lect_09_bjt ke bichhye baitha hu, daru – e – mahak nhi kis m

7/30/2019 lect_09_bjt ke bichhye baitha hu, daru e mahak nhi kis m

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Wei 1

Lecture 9

Bipolar J unction Transistors (BJ Ts)

Gu-Yeon Wei

Division of Engineering and Applied Sciences

Harvard University

[email protected]


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ES154 - Lecture 9Wei 2

Overview

Reading

Chapter 7

Supplemental Reading

Sedra&Smith: Chapter 4.1~3

Background

This lecture looks at another type of transistor called the bipolarjunction transistor (BJ T). We will spend some time understandinghow the BJ T works based on what we know about PN junctions.One way to look at a BJ T transistor is two back-to-back diodes, but

it has very different characteristics.Once we understand how the BJ T device operates, we will take alook at the different circuits (amplifiers) we can build with them.


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Bipolar J unction Transistor

NPN BJ T shown 3 terminals: emitter, base, and collector

2 junctions: emitter-base junction (EBJ ) and collector-base junction (CBJ )

These junctions have capacitance (high-frequency model)

Depending on the biasing across each of the junctions, different modes ofoperation are obtained cutoff, active, and saturation

ForwardForwardSaturation

ReverseForwardActive

ReverseReverseCutoff

CBJEBJMODE


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BJ T in Active Mode

Two external voltage sources set the bias conditions for active mode

EBJ is forward biased and CBJ is reverse biased

Operation

Forward bias of EBJ injects electrons from emitter into base (small numberof holes injected from base into emitter)

Most electrons shoot through the base into the collector across the reversebias junction (think about band diagram)

Some electrons recombine with majority carrier in (P-type) base region


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Band Diagrams (1)

In equilibrium

No current flow

Back-to-back PN diodes

c

v

f


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Band Diagrams (2)

Active Mode

EBJ forward biased Barrier reduced and

so electrons diffuseinto the base

Electrons get sweptacross the base intothe collector

CBJ reverse biased

Electrons roll downthe hill (high E-field)

Ec

Ev

Ef

Emitter Base Collector

N P N


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Minority Carrier Concentration Profiles

Current dominated by electrons from emitter to base (by design) b/c of the forward bias andminority carrier concentration gradient (diffusion) through the base

some recombination causes bowing of electron concentration (in the base)

base is designed to be fairly short (minimize recombination)

emitter is heavily (sometimes degenerately) doped and base is lightly doped

Drift currents are usually small and neglected


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Diffusion Current Through the Base

Diffusion of electrons through the base is set by concentration profile at the EBJ

Diffusion current of electrons through the base is (assuming an ideal straight line case):

Due to recombination in the base, the current at the EBJ and current at the CBJ are notequal and differ by a base current


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Collector Current

Electrons that diffuse across the base to the CBJ junction are swept across theCBJ depletion region to the collector b/c of the higher potential applied to thecollector.

Note that iC is independent ofvCB (potential bias across CBJ ) ideally

Saturation current is

inversely proportional toW and directly proportional toAE

Want short base and large emitter area for high currents

dependent on temperature due to ni2 term


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Base Current

Base current iB composed of two components:

holes injected from the base region into the emitter region

holes supplied due to recombination in the base with diffusing electrons and

depends on minority carrier lifetime b in the base

And the Q in the base is

So, current is

Total base current is


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Beta

Can relate iB and iC by the following equation

and is

Beta is constant for a particular transistor

On the order of 100-200 in modern devices (but can be higher)

Called the common-emitter current gain

For high current gain, want small W, lowNA, highND


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Emitter Current

Emitter current is the sum ofiC and iB

is called the common-base current gain


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BJ T Equivalent Circuits


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Vertical BJ T

BJ Ts are usually constructed vertically

Controlling depth of the emitters n doping sets the base width


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Circuit Symbols and Conventions

BJ Ts are not symmetric devices

doping and physical dimensions are different for emitter andcollector

npn pnp

IC

IE IC

IE

IB

IB


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I-V Characteristics

Collector current vs. vCB shows the BJ T looks like a current

source (ideally) Plot only shows values where BCJ is reverse biased and so

BJ T in active region

However, real BJ Ts have non-ideal effects

VCE

IC

VBE1

VBE2

VBE3

VBE3 > VBE2 > VBE1

VBE

IC

VCE


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Early Effect

Early Effect

Current in active region depends (slightly) on vCE

VA is a parameter for the BJ T (50 to 100) and called the Early voltage Due to a decrease in effective base width W as reverse bias increases

Account for Early effect with additional term in collector current equation

Nonzero slope means the output resistance is NOT infinite, but IC is collector current at the boundary of active region

C

A

o

I

Vr

VCE

VBE1

VBE2

VBE3

Active region

Saturation region

-VA


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Early Effect Contd

What causes the Early Effect?

Increasing VCB causes depletion region of CBJ to grow and so theeffective base width decreases (base-width modulation)

Shorter effective base width higher dn/dx

EBJ CBJ

dn/dx

VCB >VCB

Wbase


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BJ T DC Analysis

Use a simple constant-VBE model

Assume VBE = 0.7-V regardless of exact current value

reasonable b/c of exponential relationship

Make sure the BJ T current equations and region of operation match So far, we only have equations for the active region

Utilize the relationships (and ) between collector, base, and emitter currentsto solve for all currents


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BJ T Amplifier

To operate as an amplifier, the BJ T must be biased to operate in active modeand then superimpose a small voltage signal vbe to the base

Under DC conditions,

DC DC + small signal


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The DC condition biases the BJ T to the pointQ on the plot.

Adding a small voltage signal vbe translatesinto a current signal that we can write as

Ifvbe


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Small-Signal Model

We can model the BJ T as a voltage controlled current source, butwe must alsoaccount for the base current that varies with vbe

so, the small-signal resistance looking into the base is denoted by randdefined as

looking into the emitter, we get an effective small-signal resistance between

base and emitter, re


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To convert the voltage-controlled current source into a circuit that providesvoltage gain, we connect a resistor to the collector and measure the voltagedrop across it

So, the small-signal voltage gain is

Remember thatgm depends on IC

We can create an equivalent circuit to model the transistor for small signals

Note that this only applies for small signals (vbe

< VT)


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Hybrid- Model

We can represent the small-signal model for the transistor as a voltage-controlled current source or a current-controlled current source

Add a resistor (ro) in parallel with the dependent current source to

model the Early effect From our previous example,


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T Model

Sometimes, other small signal models canmore convenient to use


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Using Small-Signal Models

Steps for using small-signal models

1. Determine the DC operating point of the BJ T

in particular, the collector current

2. Calculate small-signal model parameters: gm, r, re3. Eliminate DC sources

replace voltage sources with shorts and current sources with opencircuits

4. Replace BJ T with equivalent small-signal models

Choose most convenient one depending on surrounding circuitry

5. Analyze


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Graphical Analysis

Can be useful to understand the operation of BJ T circuits

First, establish DC conditions by finding IB (orVBE)

Second, figure out the DC operating point for IC


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Apply a small signal input voltage and see ib See how ib translates into VCE Can get a feel for whether the BJ T will stay in active region of operation

What happens ifRC

is larger or smaller?


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BJ T Current Mirror

We can build current mirrors using BJ Ts

Q2 must be in active mode

What is IC2

? (Assuming Q1

and Q2

are identical)

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lect_09_bjt ke bichhye baitha hu, daru – e – mahak nhi kis m