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

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J. Kim

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.

8

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

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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.

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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.

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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.

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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.

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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.

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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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