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 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?

(A transistor in the active mode of operation)

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!

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