(Razavi 5.2 and 9.2.2) Biasing: Making sure BJT has desired large...

Lect. 14: BJT Biasing Circuits and Current Mirrors

Prof. Woo-Young ChoiElectronic Circuits 1 (13/2)

Biasing: Making sure BJT has desired large-signal (DC, quiescent) currents

(Razavi 5.2 and 9.2.2)

Determines small-signal parameter values

Problem?



Better solution:

Problem?

Determine IB

Determine IC

Fluctuations in R1,R2 values (especially in IC)

Even Better solution?

Fluctuation in VBE and IC



(Razavi 5.2)

Why better?

If VBE increases due to R fluctuation

Decrease in VBE

Increase in VE

Increase in IC and IE

RE stabilizes VBE fluctuation by negative feedback

Popular biasing technique especially for discrete TR design Emitter Degeneration



(Razavi 5.2)

Example (Lect. 11)2

1 2

1 2

50V 15 15 5 V100 50

( // ) (100 // 50) 33.3 k

BBB

B B

BB B B

RR R

R R R

V -V 0

1

BB B BB BE E E

EB

I R I RI

I

Can we minimize R dependence for IC applications



Current source biasing How can we make the current source with TRs?

1O CI I

REFV VCC BEI

R

REF 1 2C BI I I 1

1 2 CC

II

Current mirror

Much preferred biasing technique in IC becausetransistors are much smaller (cheaper) and better-controlled in IC

121

O REFI I

Requirements: Q1, Q2 are in active mode Q1, Q2 are identical



How good is the current source?

Does VO affect IO?

11

O CO C

O

V VI I

r

VC1VO

IC1

1121

O CREF

O

V VI

r

Remember Early Effect or rO



(m x Q1)

Can we different values for IC1 and IO?

Q1, Q2 can have different B-E junction areas

m times larger E-B junction than Q1

If area(Q2) = m x area(Q1)

IC2 = m x IC1 for same VBE



111

n REFI I N

With equal size transistorsand no rO consideration

Using one QREF for many current sources



Reduce dependence

3 1CC BE BEREF

V V VI

R

OIgnoring r , O CI I

3 ,REF C BI I I

2~2 1 2 /1( 1)

REF REFO

I II

2 2(1 )( 1) ( 1)

CREF C C

II I I

32

( 1)C

BI

I



Determine R2 and R3 for IO= 10A. Assume Q1,Q2 have VBE=0.7V for IC=1mA and very large

ln ,CBE T

CS

IV V

I

Widlar current source

1 ln ,REFBE T

CS

IV V

I

1 2 ln REFBE BE T

O

IV V V

I

2 ln OBE T

CS

IV V

I

1 2 3BE BE OV V I R 3 ln REF

O TO

II R V

I

1For 0.7 , 1mABE REFV V I

3 6

1 1mA0.025ln 11.5k10 10 10μA

R

210 0.7Then, 9.3k0.001

R For very small current

VBE2 is smaller than VBE1due to R3

VBE1-VBE2=?



Homework1:Determine R1 so that IC=0.25mA. Use VBE,on=0.7V



Homework2:Determine Icopy in terms of IREF and for each of following circuits

(Razavi 5.2 and 9.2.2) Biasing: Making sure BJT has desired large...

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