Chapter 7 Building Blocks of Integrated Circuit Amplifiers: Current Mirrors...

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ECE 3120 Microelectronics II Dr. Suketu Naik

Chapter 7

Building Blocks

of Integrated

Circuit Amplifiers:

Current Mirrors

and Biasing

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Operational Amplifier Circuit Components

1. Ch 7: Current Mirrors and Biasing

2. Ch 9: Frequency Response

3. Ch 8: Active-Loaded Differential Pair

4. Ch 10: Feedback and Compensation

5. Ch 11: Output Stages

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Current Mirror

Two Stage

Op Amp

(MOSFET)

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Discrete Amplifier Design: Gain Resistor

CS (Common Source) Amplifier

Resistor RD provides the gain

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IC Amplifier Design: Current Source

Constant current source to provide gain and bias

the transistor

CS (Common Source)

gain cell

CE (Common Emitter)

gain cell

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7.2 The Basic Gain Cell

▪ Two types of basic gain cells on IC:

▪ Common-source (CS)

▪ Common-emitter (CE)

▪ Both are loaded with constant-current source

▪ Resistor-on-chip is difficult to fabricate (tolerance and area)

▪ Current source provides higher output resistance than

discrete resistor and will increase the gain

Av=-gmro||RD for resistor-loaded CS amplifier

Av=-gmro||ro_currentsource for active-loaded CS amplifier

▪ These circuits are referred to as current-source loaded or

active-loaded

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Part A: Cascode Amplifiers

Goals:

1) Learn how it is implemented in IC using an active

load

2) Use these lessons to learn about Current Mirrors (CM)

both in IC and discrete design

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Current Source in IC

Current-source of the CS amplifier can be implemented

using a PMOS transistor biased in the saturation

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7.2.3 Effect of Active Load

The current source, which is implemented by the active load,

will decrease the output resistance and voltage gain

Av=-gmro1||ro2

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7.3 Cascode Amplifier

How do you increase gain?

▪ A: Increase output resistance

How do you increase output resistance?

▪ A: Current buffer

What does a current buffer do?

▪ A: it passes the current through and increases the

output resistance

How do you make a current buffer?

▪ A: Place CG (CB) stage on top of the CS (CE) stage

▪ Also called Cascoding

The gain of the basic gain cell can be increased by cascoding

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Cascoding: Improving gain and output resistance

Cascode Amplifier

with CS-CG

configuration

With an ideal current source load, total gain = -(gm ro)2

Cascode-stage gain

(1) CS amp gain, Avo=-gm1 Ro

where, Ro=total output resistance

=(gm2 ro2 ) ro1

CS amp gain, Avo=-gm1 (gm2 ro2 ) ro1

(2) If identical transistors then,

CS amp gain=

Avo=-(gm ro)2 …(eq7.27)

Total output resistance

Ro=(gm2 ro2 ) ro1=gm ro2 …(eq7.25)

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Cascode Amplifier

with CS-CG

configuration

With an ideal current source load, total gain = -(gm ro)2

Cascode-stage gain

(1) CS amp gain, Avo=-gm1 Ro

where, Ro=total output resistance

=(gm2 ro2 ) ro1

CS amp gain, Avo=-gm1 (gm2 ro2 ) ro1

(2) If identical transistors then,

CS amp gain=

Avo=-(gm ro)2 …(eq7.27)

Total output resistance

Ro=(gm2 ro2 ) ro1=gm ro2 …(eq7.25)

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Cascode Amplifier with CS-CG configuration and Active load

Ro = Rop || Ron= ro3 || ((gm2 ro2 ) ro1)

Av = - gm1 Ro= - gm1 (ro3 || ((gm2 ro2 ) ro1))

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7.3 Cascode Amplifier

Cascode Amplifier with Cascode Current Source

Q3 (CG stage) raises the output resistance of

the current source Q4 (CS Stage)

Q2 (CG stage) raises the output resistance of

the amplifier Q1 (CS Stage)

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Double Cascoding

Q: What are the

limitations to

Cascoding?

With an ideal current source load,

total gain = ? total output resistance = ?

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What is the impact of active load on each circuit?

CS Amplifier

with Active-load

Cascode CS Amplifier

with Active-load

Cascode CS Amplifier

with Cascode Active-

loadCircuit

Rin

Ro

Av

Av (identical

transistors)

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BJT Cascode

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BiCMOS (Bipolar + CMOS) Cascode

(a) MOS as amplifier, BJT as cascade stage:

Infinite input impedance and increased output resistance

(b) MOS for double-cascoding the BJT amplifier:

Why? Limitation of BJT: max possible Ro with BJT cascode is β2 ro2

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Folded Cascode Amplifier

What is the advantage?

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Advantages

Disadvantages

Cascoding: What are the advantages and disadvantages?

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In Class Practice Problems

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p7.24: MOS cascode amplifier

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p7.26: MOS cascode amplifier

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Part B:

Biasing and Current Mirrors

Goals:

1) Learn what current mirror does

2) Learn the effect on the amplifier

3) Learn how to analyze a current mirror

and how it is implemented

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7.4 IC Biasing

▪ Biasing in Integrated-Circuit (IC) design is based on the use of constant-current sources

▪ Goal: create stable constant-current source

▪ On an IC chip with a number of amplifier stages, a constant dc current (reference current) is generated at one location and is then replicated at various other locations for biasing

▪ This is known as current steering

Current Sources, Current Mirrors, and Current-Steering

Circuits

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7.4.1 The Basic MOSFET Current Source

Diode-connected

transistor

Q1 is always in saturation!

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2

1

1

1

2

2

2

2

1

1(7.52)

2

(7.53)

1(7.54)

2

( / )(7.55)

( / )

D n GS tn

DD GSD REF

O D n GS tn

O

REF

WI k V V

L

V VI I

R

WI I k V V

L

I W L

I W L


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22

2

1 2

(7.56)

(7.57)

(7.58)

( / )(7.58) 1

( / )

O GS tn

O OV

O Ao o

O O

O GSo REF

A

V V V

V V

V VR r

I I

V VW LI I

W L V


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expl7.5: MOS Current Mirror

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7.4.2 MOS Current-Steering Circuits

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▪ Once a constant current has been generated, it can be replicated to provide dc bias or load current for the various stages of the amplifier in an IC

▪ Current mirrors can be used to achieve this goal

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1

33

1

2 3 1

2 3 1

55 4

4

5 5

( / )(7.60)

( / )

( / )(7.61)

( / )

(7.62) ,

(7.63) ,

( / )(7.64)

( / )

(7.65)

REF

REF

D D SS GS tn

D D SS OV

D DD OV

W LI I

W L

W LI I

W L

V V V V V

V V V V

W LI I

W L

V V V

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Source Follower

(Unity gain amplifier)

Common Source

Amplifier

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How do you make Current Mirror on an IC?

Single Gate Layout Circuit

Parallel Gate Layout

Active Region DrainSource

Gate

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Very similar to the MOS mirror. However, with two important differences:

▪ The non-zero bias current causes an error in current mirroring (magnitude of current conducted)

▪ The current transfer ratio is determined by the relative areas of the emitter-based junctions of Q1 and Q2

7.4.3 BJT Current Mirrors

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Application: Common Emitter Amplifier with Current Mirror

NPN/PNP (NMOS/PMOS) work as complimentary pair: e.g. if PNP

is used as amplifier, the current source is provided by NPN based

current mirror

PNP-based

Common Emitter Amplifier

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ex7.18: BJT Current Mirror

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p7.49: MOS Current Mirror

Simulate

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p7.58: BJT Current Mirror

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Part C:

Advanced Current Mirror Circuits

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7.5 Current-Mirror Circuits with Improved Performance

Cascode MOS Mirror

▪ Cascoding of transistors

can be used to increase gain

and acquire better

performance

▪ Very popular biasing

circuit in IC design

Q4 and Q1 are always in

saturation

Design Equations?

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Wilson Current Mirror (BJT based)

▪ Addition of a diode-connected transistor in series with Q2 can reduce the effect of β on output resistance


Basic Current Mirror Wilson Current Mirror

Design Equations?

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Wilson Current Mirror (MOSFET based)

▪ Wilson current mirror can be used to increase output

resistance and gain


Design Equations?

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Widlar Current Mirror

▪ A resistor RE is included in the emitter lead of Q2


Design Equations?

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p7.65: MOS Cascode Current Mirror

Also calculate change in current given change in Vo=10V

Q: How to provide Iref?

A: Iref=(VDD-2VBE)/R

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p7.70: Wilson Current Mirror

Find R if VCC=VEE=2.5 V. Compare change in current with 7.58 and 7.65.

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expl7.6 & ex7.22: Widlar Current Mirror

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Summary

▪ An overriding concern for IC designers is the minimization of chip area or “silicon real estate.” As a result, large-valued resistors and capacitors are virtually absent.

▪ We can use an active load instead of passive resistors.

▪ The basic gain cell of IC amplifier is the CS (CE) amplifier with a current-source load. For an ideal current-source load (i.e. one with infinite output resistance), the transistor operates in an open-circuit fashion and thus provides the maximum gain possible: Avo = -gmro = -A0.

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▪ The intrinsic gain A0 is given by A0 = VA / VT for a BJT and A0

= VA/(VOV/2) for a MOSFET. For a BJT, A0 is constant

independent of bias current and device dimensions. For a

MOSFET, A0 is inversely proportional to ID1/2. See equation

7.15

▪ Simple current-source loads reduce the gain realized in the

basic gain cell because of their finite resistance (usually

comparable to the value of ro of the amplifying transistor)

▪ To raise the output resistance of the CS or CE transistor, we

stack a CG or CB transistor on top. This is called cascoding.

The CG or CB transistor in the cascode passes the current

gm1vi provided by the CS or CE transistor.

Summary

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▪ A MOS cascode amplifier operating with an ideal current source load achieves a gain of (gmro)

2 = A02

▪ To realize the full advantage of cascoding, the load current-source must also be cascoded, in which case a gain as high as 1/2A0

2 can be obtained

▪ Double cascoding is possible in the MOS case only. However, the large number of transistors in the stack between the power-supply rails results in the disadvantages of a severely limited output-signal swing. The folded-cascode configuration helps to resolve this issue.

Summary

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▪ A CS amplifier with a resistance Rs in its source lead has an output resistance Ro = (1+gmRS)ro. The corresponding formula for the BJT case is Ro = [1+gm(Re||rp)]ro

▪ Biasing in integrated circuits utilizes current sources. As well, current sources are used as load devices. Typically an accurate and stable reference current is generated and then replicated to provide bias current for the various amplifier stages on the chip. The heart of the current-steering circuitry utilized to perform this function is the current mirror

▪ The MOS current mirror has a current transfer ratio of (W/L)2/(W/L)1. For a bipolar mirror, the ratio is IS2/IS1.

Summary

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Summary

▪ Bipolar mirrors suffer from the finite β, which reduces the

accuracy of the current transfer ratio

▪ Both bipolar and MOS mirrors of the basic type have a finite

output resistance equal to ro of the output device. Also, for

proper operation, a voltage of at least 0.3V is required across

the output transistor of a simple bipolar mirror (|VOV| for the

MOS case)

▪ Cascoding can be applied to current mirrors to increase their

output resistances. An alternative that also solves the bproblem is the bipolar case is the Wilson circuit.

Chapter 7 Building Blocks of Integrated Circuit Amplifiers: Current Mirrors...

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