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CERMIN ARUS
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KONSEP CERMIN ARUS
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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Pemanjaran (biasing ) transistor dalam IC biasanya
berbasis pada constant-current sources.
Dalam sebuah chip IC yang mempunyai banyakpenguat, sebuah arus DC konstan (Arus referensi)
dibangkitkan pada satu lokasi dan kemudian di-kopi
ke lokasi-lokasi yang lain.
Dikenal dengan nama current steering.
Keuntungan teknik ini usaha membangkitkan
sumber arus yang bisa diprediksi serta stabil cukup
dilakukan sekali saja.
CERMIN ARUS
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Bipolar Current Mirror Circuitry
The diode-connected QREF produces
an output voltage V1 that forces Icopy1= IREF, if Q1 = QREF.
REF
REF S
S copy I
I
I I
,
1
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Bad Current Mirror Example I
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.
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Bad Current Mirror Example II
Although a path for base currents exists, this technique of
biasing is no better than resistive divider.
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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.
REF REF S
jS
jcopy I I
I
I ,
,
,
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Current Scaling
By scaling the emitter area of Q j 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.
REF jcopy nI I ,
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Example: Scaled Current
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Fractional Scaling
A fraction of IREF can be created on Q1 by scaling up theemitter area of QREF.
REF copy I I 31
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Example: Different Mirroring Ratio
Using the idea of current scalingand fractional scaling, Icopy2 is0.5mA and Icopy1 is 0.05mArespectively. All coming from asource of 0.2mA.
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Mirroring Error Due to Base Currents
11
1
n
nI I REF copy
Jika sangat besar dan n kecil makaIcopy IREF, tapi jika n semakin besar,maka error menjadi makin besar
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Improved Mirroring Accuracy
Because of QF, the base currents of QREF and Q1 are mostlysupplied by QF rather than IREF. Mirroring error is reducedtimes.
1112
n
nI I REF
copy
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Example: Different Mirroring Ratio Accuracy
2
2
2
1
154
10
15
4
REF copy
REF copy
I I
I I
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PNP Current Mirror
Penguat dengan beban sumber arus (a) dapat direalisasikan
dengan PNP bekerja di daerah aktif (b), maka arus bias bagi Q2
harus di rancang secara cermat (c)
PNP digunakan sebagai beban sumber arus bagi penguat NPN.
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Example: PNP Current Mirror
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Generation of IREF for PNP Current Mirror
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Let QREF and Q1 be
discrete NPN
devices. IREF
and
Icopy1 can vary in
large magnitude
due to IS mismatch.
Example: Current Mirror with Discrete Devices
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MOS Current Mirror
The same concept
of current mirrorcan be applied to
MOS transistors as
well.
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Bad MOS Current Mirror Example
This is not a current mirror since the relationship between VX andIREF 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
The idea of combining NMOS and PMOS to produce CMOS
current mirror is shown above.
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PR
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PR