• Current flow• DC gain

• Voltages in transistor circuits• I-V Characteristics curves

• Load line

Operation of Common Emitter Amplifiers

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vin

IB

IC

IE

VCC

VCE

VBE

RB

RC

Current flowHOME

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Common Emitter Amplifier Circuit

• There are three different currents in transistors : collector current IC, base current IB and emitter current IE.

• Since the emitter is the source of the electrons, it has the largest of the three currents.

• The collector current is slightly lower than the emitter current, while the base current is usually very small.

• The relationship is expressed as follows:

IE = IB + IC

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Current flow

• The relationship between the collector current and the base current is expressed in the following formula

• β = IC / IB

• Current gain(HFE)• The ratio between collector current and emitter current

is referred as α (alpha) and is shown in the following relationship :

• α = IC/IE

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DC Gain

• Emitter voltage,VE

• Base voltage,VB

• Collector voltage,VC

• Collector emitter voltage,VCE

• Base emitter voltage,VBE

• Collector Base voltage,VCB

• DC supply voltage,VCC

VCCVCE

VBEVC

VB

VE

RB

RC

B

C

E

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Voltages in transistor circuits

I - V characteristics are used to explain the operation of transistors.

Output I-V characteristics curvesInput I-V characteristics

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I-V Characteristics

• A load line is a line drawn over the collector curves to show each and every possible operating point of a transistor.

Load line

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Load Line

saturation

active region

Cut-off

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Operating region of transistors

Transistor can be in either saturation, cut-off or active state.

• It represents normal operation for transistors and graphically, the active region is the middle part of the load line.

• The emitter base junction is forward biased and the collector base junction is reversed biased. • The collector is gathering all the electrons into the base. As such, changes in the collector voltage will have no effect on the emitter current.

P

N

N

IB

IC

IE

VCC > VBE

VBE

VCC

C

B

E

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Active Region

• The collector current reaches its maximum value and will not change even though the base current increases.

• The value for collector emitter voltage is almost zero. The base emitter junction is forward biased and the collector base junction is reversed biased.

• Transistors must operate in this region to function as a close switch.

N

N

P

IB

IC(sat)

IE

VCC < VBE

VBB= VBE

C

BVCC

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Saturation Region

• The value of collector current is almost zero and the value of collector emitter voltage equates the value of DC voltage supply,VCC.

• Both base emitter junction and collector base junction are reversed biased.The widening depletion region will not allow current to flow from the collector to the emitter.

• IC becomes zero and transistors will then function as an open switch.

N

N

P

IB = 0

IC = 0

IE = 0

VBB

VCC

C

B

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Cut off Region

• The point at which the base current intersects the load line is referred to as the operating point or the quiescent point.

QHOME

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Q point