전자회로 1 (Fundamentals of Microelectronics 1)
Cascode Stage and Current Mirror
Instructor: Prof. Jintae Kim
Mixed-Signal Electronics Group
Konkuk University
J. Kim
What we will learn
• Bipolar cascode stage
• MOS cascode stage
• Bipolar current mirror
• MOS current mirror
2
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Boosting Output Impedance
3
SOSmout
EOEmout
RrRgR
rRrrRgR
1
||||1
2
1
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Bipolar Cascode Stage
4
1211
12112
||
||)]||(1[
rrrgR
rrrrrgR
OOmout
OOOmout
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Maximum Bipolar Cascode Rout
• The maximum output impedance of a bipolar cascode is
bounded by the ever-present r between emitter and
ground of Q1.
5
11max,
11max, 1
Oout
Omout
rR
rrgR
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Example
• Typically r is smaller than rO, so in general it is
impossible to double the output impedance by
degenerating Q2 with a resistor.
6
21
122
O
OoutA
rr
rrR
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PNP Cascode Stage
7
1211
12112
||
||)]||(1[
rrrgR
rrrrrgR
OOmout
OOOmout
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False Cascode Stages
• When the emitter of Q1 is connected to the emitter of Q2,
it’s no longer a cascode since Q2 becomes a diode-
connected device instead of a current source.
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1
2
1
2
1
12
2
112
2
1
21
1
||||1
||||1
1
O
m
O
m
mout
O
m
OO
m
mout
rg
rg
gR
rrg
rrrg
gR
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MOS Cascode Stage
9
211
21211
OOmout
OOOmout
rrgR
rrrgR
• Similar to its bipolar counterpart, MOS cascode can be thought of as stacking a transistor on top of a current source.
• Unlike bipolar cascode, the output impedance is not limited by .
J. Kim
PMOS Cascode
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211
21211
OOmout
OOOmout
rrgR
rrrgR
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Transconductance and Gain
• By representing a linear circuit with its Norton equivalent,
the relationship between Vout and Vin can be expressed
by the product of Gm and Rout.
- Gm can be found by shorting output to gnd and find iout
- Rout is the output impedance of the amp
11
outminout
outinmoutoutout
RGvv
RvGRiv
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Voltage gain of Bipolar Cascode Amp
• Since rO is much larger than 1/gm, most of IC,Q1 flows into
the diode-connected Q2. Using Rout as before, AV is
easily calculated.
12
)||( 21111
1
rrgrgA
gG
OmOmv
mm
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Alternate View of Cascode Amp
• A bipolar cascode amplifier is also a CE stage in series
with a CB stage.
13
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Practical Cascode Stage
• Since no current source can be ideal, the output
impedance drops.
14
)||(|| 21223 rrrgrR OOmOout
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Improved Cascode Stage
• In order to preserve the high output impedance, a
cascode PNP current source is used.
15
)||(||)||( 21223433 rrrgrrrgR OOmOOmout
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MOS Cascode Amplifier
16
2211
21221 )1(
OmOmv
OOOmmv
outmv
rgrgA
rrrggA
RGA
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Improved MOS Cascode Amp
• Similar to its bipolar counterpart, the output impedance of
a MOS cascode amplifier can be improved by using a
PMOS cascode current source.
17
oponout
OOmop
OOmon
RRR
rrgR
rrgR
||
433
122
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Concept of Current Mirror
• The motivation behind a current mirror is to sense the
current from a “golden current source” and duplicate this
“golden current” to other locations.
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MOS Current Mirror
• The same concept of current mirror can be applied to
MOS transistors as well.
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Bad MOS Current Mirror
• This is not a current mirror since 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.
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Example: Current Scaling
• Similar to their bipolar counterpart, MOS current mirrors
can also scale IREF up or down (I1 = 0.2mA, I2 = 0.5mA).
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CMOS Current Mirror
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Bipolar Current Mirror
• The diode-connected QREF produces an output voltage V1
that forces Icopy1 = IREF, if Q1 = QREF.
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REF
REFS
Scopy I
I
II
,
1
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Bad Current Mirror Example 1
• Without shorting the collector and base of QREF together,
there will not be a path for the base currents to flow,
therefore, Icopy is zero.
24
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Multiple Copies of IREF
• Multiple copies of IREF can be generated at different
locations by simply applying the idea of current mirror to
more transistors.
25
REF
REFS
jS
jcopy II
II
,
,
,
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Current Scaling
• By scaling the emitter area of Qj n times with respect to
QREF, Icopy,j is also n times larger than IREF. This is
equivalent to placing n unit-size transistors in parallel.
26
REFjcopy nII ,
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Fractional Scaling
• A fraction of IREF can be created on Q1 by scaling up the
emitter area of QREF.
27
REFcopy II3
1
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Mirroring Error Due to Base Currents
28
11
1
n
nII REF
copy
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Improved Mirroring Accuracy
• Because of QF, the base currents of QREF and Q1 are
mostly supplied by QF rather than IREF. Mirroring error is
reduced times.
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11
12
n
nII REF
copy
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Example: Different Mirroring Ratio Accuracy
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2
2
2
1
154
10
154
REFcopy
REFcopy
II
II
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PNP Current Mirror
• PNP current mirror is used as a current source load to an
NPN amplifier stage.
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Generation of IREF for PNP Current Mirror
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