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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?
Lect. 14: BJT Biasing Circuits and Current Mirrors
Prof. Woo-Young ChoiElectronic Circuits 1 (13/2)
Better solution:
Problem?
Determine IB
Determine IC
Fluctuations in R1,R2 values (especially in IC)
Even Better solution?
Fluctuation in VBE and IC
Lect. 14: BJT Biasing Circuits and Current Mirrors
Prof. Woo-Young ChoiElectronic Circuits 1 (13/2)
(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
Lect. 14: BJT Biasing Circuits and Current Mirrors
Prof. Woo-Young ChoiElectronic Circuits 1 (13/2)
(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
Lect. 14: BJT Biasing Circuits and Current Mirrors
Prof. Woo-Young ChoiElectronic Circuits 1 (13/2)
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
Lect. 14: BJT Biasing Circuits and Current Mirrors
Prof. Woo-Young ChoiElectronic Circuits 1 (13/2)
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
Lect. 14: BJT Biasing Circuits and Current Mirrors
Prof. Woo-Young ChoiElectronic Circuits 1 (13/2)
(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
Lect. 14: BJT Biasing Circuits and Current Mirrors
Prof. Woo-Young ChoiElectronic Circuits 1 (13/2)
111
n REFI I N
With equal size transistorsand no rO consideration
Using one QREF for many current sources
Lect. 14: BJT Biasing Circuits and Current Mirrors
Prof. Woo-Young ChoiElectronic Circuits 1 (13/2)
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
Lect. 14: BJT Biasing Circuits and Current Mirrors
Prof. Woo-Young ChoiElectronic Circuits 1 (13/2)
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=?
Lect. 14: BJT Biasing Circuits and Current Mirrors
Prof. Woo-Young ChoiElectronic Circuits 1 (13/2)
Homework1:Determine R1 so that IC=0.25mA. Use VBE,on=0.7V
Lect. 14: BJT Biasing Circuits and Current Mirrors
Prof. Woo-Young ChoiElectronic Circuits 1 (13/2)
Homework2:Determine Icopy in terms of IREF and for each of following circuits