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BIPOLAR SUB CIRCUITS Bipolar Current Sinks and -Sources
Transistor operates in forward-active region.
maxCECN
satCE VVV << max
CEBNCNBN VVVV +<<
Current sink Current source
N
N N
Current sink with higher output resistance
©Loberg University of Jyväskylä
3
NPN-Current Sink
+==
A
CEV/VESFCOUT V
V1eIII TBEα
N
VCC
IOUT
VCN
VBE
+
-
Collector characteristics IOUT
VCN
TBE V/VESF eIα
BIPOLAR SUB CIRCUITS Bipolar Current Sinks and -Sources
©Loberg University of Jyväskylä
4
BIPOLAR SUB CIRCUITS Bipolar Current Sinks and -Sources
PNP-Current Source
Collector characteristics IOUT
VCN
TBE V/VESF eIα
N
VCC
IOUT VCN
VBE +
-
+
-
VCC
©Loberg University of Jyväskylä
5
BIPOLAR SUB CIRCUITS Bipolar Current Sinks and -Sources
NPN-Current Sink with emitter resistance
N
VCC
IOUT
VCN
VBE
+
-
VBN
RE
E
BEBNFCOUT R
VVII −== α
©Loberg University of Jyväskylä
7
BIPOLAR SUB CIRCUITS
VCC
IREF
OUTi
BiOUTv+
-
1iCi
)1(iandIiwhen FOUTREFB >><< β
+=
ESF
OUTREFTOUT I
iIlnVvα
OUTi
=
ESF
REFTOUT I
IlnVvα
REFI−
OUTv
V7.0vV6.0 OUT <<
Bipolar Diode Connected Voltage Source
©Loberg University of Jyväskylä
8
BIPOLAR SUB CIRCUITS
VCC
IREF
2OUTv
1OUTv
Bipolar Diode Connected Voltage Source
©Loberg University of Jyväskylä
9
BIPOLAR SUB CIRCUITS
RVV
2I BECC
F
F1C
−+
=ββ If βF>>1 then
RBECC
1C IR
VVI =−
≈
Current Mirror
Identical tansistors
BECECE VVV =≈ 21
Assumptions
©Loberg University of Jyväskylä
10
BIPOLAR SUB CIRCUITS Current Mirror
VBE depends on the ambient temperature :
( ) 1VV1
VV
II
BECCBE
BE
R
R
−∝∆∆Sensitivity of reference current IR
is low if we have VCC >>VBE
-2.2mV/°C
Resistor R can be replaced by constant current source IREF
Reference current IR depends on the VCC , R and VBE .
Constant current sink
©Loberg University of Jyväskylä
11
BIPOLAR SUB CIRCUITS Current Mirror
If VCE1 > VCE2
+
+=
A
1CER
F
F1C V
V1I2
Iββ
V7.0V 1CE >>
©Loberg University of Jyväskylä
12
BIPOLAR SUB CIRCUITS Current Mirror
Transistors have different emitter areas A1 and A2
2
1
2ES
1ES
AA
II
=
( )( )A2CE
V/V2ESF
A1CEV/V
1ESF
2C
1C
VV1eIVV1eI
II
TBE
TBE
++
=αα
If we assume that 1andVV FCEA >>>> β REF2
1OUT I
AAI ≈
©Loberg University of Jyväskylä
13
BIPOLAR SUB CIRCUITS
Identical transistors
F
INOUT 1N1
II
β+
+=
All output transistors has same base current.
Difference between reference and output current is proportional to the number of outputs.
)N1(III BOUTIN +=−
(Multiple output current sink)
Multiple Output Current Mirror
©Loberg University of Jyväskylä
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Emitter follower is used to supply base current.
( )1N11
II INOn
++
+=
ββ
Reduced difference between reference and output current.
β++
=−1
N1III BOUTIN
(Multiple output current sink)
Modified Multiple Output Current Mirror BIPOLAR SUB CIRCUITS
©Loberg University of Jyväskylä
15
BIPOLAR SUB CIRCUITS
Multiple Collector BJT
Different values of sink currents IOn.
Output current depends on the effective value of collector area.
IN
ON
IN
2O
IN
2O
IN
1O
II
II
II
II
10II
minO
maxO ≈
(Multiple output current sink)
Multiple Output Current Mirror
©Loberg University of Jyväskylä
16
( ) TBET1BE2BE V/VV/VV
1C
2C eeII ∆== −
≈
+
=1C
2C
1C
T
1C
2C
1C
T
F
FE I
IlnIV
IIln
IV
1R
ββ
2CE1CE VV ≈Assumptions Early voltage VA is high.
RVV
1I 2BECC
F
F2C
−
+
=β
β
Low output currents with practical resistance values.
High output resistance Ro
( )Emoo Rg1rR +≈
mV60V10II
BE1C
2C =⇒= ∆
mV120V100II
BE1C
2C =⇒= ∆
Widlar Current Source BIPOLAR SUB CIRCUITS
©Loberg University of Jyväskylä
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BIPOLAR SUB CIRCUITS
1E
2E
2C
1C
RR
II
∝
>
Widlar Current Source RE1>RE2
Current Mirror RE1=RE2
Widlar Current Source
©Loberg University of Jyväskylä
18
BIPOLAR SUB CIRCUITS
Wilson Current Source
( )( )
( )( ) R
V2V22
2I22
2I BECC
FF
FFIN
FF
FF1C
−++
+=
+++
=ββββ
ββββ
( )( )
( )( ) R
V2V21
1I21
1I BECC
FF
FFIN
FF
FF1C
−++
+=
+++
=ββββ
ββββ
See Modified Multiple Output Current Sink N=1
High output resistance Ro
Wilson Current Source
©Loberg University of Jyväskylä
19
BJT or FET
(Emitter-coupled pair)
−+ = VVPower Supply typically
Identical pair
Generic Differential Stage
1Iv 2Iv
IBIAS is Constant Current Source
Sub Circuit of Operational Amplifiers Emitter Coupled Logic (ECL)
Assumption: Input currents iI1=iI2=0
BIPOLAR SUB CIRCUITS Differential Stage
1IV
1iv
2IV
2iv
©Loberg University of Jyväskylä
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BIPOLAR SUB CIRCUITS Differential Stage
1i1I1I vVv += 2i2I2I vVv +=Input signals:
1L
1ON1 R
VVI −=
+
2L
2ON2 R
VVI −=
+
Generic Differential Stage
2Iv1Iv1L11ON RIVV −= +
2L22ON RIVV −= +
Quisent values
0III BIAS21 =−+
2I1I2I1I VVvv ===(Ac-component is zero)
Summ of currents I1 and I2 is constant. ( ) 0IIRVVV 21L2ON1ONOD =−−=−=
1IV
1iv
2IV
2iv
©Loberg University of Jyväskylä
21
Differential Stage BIPOLAR SUB CIRCUITS
If VON1 is output of differential stage then:
VIN1 is inverting input VIN2 is noninverting input
Generic Differential Stage
2Iv1Iv
180° phase difference
Differential mode
2IN1IN VV =2IN1IN VV ∆∆ −=
Common mode
2IN1IN VV =2IN1IN VV ∆∆ =
1IV
1iv
2IV
2iv
©Loberg University of Jyväskylä
22
Arbitrary input voltage consists of common mode and differential mode voltages.
DMCMIN VVV +=1 DMCMIN VVV −=2
2IN1IN VV +
2IN1IN VV −( )2IN1IN VV
21
−
( ) CM2IN1IN VVV21
=+
1INV2INV
( ) CM2IN1IN VVV21
=+
( ) DM2IN1IN VVV21
=−
Where
BIPOLAR SUB CIRCUITS Differential Stage
©Loberg University of Jyväskylä
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Common Mode Gain ACM
Differential Mode Gain ADM
Linear system and superposition
CMCMDMDM1ON VAVAV +=
CMCMDMDM2ON VAVAV +−=
Arbitrary output voltages:
Common Mode Rejection Ratio CMRR
CM
DM
AACMRR = In Ideal Case the ACM is zero
Differential Amplifier
BIPOLAR SUB CIRCUITS Differential Stage
©Loberg University of Jyväskylä
24
+=
CMRRVVAV CM
DMDM1ON
−−=
CMRRVVAV CM
DMDM2ON
Arbitrary output voltages
BIPOLAR SUB CIRCUITS Differential Stage
©Loberg University of Jyväskylä
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T1BE V/VESF1C eII α=
T2BE V/VESF2C eII α=
Collector currents
( ) TDT21 V/VV/VV
2C
1C eeII
== −
IE1 IE2
Node E : 0III EE2E1E =−+F
2C
F
1CEE
IIIαα
+=
21D VVV −=
BJT Differential Stage BIPOLAR SUB CIRCUITS
(a)
(b)
©Loberg University of Jyväskylä
26
TD V/VEEF
1C e1II −+
=α
TD V/VEEF
2C e1II
+=
α
TD V/VCEEF
CC1O e1RIVV −+
−=α
TD V/VCEEF
CC2O e1RIVV
+−=α
+−
+−=−= − TDTD V/VV/VCEEF2O1OOD e1
1e11RIVVV α
DV
2OV1OV
TV2− TV2
"Linear" input voltage region TD V2V <
V
CCV
CEEFCC RIV α−
TV4
BJT Differential Stage BIPOLAR SUB CIRCUITS
(a) and (b)
©Loberg University of Jyväskylä
27
BJT Differential Stage BIPOLAR SUB CIRCUITS
9.0V
-9.0V
9.0V
+9.0V
BSX20 BSX20
BSX20 BSX20
Q1 Q2
Q3 Q4
3.8kΩ 3.8kΩ
3.3kΩ
22Ω
1.2kΩ 1.2kΩ
VCQ1 = 6.26V VCQ2 = 6.64V
VEQ = -0.69V
vIN1 vIN2
vIN2
vIN1
vC1 vC2
vE
Amplitude : 50mV or 300mV
Practical example
28
vC1
vC2
BJT Differential Stage BIPOLAR SUB CIRCUITS
Collector voltages in fully differential mode
30
Input amplitude : 50mV
vC1
vE Input amplitude : 300mV vE
vC1
BJT Differential Stage BIPOLAR SUB CIRCUITS