BJT in Saturation Mode Section 4.5. Outline Modes of Operations Review of BJT in the active Region...
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Transcript of BJT in Saturation Mode Section 4.5. Outline Modes of Operations Review of BJT in the active Region...
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BJT in Saturation Mode
Section 4.5
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Outline
• Modes of Operations• Review of BJT in the active Region• BJT in Saturation Mode
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Modes of Operation
BE\BC Forward Biased Reverse Biased
Forward Biased Saturation Active
Reverse Biased Reverse Active Mode
Cut-Off
Applications: 1. Saturation and cut-off mode are used in digital circuits.2. Active mode is used in the amplifier design.
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Extension of a PNP transistor
(NPN transistor) (PNP transistor)
1. Emitter-base junction is forward biased.2. Holes are injected into the base.3. Base-collector junction is reverseBiased.4. Injected holes in the base is sweptacross the base-collector junction bythe electric field.
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BJT Current
Assumption:BEJ: Forward BiasedBCJ: Reverse Biased
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Large Signal Model of a BJT
Called “large” signal modelbecause this model is applicable even if VBE
changes from 300 mV to 800 mV
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Large-Signal Model of BJT Transistors
(NPN) (PNP)
E
C
E
C
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Experiments
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Saturation Mode
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BJT in Saturation Mode
Key assumption so far:BE=Forward BiasedBC=Reverse Biased
What happens when these assumptions are not true?
(A transistor in the active mode of operation)
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Review: Forward Bias Diode
Depletion region shrinks due to charges from the battery.The electric field is weaker.Majority carrier can cross the junction via diffusion;Greater diffusion current.Current flows from P side to N side
E
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Hole Current into the Collector
A reverse biased BCJ keepsholes in the base.
But as BCJ becomes forwardbiased, the strong electric fieldwhich opposes of the movementof holes into the collector is weakened.
There is now a hole current into the collector.
Net Result: heavy saturation leads to a sharp rise in the base current and a rapidfall in β.
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A Large Signal Model of the BJT
The net collector current decreases as the collector enter into saturation
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General Rules
• As a rule of thumb, we permit soft saturation with VBC <400 mV because the current in the B-C junction is negligible, provided that various tolerances in the component values do not drive the device into deep saturation.
• For a device in soft saturation or active region, we approximate IC as Isexp(VBE/VT)
• In the deep saturation region, the collector-emitter voltage approaches a constant value called VCE, SAT (about 200 mV).
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Voltage and Current Polarities of NPN and PNP transistors
A “fat” voltage between collector and emitter voltage places a transistor in the active region!
A “skinny” voltage between collector and emitter voltage places a transistor in the active region!
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Design Problem
How do you get a transistor out of Saturation?
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Use 2n3904 npn BJT in Simulation
(Error!, put 2n3904 here!)
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Include 2n3904 (NPN) model