-5 m/s = ~ .02 mm/s !

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3. KVL & Applications3. KVL & Applications

• First, note that for KVL to work, there must be a closed circuit

• Kirchhoff’s Voltage Law (KVL)

vn = 0

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3. Applying KVL3. Applying KVL• Label voltages

• Pick loops

• Write loop equations

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4. Electric Power / Power 4. Electric Power / Power Ratings Ratings

• Power is work done per unit time• For an electronic element, Power is the

product of the voltage across the element and the current flowing through it

P = VI• “Positive Power” is dissipated by a load• “Negative Power” is provided by an

energy source

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4 ½. I-V Characteristics4 ½. I-V Characteristicsi

v

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5. Ohm’s Law5. Ohm’s Law

• A resistor is a passive electronic circuit element with the following property – the voltage developed across it is directly proportional to the electric current through it

V = IR• R is measured in volts per amp• 1 volt/amp = 1 ohm (Ω)• What are some ways to calculate the

power dissipated in resistor R?

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5. Working with Ohm’s 5. Working with Ohm’s LawLaw

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6. Equivalent Resistance 6. Equivalent Resistance ComputationComputation

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6. Equivalent R6. Equivalent R

• For series resistors

Req = Ri

• For parallel resistors

1 / Req = 1/Ri

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6. Equivalent R Practice6. Equivalent R Practice

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7. Voltage & Current 7. Voltage & Current DividersDividers

• A voltage divider accesses a fraction of a voltage

• A current divider accesses a fraction of a current

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8. Ideal v Real Sources8. Ideal v Real Sources

• Ideal sources are independent of loading

• Real sources have equivalent series or parallel internal resistances

I I