BJT in Saturation Mode

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BJT in Saturation Mode

• Section 4.5

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Schedule
9 2/11 Tuesday Physics of a BJT 4.1-4.3
L 2/11 Tuesday Measure Beta of a transistor  
10 2/13 Thursday PNP 4.3, 4.5-4.6
11 2/18 Tuesday BJT in saturation 4.5
L 2/18 Tuesday BJT in saturation/BJT implementation of an NAND gate  
12 2/20 Thursday Small signal model homework small eq. circuit
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Outline

• Modes of Operations
• Review of BJT in the active Region
• BJT in Saturation Mode
• Digital Integrated Circuits

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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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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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Review
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Review

• PN Junction
• Reverse Bias
• Forward Bias
• BJT in the Active Mode

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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 via diffusion Greater
diffusion current. Current flows from P side to N
side
9
Review PN Junction under Reverse Bias
Reverse Connect the terminal to the n
side. Depletion region widens. Therefore,
stronger E. Minority carriers cross the PN
junction easily through diffusion. Current is
composed mostly of drift current contributed by
minority carriers. np to the left and pn to the
right. Current from n side to p side, the
current is negative.
E
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Operation of an NPN Transistor in the Active
Region
Electrons are injected into the BC junction
Electrons are injected into the B holes to the E.
Electrons are swept across the reversed biased BC
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Thin Base Region
The base region is made thin in order to reduce
recombination as electrons travel from BE
junction to BC junction.
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Highly Doped Emitter
In order to emphasize the current
contribution due to the electrons (which can
cross the BC junction), the emitter is heavily
doped by N type materials.
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Base Current
The proportional of hole current and electron
current is determined by dopants (ND and
NA). Even though the presence of holes are
minimized, a small number holes still must enter
through the base.
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Electrons in the Base
Electrons are swept Into the collector low
electron density at x2
Electrons injected into the base high
electron density at x1.
The electron gradient allows electrons to travel
through diffusion.
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Recombination
Recombination
Base must supply holes that will enter the
emitter and for recombination with the electrons.
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Extension of a PNP transistor
(PNP transistor)
(NPN transistor)

• Base-emitter junction is forward
• biased.
• Holes are injected into the base.
• Base-emitter junction is reverse
• Biased.
• Injected holes in the base is swept
• across the base-collector junction by
• the electric field.

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BJT Current
Assumption BEJ Forward Biased BCJ Reverse
Biased
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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
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Large-Signal Model of BJT Transistors
(NPN)
(PNP)
C
C
E
E
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Experiments
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Saturation Mode
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BJT in Saturation Mode
Key assumption so far BEForward
Biased BCReverse Biased What happens when these
assumptions are not true?
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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
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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 ß.
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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 lt400 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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Use 2n3904 npn BJT in Simulation
(Error!, put 2n3904 here!)
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Include 2n3904 (NPN) model
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A NAND Gate Implemented With NPN Transistors
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Optional Slides
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BJT Inverter
(Define the input voltage as a variable)
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Run Parametric Analysis
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Parametric Analysis
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Select a Wire to Plot
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Use Calculator to Plot
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Plot with Calculator (Under Tools)
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RTL (Resistor-Transistor Logic)
Vout
VA
VB
First introduced in 1962! (50 years ago!) What
is the logic function?
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RTL Based NOR
A B Vout
3.6 V 3.6 V 34.05 mV
3.6 V 0 V 42.59 mV
0 V 3.6 V 42.59 mV
0V 0V 3.6 V
NOR is an universal gate! If you can build a
NOR, you can build any logic.
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Diode-Transistor Logic
This resistor allow charges to be drained from
the base
What is the logic function?
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Sweep VB
VS the input voltage at which the output is
approximately 2V. VS2V Condition VA4V, VC4V.
VB is swept from 0 to 4V
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Diode-Transistor Logic
This resistor allow charges to be drained from
the base
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Sweep VB
Fixed VA4V VCC4V Sweep VB from 0 to 4 V
Increase the VS by about one diode drop.
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Basic TTL Gate
Diode is replaced by TTL A relative of 7400LS
Gate
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Sweep VB
Fixed VA4V VCC4V Sweep VB from 0 to 4 V
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7400 NAND Gate
7400 Schematic We will revisit this schematic in
a couple of weeks!