EE105 Fall 2007 Lecture 12, Slide 1 Prof. Liu, UC Berkeley

Lecture 12

OUTLINE• Cascode Stage (cont’d)

– supplementary remarks• Current Mirrors

Reading: Chapter 9.2

ANNOUNCEMENTS• Review session: 3-5PM Friday (10/5) in 306 Soda (HP Auditorium)• Midterm #1 (Thursday 10/11, 3:30PM-5:00PM) location:

• 106 Stanley Hall: Students with last names starting with A-L• 306 Soda Hall: Students with last names starting with M-Z

EE105 Fall 2007 Lecture 12, Slide 2 Prof. Liu, UC Berkeley

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.

1211

12112

||

||)]||(1[

rrrgR

rrrrrgR

OOmout

OOOmout

EE105 Fall 2007 Lecture 12, Slide 3 Prof. Liu, UC Berkeley

Another View of a Cascode Stage• Instead of considering a cascode as Q2 degenerating

Q1, we can also think of it as Q1 stacked on top of Q2

(current source) to boost Q2’s output impedance.

EE105 Fall 2007 Lecture 12, Slide 4 Prof. Liu, UC Berkeley

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

EE105 Fall 2007 Lecture 12, Slide 5 Prof. Liu, UC Berkeley

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.

EE105 Fall 2007 Lecture 12, Slide 6 Prof. Liu, UC Berkeley

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

,1,

1 lnln

Current mirror concept Generation of required VBE Current Mirror Circuitry

EE105 Fall 2007 Lecture 12, Slide 7 Prof. Liu, UC Berkeley

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.

EE105 Fall 2007 Lecture 12, Slide 8 Prof. Liu, UC Berkeley

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.

EE105 Fall 2007 Lecture 12, Slide 9 Prof. Liu, UC Berkeley

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

,

,,

EE105 Fall 2007 Lecture 12, Slide 10 Prof. Liu, UC Berkeley

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 ,

EE105 Fall 2007 Lecture 12, Slide 11 Prof. Liu, UC Berkeley

Example: Scaled Currents

EE105 Fall 2007 Lecture 12, Slide 12 Prof. Liu, UC Berkeley

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

EE105 Fall 2007 Lecture 12, Slide 13 Prof. Liu, UC Berkeley

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).

EE105 Fall 2007 Lecture 12, Slide 14 Prof. Liu, UC Berkeley

Effect of Base Currents

111

n

nII REFcopy

copycopy

REFCREF

I

n

III ,

n

II copy

REFC ,

EE105 Fall 2007 Lecture 12, Slide 15 Prof. Liu, UC Berkeley

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 ,

EE105 Fall 2007 Lecture 12, Slide 16 Prof. Liu, UC Berkeley

Different Mirroring Ratio Accuracy

2

2

2

1

154

10

154

REFcopy

REFcopy

II

II

1, copyREFC II

REFCFBREF III ,, 4REFCcopycopy

FC

IIII ,21, 4

2

1,

15

copy

FB

II

EE105 Fall 2007 Lecture 12, Slide 17 Prof. Liu, UC Berkeley

PNP Current Mirror• A PNP BJT current mirror can be used as a current-

source load for an NPN BJT amplifier stage.

EE105 Fall 2007 Lecture 12, Slide 18 Prof. Liu, UC Berkeley

Generation of IREF for a PNP-BJT Current Mirror

• Neglecting base currents, the currents flowing through QM and QREF2 are the same.

EE105 Fall 2007 Lecture 12, Slide 19 Prof. Liu, UC Berkeley

Current Mirror with Discrete BJTs• If QREF and Q1 are discrete NPN BJTs, IREF and Icopy1 can

differ dramatically, due to IS mismatch.