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EECE488: Analog CMOS Integrated Circuit Design

Set 5

Current Mirrors

Shahriar Mirabbasi

Department of Electrical and Computer Engineering

University of British Columbia

shahriar@ece.ubc.ca

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Applications of Current Sources

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Simple Resistive Biasing for Current Source

2

12

2 )(2

THDDoxn

OUT VVRR

RLWCI

+

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Problems

Output current depends on:

Supply

Process

Temperature

What if the bias voltage is independent of supply voltage?

Is there a way of generating reliable currents?

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Basic Idea

Iout is a function of gate-source voltage

Typically we assume that one precisely defined current source isavailable and other current sources copy their current from this precise

source.

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Basic Idea

This structure is called current mirror

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Question

What happens if the two transistors in the basic current mirrorhave different sizes?

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Example

Assuming all the transistors are in saturation region, find Iout:

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Current Mirrors: Amplifier Bias Example

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Board Notes

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Current Mirrors: Signal Amplification Example

Find the small signal voltage gain of the following circuit.

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Effect of Channel Length Modulation

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Cascode Current Mirror

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Board Notes

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Cascode Current Mirror

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Cascode Current Mirror

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Cascode Current Mirror

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Board Notes

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Cascode Current Mirror Biasing

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Cascode Current Mirror Biasing

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Current Mirror Biasing

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Basic Circuit to Generate Supply Independent Current

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Supply Independent Current

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Board Notes

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Supply Independent Current

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Start-up Problem

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Start-up Problem

DDGSTHGSDDTHTHTH VVVVandVVVV >++

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Active Current Mirrors

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Active Current Mirrors in Differential to Single-Ended

Amplifiers

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Differential to Single-Ended Amplifiers

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Calculation of Gm

ID1 = ID3 = ID4 = gm1,2Vin / 2 ID2 = gm1,2Vin / 2

Iout = ID2 ID4 = gm1,2Vin , Gm = gm1, 2

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Small-Signal Gain

Av gm1,2 (ro2 || ro4 )

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Common Mode Characteristics

ACM =Vout

Vin ,CM

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Common Mode

ACM

1

2gm3,4||ro3,4

2

12gm1,2

+RSS

=1

1 + 2gm1,2RSS

gm1,2

gm3, 4

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Common Mode

CMRR =ADM

ACM

= gm1, 2(ro1,2 || ro3,4 )gm3,4 (1 + 2gm1,2RSS )

gm1,2

= gm3,4 (ro1, 2 || ro3,4 )(1 + 2gm1, 2RSS)