cermin arus

8/18/2019 cermin arus

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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

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