01Ch7_CascodeAmps_Currentmirrors

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EE 3120 Microelectronics II Suketu Naik

Chapter 7Building 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 5. Ch 11: Output Stages

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Current Mirror

Two StageOp Amp(MOSFET)

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Current SourceConstant current source to 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 exist: Common-source (CS) Common-emitter (CE)

Both are loaded with constant-current source. This is done instead of resistor-on-chip because of

difficulties associated with fabrication of exact resistances and available chip area

It also increases gain (recall that Avo=-gmro||RD)

These circuits are referred to as current-source loaded or active-loaded.

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Part A:Cascode Amplifier

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Current Source The current-source load of the CS amplifier can be

implemented using a PMOS transistor biased in the saturation region to provide the required current I

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7.2.3. Effect of Active LoadThe current source, which is implemented by the active load will decrease the output resistance and voltage gain

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7.2.3 Effect of the Output Resistance of the Current-Source Load

large-signal MOSFET model

current-sourceno longer hasinfinite output

resistan

212

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for PMOS implemen

(7.16)

(7.17)

tation of active-loaded CS amplifier:

p ox DD G tp

Ao

WI C V V V

LV

rI

ce

1 1 2 (7.18) ||ov m o o

i

vA g r r

v

Why is the effect of Q2 approximated by 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

resitance How do you make a current buffer?

A: Place CG (CB) stage on top of the CS (CE) stage Also called Casocoding

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

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Cascoding: Improving gain and output resistanceCascode Amplifier

With an ideal current source load, gain = (gmro)2 = A02

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p7.24: MOS cascode amplifier

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Cascoding: Improving gain and output resistanceDouble Cascoding

With an ideal current source load, gain = ?

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Cascoding: Improving gain and output resistanceCascoding: active load

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-load

Rin

Ro

Av

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7.3 Cascode AmplifierCascode 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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p7.26: MOS cascode amplifier

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Cascoding: Improving gain and output resistanceBJT Cascode

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Cascoding: Improving gain and output resistanceBiCMOS (Bipolar + CMOS) Cascode

(a) MOS as amplifier, BJT as cascode: infinite input impedance and increased output resistance(b) MOS used for double cascoding the BJT amplifier: why? max possible Ro with BJT cascode is β2ro2

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Cascoding: Improving gain and output resistanceFolded Cascode Amplifier

What is the advantage?

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Cascoding: What are the advantages and disadvantages?Advantages

Disadvantages

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Part B:Biasing and Current Mirrors

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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

This approach has the advantage that the effort expended on generating a predictable and stable reference current need not be repeated.

Current Sources, Current Mirrors, and Current-Steering Circuits

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7.4.1 The Basic MOSFET Current Source

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2

11

1

2

22

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

RW

I I k V VL

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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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(Common Drain) Common Source

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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

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p7.49: MOS Current Mirror

Simulate

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p7.58: BJT Current Mirror

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How do you make CM on an IC?

Single Gate Layout Circuit

Parallel Gate Layout

Active Region Drain SourceGate

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Part C:Advanced Current Mirror Circuits

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7.5. Current-Mirror Circuits with Improved Performance

7.5.1. Cascode MOS Mirror Cascoding of transistors may be used to increase gain and acquire better performance

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7.5.3 The Wilson Current Mirror (BJT) The addition of a diode-connected transistor in series with Q2 may

reduce the effect of β on output resistance.


Basic Current Mirror Wilson Current Mirror

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7.5.3 The Wilson Current Mirror (MOSFET) No β exists for MOS. However, Wilson's adaptation may be used

to increase output resistance and gain.


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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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7.5.4 The Widlar Current Mirror A resistor RE is included in the emitter lead of Q2


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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 ID

1/2. 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 = A0

2

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||r)]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 problem is the bipolar case is the Wilson circuit.

01Ch7_CascodeAmps_Currentmirrors

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