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!