Transcript BJT in Saturation Mode

BJT in Saturation Mode
Section 4.5
Outline
• Modes of Operations
• Review of BJT in the active Region
• BJT in Saturation Mode
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.
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 reverse
Biased.
4. Injected holes in the base is swept
across the base-collector junction by
the electric field.
BJT Current
Assumption:
BEJ: Forward Biased
BCJ: Reverse Biased
Large Signal Model of a BJT
Called “large” signal model
because this model is
applicable even if VBE
changes from 300 mV to 800 mV
Large-Signal Model of BJT
Transistors
(NPN)
C
E
(PNP)
C
E
Experiments
Saturation Mode
BJT in Saturation Mode
(A transistor in the
active mode of
operation)
Key assumption so far:
BE=Forward Biased
BC=Reverse Biased
What happens when these assumptions are not true?
Review: Forward Bias Diode
E
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
Hole Current into the Collector
A reverse biased BCJ keeps
holes in the base.
But as BCJ becomes forward
biased, the strong electric field
which opposes of the movement
of 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 rapid
fall in β.
A Large Signal Model of the BJT
The net collector current decreases as the collector
enter into saturation
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).
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!
Design Problem
How do you get a transistor out of Saturation?
Use 2n3904 npn BJT in
Simulation
(Error!, put 2n3904 here!)
Include 2n3904 (NPN) model