ECEN474: (Analog) VLSI Circuit Design Fall 2011

Lecture 24: Variable Gain Amplifiers (VGAs)

Sebastian HoyosAnalog & Mixed-Signal Centerg g

Texas A&M University

Agenda

• Variable Gain Amplifiers• Material is related primarily to Project #4

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Variable Gain Amplifier (VGA) Applications

• Variable gain amplifiers (VGAs) are l d i li ti i d temployed in many applications in order to

maximize the overall system dynamic range• Critical component of automatic-gain

control (AGC) systems

VGA [Pandey]

3Hard-Disk Drive Receiver Front-End

[ y]

Typical VGA Design Goals

• Constant bandwidth across wide gain rangeE ti l i t l (“li i dB”) f d• Exponential gain control (“linear in dB”) preferred in many applicationsLow noise low distortion low power• Low noise, low distortion, low power

Poor Performance Desired Performance[Gilbert]

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VGA Techniques

• Multipliers

• Transconductance ratio amplifiersp

• Source degeneration• Source degeneration

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Multiplier-Based VGA

oxm

Dmv

IWCg

RgA

31

1

[Razavi]

Tcontox

oxm

VVLWCI

Lg

23

31

1

2

?V by affected I is How cont3

TcontoxTcontox

m

WW

LW

LWVVCVV

LW

LWCg

L

31

2

31

2

1

3

1

21

2

2

Gain can be linea l cont olled b V

DTcontoxv RLW

LWVVCA

3121

• Gain can be linearly controlled by Vcont

• Circuit only operates with positive Vcont (2-quadrant), which is generally OK for VGA applicationsg y pp

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4-Quadrant Multiplier• Allows multiplication in all 4-

quadrants

[Razavi]

• Differential Vcont allows the sign of the gain to be inverted

• Can also use for VGAs, although 4-quadrant operation is not necessary

• Often used in RF transceivers as a frequency translator (mixer)

• Also called the “Gilbert Cell”, after Barrie Gilbert who is the inventor of the bipolar version

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Transconductance Ratio VGA #1

• Diode-load transconductance (g ) cantransconductance (gm2) can be altered by stealing current with a parallelcurrent with a parallel current source M3, thus altering the gain

• Issues• Gain is a ratio of nmos and

pmos transconductance, which can be sensitive to process variations2

1

m

mv g

gA

p ocess a at o s• Bandwidth changes with gain

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Transconductance Ratio VGA #2

ISSCC 1997

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Transconductance Ratio VGA #2

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Transconductance Ratio VGA #2

• gmi is from M1• g is from M2• gmo is from M2• M4 source-follower

output buffers• Both the gmi and gmo

transistors are segmented into multiplesegmented into multiple parallel transistors

• Gain is controlled by yswitching off bias current to these segments

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Transconductance Ratio VGA #2

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Source Degeneration VGA

ISSCC 1999

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Source Degeneration VGA

Gm-OpAmp-C Integrator

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Gm OpAmp C Integrator

Source Degeneration VGA

B d idth d d l di l i t t

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• Bandwidth and group delay display consistent performance over gain range

Digitally Controlled VGA

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VGA Based on Analog Multiplier & Current Mirror Amplifiers& Current Mirror Amplifiers

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Analog Multiplier

vvWCi 4 xyoxout vvL

Ci 4

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VGA Based on Analog Multiplier & Current Mirror Amplifiers& Current Mirror Amplifiers

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Low-Voltage Cascode Current Mirrors

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Basic Current Amplifier Frequency Response

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Frequency Compensation Scheme

• Parallel transconductancet i t MC ith ititransistor MC with capacitive degeneration introduces a zero which provides frequency

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p q ycompensation

Measurement Results

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Next Time

• Analog Applications• Switch-Cap Filters, Broadband Amplifiers

• Bandgap Reference Circuits• Distortion

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