(course code: ee314) lecturehome.iitj.ac.in/~sptiwari/ee314/lecture24_currentmirror.pdf ·...
TRANSCRIPT
10/2/2018
1
Analog Electronics(Course Code: EE314)
Lecture 24: Current Mirror
Indian Institute of Technology Jodhpur, Year 2018
Course Instructor: Shree PrakashTiwari
Email: [email protected]
b h //h / /Webpage: http://home.iitj.ac.in/~sptiwari/
Course related documents will be uploaded on http://home.iitj.ac.in/~sptiwari/EE314/
1
Note: The information provided in the slides are taken form text books for microelectronics (including Sedra & Smith, B. Razavi), and various other resources from internet, for teaching/academic use only
Temperature and Supply‐Voltage Dependence of Bias Current
• Circuits should be designed to operate properly over a range of supply voltages and temperatures.
• For the biasing scheme shown below, I1 depends on the temperature as well as the supply voltage, since VT and IS depend on temperature.
VVSeII TBE /
1
CCBE VRR
RV
21
2
10/2/2018
2
Temperature and Supply Dependence of Bias Current
1212 )ln()( STCC IIVRRVR
3
• Since VT, IS, n, and VTH all depend on temperature, I1 for both bipolar and MOS depends on temperature and supply.
2
21
21 2
1
THDDoxn VV
RR
R
L
WCI
Concept of a Current Mirror• Circuit designs to provide a supply‐ and temperature‐independent current exist, but require many transistors to implementtransistors to implement.
“golden current source”
• A current mirror is used to replicate the current from a “golden current source” to other locations.
10/2/2018
3
Current Mirror Circuitry
• Diode‐connected QREF produces an output voltage VX that forces Icopy1 to be equal to IREF, if Q1 is identical to QREF.
Current mirror concept Generation of required VBE Current Mirror Circuitry
REFREFS
Scopy I
I
II
,
1,1
REFS
REFT
S
copyTX I
IV
I
IVV
,1,
1 lnln
Bad Current Mirror Example 1
f h ll d b f h d• If the collector and base of QREF are not shorted together, there will not be a path for the base currents to flow, so that Icopy is zero.
10/2/2018
4
Bad Current Mirror Example 2
• Although it provides a path for base currents to flow, this biasing approach is no better than a resistive voltage divider.
Multiple Copies of IREF• Multiple copies of IREF can be generated at different locations by applying the current mirror concept to multiple transistorsmultiple transistors.
REFREFS
jSjcopy I
I
II
,
,,
10/2/2018
5
Current Scaling
• By scaling the emitter area of Qj by a factor of n with respect to the emitter area of QREF, Icopy,j is scaled by a factor of n with respect to Ifactor of n with respect to IREF.
– This is equivalent to placing n unit‐sized transistors in parallel.
REFjcopy nII ,
Example: Scaled Currents
10/2/2018
6
Fractional Scaling
• A fraction of IREF can be created in Q1 by scaling up the emitter area of QREF.
T
XSREF V
VII exp3
T
XScopy V
VII exp
REFcopy II3
1
Example: Different Mirroring Ratios
• Using the concept of current scaling and fractional scaling, Icopy1 = 0.05mA and Icopy2 = 0.5mA, derived from a single 0 2mA reference current source (I )from a single 0.2mA reference current source (IREF).
10/2/2018
7
Effect of Base Currents
copycopy
REFCREF
I
n
III ,
n
II copy
REFC ,
11
1
n
nII REF
copy
Improved Mirroring Accuracy
• Use QF (rather than IREF) to supply the base currents of QREF and Q1, reduce the mirroring error by a factor of .
copycopy
FEFC
I
n
III ,,
1
12, n
II copy
FB
REFCFBREF III ,,
n
II copy
REFC ,
11
12
n
nII REF
copy
10/2/2018
8
Different Mirroring Ratio Accuracy
REFCFBREF III ,, 4REFCcopycopy
FC
IIII ,21
, 4
2
1 154
REFcopy
II
2
1,
15
copy
FB
II
2
2 154
10
REF
copy
II
1, copyREFC II
PNP Current Mirror
• A PNP BJT current mirror can be used as a current‐source load for an NPN BJT amplifier stage.
10/2/2018
9
Generation of IREF for a PNP‐BJT Current Mirror
• Neglecting base currents, the currents flowing through QM and QREF2 are the same.
Current Mirror with Discrete BJTs
• If QREF and Q1 are discrete NPN BJTs, IREF and Icopy1 can differ dramatically, due to IS mismatch.
10/2/2018
10
MOS Current Mirror• The motivation behind a current mirror is to duplicate a
(scaled version of the) “golden current” to other locations.
Current mirror concept Generation of required VGS Current Mirror CircuitryCurrent mirror concept Generation of required VGS Current Mirror Circuitry
1 W 1 W
REF
REFcopy I
LW
LWI
/
/ 11 1
1/
2TH
oxn
REFX V
LWC
IV
22
1THX
REFoxnREF VV
L
WCI
2
11 2
1THXoxncopy VV
L
WCI
MOS Current Mirror – NOT!• This is not a current mirror, because the relationship between
VX and IREF is not clearly defined.
• The only way to clearly define VX with IREF is to use a diode‐connected MOS since it provides square‐law I‐V relationship.
10/2/2018
11
Example: Current Scaling
• MOS current mirrors can be used to scale IREF up or down – I1 = 0.2mA; I2 = 0.5mA
:0
Impact of Channel‐Length Modulation
0
satDDSTHXoxncopy
VVVVVW
C
VVVVL
WCI
2
,12
11
11
12
1
THTHXREF
oxn
satDGSTHXREF
oxnREF
VVVL
WC
VVVVL
WCI
12
1
12
1
2
,2
THGSDSTHXoxn VVVVVL
C
1
2
1
12
REF
TH
GSDSREF
REFTH
THGSDSREF
REFcopy V
VVI
LW
LW
V
VVVI
LW
LWI
11
/
/
1
1
/
/ 11111
10/2/2018
12
What next
• Differential Amplifiers