BJT in Saturation Mode Section 4.5

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

4.3, 4.5 4.6

4.5

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 Reverse Active Mode Reverse Biased Active 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 the n side.

E 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.

n p to the left and p n to the right.

Current from n side to p side, the current is negative.

Operation of an NPN Transistor in the Active Region Electrons are injected into the B; holes to the E.

Electrons are injected into the BC junction Electrons are swept across the reversed biased BC

Thin Base Region The base region is made thin in order to reduce recombination as electrons travel from BE junction to BC junction.

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.

Base Current The proportional of hole current and electron current is determined by dopants (N D and N A ).

Even though the presence of holes are minimized, a small number holes still must enter through the base.

Electrons in the Base Electrons injected into the base; high electron density at x1.

Electrons are swept Into 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 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 V BE changes from 300 mV to 800 mV

Large-Signal Model of BJT Transistors (NPN) (PNP) C E C E

Experiments

Saturation Mode

BJT in Saturation Mode 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 fall in β .

in the base current and a rapid

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 I C as I s exp(V BE /V T ) In the deep saturation region, the collector-emitter voltage approaches a constant value called V CE, 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) Vout VA VB First introduced in 1962! (50 years ago!) What is the logic function?

RTL Based NOR A 3.6 V 3.6 V 0 V 0V B 3.6 V 0 V 3.6 V 0V Vout 34.05 mV 42.59 mV 42.59 mV 3.6 V NOR is an universal gate! If you can build a NOR, you can build any logic.

Diode-Transistor Logic What is the logic function?

This resistor allow charges to be drained from the base

Sweep VB VS: the input voltage at which the output is approximately 2V.

VS~2V Condition: VA=4V, VC=4V. VB is swept from 0 to 4V

Diode-Transistor Logic This resistor allow charges to be drained from the base

Sweep VB Fixed VA=4V VCC=4V Sweep VB from 0 to 4 V Increase the VS by about one diode drop.

Basic TTL Gate Diode is replaced by TTL A “relative “ of 7400LS Gate

Sweep VB Fixed VA=4V VCC=4V Sweep VB from 0 to 4 V

7400 NAND Gate 7400 Schematic We will revisit this schematic in a couple of weeks!