Current Mirrors. OUTLINE Cascode Stage (cont’d) supplementary remarks Current Mirrors Reading:...
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Transcript of Current Mirrors. OUTLINE Cascode Stage (cont’d) supplementary remarks Current Mirrors Reading:...
Current Mirrors
OUTLINE
Cascode Stage (cont’d) supplementary remarks
Current Mirrors
Reading: Chapter 9.2
Review: Cascode Stage Rout
The impedance seen looking into the collector can be boosted significantly by using a BJT for emitter degeneration, with a relatively small reduction in headroom.
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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.
CCBE
VVS
VRR
RV
eII TBE
21
2
/1
Concept of a Current Mirror Circuit designs to provide a supply- and
temperature-independent current exist, but require many transistors to implement. “golden current source”
A current mirror is used to replicate the current from a “golden current source” to other locations.
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.
REFREFS
Scopy I
I
II
,
1,1
REFS
REFT
S
copyTX I
IV
I
IVV
,1,
1 lnln
Current mirror concept Generation of required VBE Current Mirror Circuitry
Bad Current Mirror Example 1
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.
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 transistors.
REFREFS
jSjcopy II
II
,
,,
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 IREF. This is equivalent to placing n unit-sized transistors in parallel.
REFjcopy nII ,
Example: Scaled Currents
Fractional Scaling
A fraction of IREF can be created in Q1 by scaling up the emitter area of QREF.
REFcopy II3
1
T
XSREF V
VII exp3
T
XScopy V
VII exp
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 (IREF).
Effect of Base Currents
111
n
nII REFcopy
copycopy
REFCREF
I
n
III ,
n
II copy
REFC ,
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 .
111
2
n
nII REFcopy
copycopy
FEFC
I
n
III ,,
11
2, n
II copy
FB
REFCFBREF III ,,
n
II copy
REFC ,
Different Mirroring Ratio Accuracy
2
2
2
1
154
10
154
REFcopy
REFcopy
II
II
1, copyREFC II
REFCFBREF III ,, 4REFCcopycopy
FC
IIII ,21, 4
2
1,
15
copy
FB
II
PNP Current Mirror
A PNP BJT current mirror can be used as a current-source load for an NPN BJT amplifier stage.
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.