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

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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

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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

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REFcopy

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II

1, copyREFC II

REFCFBREF III ,, 4REFCcopycopy

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2

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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.