ECE 201 Circuit Theory I 1

Voltage and Current Sources

Circuit Symbols

Vbattery12 V

Isource1 A

ECE 201 Circuit Theory I 2

“IDEAL” or “Independent” Sources

• IDEAL voltage source– maintains the

prescribed voltage across its terminals regardless of the current drawn from it

• IDEAL current source– maintains the

prescribed current through its terminals regardless of the voltage across those terminals

Vs12 V

LOAD

Z=A

+jB

LOADCurrent

DC 1e-009Ohm

0.012 A+ -

LOAD

Z=A

+jB

Is1 A

LOADVoltageDC 1MOhm 999.001 V

+

-

ECE 201 Circuit Theory I 3

“Controlled” or “Dependent” Sources

• A voltage or current source whose value “depends” on the value of voltage or current elsewhere in the circuit

• Use a diamond-shaped symbol

Vs1 V/V Vs=uVxVx

+

-

+

-

ECE 201 Circuit Theory I 4

Voltage-Controlled Voltage Source VCVS

• Vs is the “output” voltage

• Vx is the “reference” voltage

• u is the “multiplier”

• Vs = uVx

Vs1 V/V Vs=uVxVx

+

-

+

-

ECE 201 Circuit Theory I 5

Current-Controlled Voltage Source CCVS

• Vs is the “output” voltage

• Ix is the “reference” current

• p is the multiplier

• Vs = pIx

Vs1 Ohm Vs=pIx

-

+

Ix

ECE 201 Circuit Theory I 6

Voltage-Controlled Current Source VCCS

• Is is the “output” current

• Vx is the “reference” voltage

• a is the multiplier

• Is = aIx

Is1 Mho Vx

+

-

Is = aVx

ECE 201 Circuit Theory I 7

Current-Controlled Current Source CCCS

• Is is the “output” current

• Ix is the “reference” current

• b is the multiplier

• Is = bIx

Is1 A/A Ix Is=bIx

ECE 201 Circuit Theory I 8

Common-Emitter Transistor Amplifier

Q1

2N2222A

R11kOhm

R247kOhm

Rcollector3.3kOhm

Remitter560 Ohm

VCC

12V

BASE

COLLECTOR

EMITTER

Cc1

1uF

Cc2

1uF

Rload1kOhm

V1

1 V 1kHz 0Deg Ce

1uF

ECE 201 Circuit Theory I 9

The transistor and its equivalent circuit

Q

2N2222ABASE

COLLECTOR

EMITTER

I11 A/A

rbe

1kOhm COLLECTOR

EMITTER

BASE

ECE 201 Circuit Theory I 10

Substituting the equivalent circuit for Q1

R11kOhm

R247kOhm Rcollector

3.3kOhm

Remitter560 Ohm

VCC

12V

BASECOLLECTOR

EMITTER

Cc1

1uF

Cc2

1uF

Rload1kOhm

V1

1 V 1kHz 0Deg Ce

1uF

BETAib50 A/A

rbe

1kOhm

ib --->

ECE 201 Circuit Theory I 11

Active and Passive Elements

• Active element– a device capable of

generating electrical energy

• Voltage source• Current source• Power source

– Batteries– Generators

– a device that needs to be powered (biased)

• transistor

• Passive element– a device that absorbs

electrical energy• Resistor• Capacitor• Inductor• motor• light bulb• heating element