Operation Bj t
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Transcript of Operation Bj t
• 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