BJT Intro and Large Signal Model - Penn Engineeringese319/Lecture_Notes/Lec_3_BJTLgSig_08.pdf · BJT Intro and Large Signal Model. ESE319 Introduction to Microelectronics 2008 by

ESE319 Introduction to Microelectronics

12008 by Kenneth R. Laker (based on P.V. Lopresti 2006) update 09Sep08 KRL)

BJT Intro and Large Signal Model



Why BJT?What's the competition to BJT and bipolar technologies?

What advantages does the competition have over BJT?

What advantages does BJT and bipolar have over their com-petition?

What circuit applications benefit from BJT and bipolar tech-nologies?



Why BJTWhat's the competition to BJT and bipolar technologies?

MOSFET, in particular CMOS is the leading competitor

What advantages does the competition have over BJT?Small size (die area), low cost and low power dissipation

What advantages does BJT and bipolar have over their competi-tion?

High frequency operation, high current drive, high reliability in severe environmental conditions.

What circuit applications benefit from BJT and bipolar technolo-gies?

RF analog and digital circuits,power electronics and power am-plifiers, automobile electronics, radiation hardened electronics



BJT Physical Configuration

Each transistor looks like two back-to-back diodes, but each behaves much dif-ferently!

NPN PNP

Closer to actual layout



BJT Symbols and ConventionsNPN PNP

Note reversal in current directions and voltage signs for PNP vs. NPN!



NPN BJT Modes of Operation

ModeForward-ActiveReverse-ActiveCutoffSaturation

VBE

VBC

> 0 < 0< 0 > 0< 0 < 0> 0 > 0

Forward-Active ModeEBJ forward bias (V

BE > 0)

CBJ reverse bias (VBC

< 0)

Not Useful!

VBC

= -VCB

vCE

= vCB

+ vBE

iE = i

C + i

B



NPN BJT Forward-Active Mode Basic ModelCollector-base diode is reverse biasedV CB0

Base-emitter diode is forward biased

V BE≈0.7

iC≈ I S evBEV T

iB=iC

iE=iBiC=1iB

V T=kTq≈25mV @ 25oCI s=

AEq Dnni2

N AWArea of base-emitter junctionWidth of base regionDoping concentration in baseElectron diffusion constant Intrinsic carrier concentration = f(T) =common−emitter current gain

( or VBC

< 0)

saturation current

AEWN A

niDn



Note that the iC equation looks like that of a forward-biased

diode.

Is it?



Note that the iC equation looks like that of a forward-biased

diode. Is it?

iC=I S ev BEV T−1

By using the other two equations the question can be answered

iE=iBiC=11 iC=

1iC

and write:

iE=1I S e

v BEV T−1= 1

I S e

vBEV T−1

AhHa! This iE equation describes a forward-biased emitter-base

“diode”.

=common−emitter current gain=common−base current gain



So the new set ofequations is:

iC=I S ev BEV T−1= iE

iE=I Se

vBEV T−1

iB=iC

=

1

Where:

50200

10−18 I S10−12 A.

Typically:

Is is strongly temperature-

dependent, doubling for a 5 degree Celsius increase in ambient temperature!

50200

I s=AE q Dnni

2

N AW



Two equivalent large signal circuit models for the forward-active mode NPN BJT:

Nonlinear VCCSNonlinear CCCS

iC≈ I S evBEV T=F iE

iE≈I SFeV BE

V T

Key Eqs.=F



Yet another NPN BJT large signal model

iC= iB≈ I S evBEV T ⇒ iB≈

I SevBEV T

This “looks like” a diode between base and emitter and the equivalent circuit becomes:

Note that in this model, the diode current is represented in terms of the base current. In the previous ones, it was rep-resented in terms of the emitter current.

iB i

C

iE



NPN BJT Operating in the Reverse-Active Mode

● We also have transistor action if we:● Forward bias the base-collector junction and● Reverse bias the base emitter junction

● The physical construction of the transistor, how-ever, leads to betas on the order of 0.01 to 1 and correspondingly smaller values of alpha, e. g.:

R=R

R1≈0.11.1

≈0.091 for R=0.1

Recall for NPN Reverse-Active Mode VBE

< 0 & VBC

> 0



The equivalent large signal circuit model for the reverse-active mode NPN BJT:

iC≈−I SRevBCV T

iE≈−I S evBCV T

iB

iC

iE

RVRS Active

FWD Active

Key Eqs.

F I SE=R I SC=I SSat. or Scale Current Eq.

ACAE

BJT is non-symmetricalR≪F R≪F



The Ebers-Moll Large Signal ModelThe E-M model combines the FWD & RVRS Active equivalent circuits:

Note that the lower left diode and the upper right controlled current source form the forward-active mode model, while the up-per left diode and the lower right source represent the reverse-active mode model.

iC=F iDE−iDCiE=iDE−R iDCib=1−F I DE1−R I DC

iDE=I SE ev BEV T−1

iDC=I SC evBCV T −1

F I SE=R I SC=I S



Operation in the Saturation Mode

Consider the E-M model for collector current. We will include the “-I

S” term in the ideal diode equation.iC=F iDE−iDC

The first term is the forward mode collector current:

F iDE= I S evBEV T −1

The second is the reverse mode collector current:

iDC=I SR

evBCV T −1

Recall for Saturation Mode VBE

> 0 & VBC

> 0 (or VCB

< 0)

iC= I S evBEV T−1−

I SR

ev BCV T−1



Combining terms:

iC=[e vBEV T −1− R1R

e−vCBV T −1] I S

Using typical values:

R=0.1

I S=10−14 A.

V T=0.025V.

iC=[e40 vBE−1−11e−40 vCB−1 ]10−14We obtain:

1R

=R1R


I SR

ev BCV T−1

Let's plot iC vs. v

BC (or v

CB) with v

BE = 0.7 V



Scilab Saturation Mode Calculation//Calculate and plot npn BJT collector//current in saturation modevBE=0.7;VsubT=0.025;VTinv=1/VsubT;betaR=0.1;IsubS=1E-14;alphaR=betaR/(betaR+1);alphaInv=1/alphaR;ForwardExp=exp(VTinv*vBE)-1;vCB=-0.7:0.001:-0.1;vBC=-vCB;ReverseExp=alphaInv*(exp(VTinv*vBC)-1);iC=(ForwardExp-ReverseExp)*IsubS;signiC=sign(iC);iCplus=(iC+signiC.*iC)/2; //Zero negative valuesplot(vCB,1000*iCplus); //Current in mA.



Saturation Mode Plot

Forward-active

Saturation

Recall for Sat. Mode V

BE > 0

& V

BC > 0

(or VCB

< 0)

A



The PNP Transistor npnpnp

ModeForward-ActiveReverse-ActiveCutoffSaturation

VEB

VCB

> 0 < 0< 0 > 0< 0 < 0

> 0

Not Useful!

VCB

= -VBC

> 0



PNP BJT Forward-Active Mode Basic Model

Collector-base diode is reverse biased

V BC0

Emitter-base diode is forward biased

V EB≈0.7 iC=I S ev EBV T−1

iB=iC

iE=iBiC=1iB

Note reversal in voltage polarity and in current directions!



PNP BJT Large Signal Model FWD. Active

Note reversal in current directions!

iC=I S ev EBV T−1

iB=iC

iE=iBiC=1iBSubstituting, as in the npn case, we get:

iE=I Se

vEBV T−1



Yet another PNP BJT large signal model

This “looks like” a diode between base and emitter and the equivalent circuit becomes:

Again, in this model, the diode carries only base current, not emitter current.

iB

iC

iC= iB= I S evEBV T−1⇒ iB=

I Se

vEBV T−1≈

I Sev EBV T



Scilab Plot of NPN Characteristic (i

C vs. v

CE and v

BE)

//Calculate and plot npn BJT collector//characteristic using Ebers-Moll modelVsubT=0.025;VTinv=1/VsubT;betaR=0.1;alphaInv=(betaR+1)/betaR;IsubS=1E-14;for vBE=0.6:0.02:0.68 ForwardExp=exp(VTinv*vBE)-1; vCE=-0:0.001:10; vBC=vBE-vCE; ReverseExp=alphaInv*(exp(VTinv*vBC)-1); iC=(ForwardExp-ReverseExp)*IsubS; signiC=sign(iC); iCplus=(iC+signiC.*iC)/2; //Zero negative vals plot(vCE,1000*iCplus); //Current in mA.end

iC=F iDE−iDC

iDC= I SC evBCV T−1

where

iDE=I SE ev BEV T−1

vBC=vBE−vCENo Early effect



Plot Output

vBE=0.68V

vBE=0.66V

vBE=0.64V

Early effect not included.

forward-active modesaturation mode



More on NPN Saturation● The base-collector diode has much larger

area than the base-emitter one.● Therefore, with the same applied voltage, it will

conduct a much larger forward current than will the base-emitter diode.

● When the collector-emitter voltage drops below the base-emitter voltage, the base-collector diode is forward biased and conducts heavily.

vCE≈V CE sat vCB=vCE−vBE when



IC Expansion Around Zero V

CE

Note that the collector current is zero at about VCE

0.06 V., not 0 V.! Also note the large reverse collector-base current below 0.06 V.

IC

(mA)

VCE

(V)

IC

= 0



Voltage at Zero Collector Current

IC = 0 =>I SR

evBCV T−1= I S e

v BEV T−1

R ev BEV T−1=e

v BCV T −1

R ev BEV T−1=e

vBE−vCE V T −1

R 1−e−vBEV T =e

−vCEV T−e

−vBEV T

V BE

V T≈40∗0.7⇒ e

−vBEV T ≪1

R=e−vCEV T ⇒ vCE=−V T ln R

vCE=−0.025⋅ln 0.10.11

=0.0599V.


I SR

ev BCV T−1

Documents

BJT Intro and Large Signal Model - Penn Engineeringese319/Lecture_Notes/Lec_3_BJTLgSig_08.pdf · BJT Intro and Large Signal Model. ESE319 Introduction to Microelectronics 2008 by