EEEB143 Chapter 5 --BJT.pdf

EEEB143 Electronics Analysis & Design 1 CHAPTER 5:

BIPOLAR JUNCTION TRANSISTOR

Course OutcomesCourse Outcome 6

Understand the characteristics of BJT, and be able to apply its models and behavior to solve BJT-related DC circuits.

DR FAZRENA HAMID EEEB143 SEM2 14/15 2

Content Physical Structure: npn, pnp, current relationships

I-V Characteristics

Circuit Symbols and Conventions

BJT DC Circuit Analysis: Circuit Configurations, Load Line and Operation Mode

BJT DC Biasing strategies


Part 1: Basic Bipolar Junction Transistor


BASIC BIPOLAR JUNCTION TRANSISTORBJT has three separately doped regions and two pn junctions◦ 4 possible modes of operations: depends upon DC biasing

◦ two types: npn & pnp

BJT: voltage-controlled current source. Collector current IC controlled by VBE voltage

Modes of operation

cut-off: VBB<VBE(on), IB=IC=0

saturation: ↑ VBB >VBE(on), IB↑, Q-point moves up load line until IC can no longer ↑ at VCE < VCE(sat) VCE(sat): 0.1 – 0.3V


Transistor Structures

Fig simplified block diagram of npn & pnp BJT

Note: emitter, base & collector regions

Fig 5.2: Cross-sectional area of npn BJT

Asymmetrical device:◦ Different geometries

◦ Different impurity doping concentrations (e:1019, b: 1017, c:1015 cm-3)

npn Transistor: Forward-Active Mode Operation

Active region: transistor used as an amplifier:◦ B-E junction forward biased

◦ B-C junction reverse biased

B-E junction FB electrons excited along the edge of emitter & injected into the base

In the base region, electrons are quickly accelerated through the base due to B-C junction RB

Flow of electrons being emitted by the emitter and collected by collector.

Figure 5.4: Minority carrier concentration across base region of an npn bipolar transistor biased in the active mode

B-E junction FB electrons injected into the base: excess minority carrier concentration at base (point A)

B-C junction RB zero electron concentration (point B)

Minority carrier concentration: linear no recombination of electron with holes in base

npn Transistor: Forward-Active Mode Operation

B-E junction FB

I-V relationship similar to that of an ideal diode

IEO contains electrical parameters of the junction as well as being proportional to the active B-E cross-sectional area

TBETBE Vv

EO

Vv

EOE eIeIi//

1

Emitter current

Majority of emitter current due to injected electrons into the base these electrons form the major component of collector current

Ic proportional to exp (vbe/vt) & independent of B-C voltage

Behaves as constant current source:◦ iC controlled by B-E voltage

◦ iC slightly smaller than iE

where

α: common-base current gain (value slightly less than 1)

TBE Vv

SC eIi/

EC ii

EOS II

Collector current

Current in base is due to two mechanisms:

◦ 1) B-E junction FB: holes from base flow across B-E junction into emitter:

◦ 2) Recombination of electrons from emitter with holes in base means holes need to be externally supplied to the base. Recombination current is also proportional to injected electrons

Base current

Common-Emitter Current Gain

Collector and base currents are linearly related:

β: common-emitter current gain

dependent on fabrication techniques & process tolerances

vary between transistor types

B

C

i

i

TBETBE VvSCVv

BOB eIi

eIi//

Current relationship

Treating the BJT as a single node, using KCL:

if the transistor is in active region,

Thus, and

As then by comparing:

or

If β >>1, then and

BCE iii

BC ii BE ii 1

EC ii

1

EC ii

1

1

1EC ii

The currents in the pnp transistors are as follows:

pnp Transistor: Forward-Active Mode

TEB Vv

EOE eIi/

TEB Vv

SEC eIii/

TEBTEB VvSCVv

BOB eIi

eIi//

Circuit Symbols and Conventions

npn pnp

Current-Voltage Characteristics


Common-base circuit configurationBase : common terminal

Current source provide iE

Provided B-C junction is reverse biased, iC independent of B-C voltage, thus

◦ B-C junction RB:

◦ +ve VCB (for npn),

◦ +ve vBC (for pnp)

◦ Vary B-C voltage by varying V+ for figure (a) or V- for figure (b)


EC ii

Typical common-base I-V characteristic: (iC vs B-C voltage)

when B-C junction is RB ideal constant current source

If B-C junction FB by 0.2-0.3V

iC drops to zero as FB voltage is increased.


EC ii

EC ii

Common-emitter circuit configurationEmitter is common connection

◦ VBB forward-biases B-E junction and control iB◦ VCC varies C-E voltage

Common-emitter I-V characteristics (iC vs C-E voltage, varying iB values)

For npn transistor to be in forward-active mode:

B-C junction zero or reverse biased:◦ VCE > VBE(on)

For VCE > VBE(on): finite slope, due to base-width modulation Early effect, finite VA , Early voltage

• iC dependent on both vBE & vCE (if VA finite)

For VCE < VBE(on): B-C junction FB, iC drops to zero

A

CEVv

SCV

veIi TBE 1

/

Common-emitter I-V characteristics (iC vs C-E voltage, varying iB values)VA: Early voltage : a point at negative voltage axis where all curves meet

Nonzero slope: finite output resistance ro looking into collector

Normally, in dc analysis, dependence of iC on vCE is not critical.


.

1

constvCE

C

oBE

v

i

r

C

Ao

I

Vr

Part 2: DC Analysis of Transistor Circuits


Common-emitter circuit with npn transistor


If B-E junction is FB, its voltage drop = =VBE(on)

Transistor is in forward active mode:

IC is a dependent current source:

For transistor to be in active mode: ◦ VBB > VBE(on) ◦ VCE > VBE(on)

KVL in base-emitter portion:

Base current:

KVL in collector-emitter portion:

or

V

B

BEBBB

R

onVVI

)(

BC II

Power dissipated in the transistor:

As usually IC>>IB and VCE>VBE(on), thus

(Analysis neglecting Early effect: assume VA = ∞)CECCCC VRIV

CCCCCE RIVV

CECBEBT VIonVIP )(

CECT VIP

Example 5.3Calculate transistor currents, VCE and transistor power dissipation


Common-emitter circuit with pnp transistor VBB and VCC polarity reversed

Analysis as previous circuit, with

and


B

EBBBB

R

onVVI

)( CCCCEC RIVV

Example 5.4pnp using positive voltage sources


DC Load LineDc analysis to obtain quiescent values or bias point of the transistor

Intersection with i-v characteristics gives the Q-point

Fig (a) Base-emitter junction piecewise linear i-v characteristic & input load line KVL around B-E loop

Q point: IBQ

Fig (b): Common-emitter transistor characteristics and C-E load line

◦ KVL around C-E loop

◦ Q point: ICQ and VCE


Example 5.5


Calculate currents & voltages in a circuit when BJT is in saturation

Circuit with emitter resistor


Example 5.7: Circuit containing positive & negative supplies


Example 5.9: pnp BJT design


Example 5.10: npn BJT with load resistance


Part 3: Bipolar Transistor Biasing


Single Base Resistor BiasingBiasing: establish Q-point near center of load line

signal source vS connected to ground

coupling capacitor CC to isolate dc base current with vS

Fig.: (a) common-emitter circuit with single bias resistor (b) dc equivalent circuit

Coupling capacitor CC ~ 1 to 10 μF, for large capacitors & high frequencies signal is coupled to base with minimal attenuation

# Design example 5.14: Q-point unstable with variations of β


Voltage Divider Biasing and Bias StabilityR1 & R2 replaces RB, and now RE included

Thevenin equivalent circuit for the base portion

# Example 5.15:

Biasing the BJT in active region done using resistor in kilohm range

ICQ and VCEQ less sensitive to variation in β stabilising effect of RE


CCTH VRR

RV .

21

2

21 RRRTH

Bias stabilityNeed RTH<< (1+ β)RE , Thus :

Usually β>>1 so β/(1+ β) ≈ 1

Hence ICQ not dependent on β :

For bias stability: RTH = 0.1 (1+ β)RE


E

BETHBQCQ

R

onVVII

1

)(

E

BETHCQ

R

onVVI

)(

Positive and Negative Voltage BiasingUsing both positive & negative dc supply voltage


Example 5.17


Positive and Negative Voltage Biasing

21 RRRTH

VVVRR

RVTH .

21

2

EEEB143 Chapter 5 --BJT.pdf

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