bjt in saturation mode section 4.5. schedule 92/11tuesdayphysics of a bjt4.1-4.3 l2/11tuesdaymeasure...
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BJT in Saturation Mode
Section 4.5
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]
Outline
• Modes of Operations• Review of BJT in the active Region• BJT in Saturation Mode• Digital Integrated Circuits
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.
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!
Review
Review
• PN Junction– Reverse Bias – Forward Bias– BJT in the Active Mode
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
Review: PN Junction under Reverse Bias
Reverse: Connect the + terminal to then side.
Depletion region widens.Therefore, stronger E.
Minority carriers cross the PN junction easilythrough diffusion.
Current is composed mostly of drift current contributedby minority carriers.
np to the left and pn to the right.
Current from n side to p side,the current is negative.
E
Operation of an NPN Transistor in the Active
Region
Electronsare injectedinto the BC junction
Electrons are injected into the B; holes to the E.
Electronsare swept acrossthe reversed biased BC
Thin Base Region
The base region is made thin in order to reduce recombinationas electrons travel from BE junction to BC junction.
Highly Doped Emitter
In order to emphasize the current contributiondue to the electrons (which can cross the BC junction),the emitter is heavily doped by N type materials.
Base Current
The proportional of hole current and electron currentis determined by dopants (ND and NA).
Even though the presence of holes are minimized, a smallnumber holes still must enter through the base.
Electrons in the Base
Electrons injected into the base; high electrondensity at x1.
Electrons are sweptInto the collector; low electron density at x2
The electron gradient allows electrons to travel through diffusion.
Recombination
Recombination
Base must supply holes that will enter the emitter and for recombination with the electrons.
Extension of a PNP transistor
(NPN transistor) (PNP transistor)
1. Base-emitter junction is forward biased.2. Holes are injected into the base.3. Base-emitter junction is reverseBiased.4. Injected holes in the base is sweptacross the base-collector junction bythe electric field.
BJT Current
Assumption:BEJ: Forward BiasedBCJ: Reverse Biased
Large Signal Model of a BJT
Called “large” signal modelbecause this model is applicable even if VBE
changes from 300 mV to 800 mV
Large-Signal Model of BJT Transistors
(NPN) (PNP)
E
C
E
C
Experiments
Saturation Mode
BJT in Saturation Mode
Key assumption so far:BE=Forward BiasedBC=Reverse Biased
What happens when these assumptions are not true?
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
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.
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!
Use 2n3904 npn BJT in Simulation
(Error!, put 2n3904 here!)
Include 2n3904 (NPN) model
A NAND Gate Implemented With NPN Transistors
Optional Slides
BJT Inverter
(Define the input voltage as a variable)
Run Parametric Analysis
Parametric Analysis
Select a Wire to Plot
Use Calculator to Plot
Plot with Calculator (Under Tools)
RTL (Resistor-Transistor Logic)
First introduced in 1962! (50 years ago!)What is the logic function?
Vout
VAVB
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.
Diode-Transistor Logic
This resistor allow chargesto be drained from the base
What is the logic function?
Sweep VB
VS: the input voltage at which the output is approximately 2V.VS~2VCondition: VA=4V, VC=4V. VB is swept from 0 to 4V
Diode-Transistor Logic
This resistor allow chargesto be drained from the base
Sweep VB
Fixed VA=4VVCC=4VSweep VB from 0 to 4 V
Increase the VS by about one diode drop.
Basic TTL Gate
Diode is replaced by TTLA “relative “ of 7400LS Gate
Sweep VB
Fixed VA=4VVCC=4VSweep VB from 0 to 4 V
7400 NAND Gate
7400 Schematic We will revisit this schematic in a couple of weeks!
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