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8/6/2019 Amps 1 Final

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Part I: Amplifier FundamentalsPart I: Amplifier Fundamentals


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Agenda

Ideal Amplifiers

Configurations and Operation of Amplifiers

Common Amplifier Source Errors

Understanding Amplifier Specification


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3


Why So MANY AMPS???

Lots of Specifications

Some are Important for Different Applications

Each Amplifier is Designed to Improve or Optimize One or aCombination of Specifications

No Ideal Op Amp; YET? Specialty Amps for a Variety of Applications and Functions

Current Amplifier Trends

Power Consumption - Driven by portable applications

Rail-to-Rail Higher Dynamic range on lower supply voltage Smaller Packaging Circuit density in portable applications

Price Higher Performance at lower Price


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What is an Ideal Op Amp?

Amplifies a small signal (X) to a larger signal (Y) by Gain of G

Ideal Op Amp Characteristics

Voltage at + Input = Voltage at - Input Infinite Input Impendence Zero Output Impendence

Infinite Open Loop Gain In closed loop Negative Input=Positive Input

Infinite Bandwidth

+

-GX Y

VIN

VOUT


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Standard Configurations

+

-R1

R1

+

-

Non-Inverting

1

21

R

R

V

V

IN

OUT

+=

1

2

R

R

V

V

IN

OUT

=

Inverting

R2

R2

VIN

VOUT

VIN

VOUT


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Operation of an Ideal Inverting Amplifier

+

-

R2

R1

Vin Vout

I2

I1

1

1

R

VI

in

=

)(

0

1

2

2

1

RRVV

RR

VV

inout

in

out

=

=

Virtual GroundBecause +VIN = -VIN

21 II=


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Operation of an Ideal Non-Inverting

Amplifier

+

-

VinVout

I1

R1

R2

V1

1VVin =

1

1

1 R

VI =

)1(1

2

1

2

1

1

1

R

RVV

RR

VVV

out

out

+=

+=


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Nothing is ideal, friends..

Real Characteristics

Finite open loop gain Offset voltage Input bias & offset currents Finite bandwidth And, these amplifiers are not free

IDEAL

REAL

-+


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Input Error Sources

VOS The difference in voltage between the inputs [~mV]

Ideal

Offset Voltage (Vos )

Input Impedance (ZIN)

Input Bias Current (Ib)Input Offset Current (Ios )

A

+

--+

Output Impedance

(ZOUT )

IB The Current into the Inputs [~pA to A]

IOS The difference between the + IB and IB [~IB /10]

ZIN Input Impedance [M to G ]

ZOUT Output Impedance [


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Bias Current Drift and Offset Voltage Drift

Offset Voltage is affected by the temperature

Drift is Usually in Units of V/ C Often a minimum and maximum VOS is Specified over the Temperature

Range of the amp

Bias Current is also affected by temperature

Drift is Usually in Units of nA/ C

Often a minimum and maximum IBIAS is Specified over the Temperature

Range of the amp

FET amplifiers have the lowest input Bias current


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Very Low Bias Current Fast FETs

Amplifier Family Applications

Photodiode Isc

is linear over 6-9 decades and is usually in the rangeof pA- A

Sensitivity is determined by amount of Isc

multiplied by R2

Minimizing Ibwill ensure the highest possible sensitivity of the system

Additionally, maximizing the bandwidth minimizes the effects of Ib

PrecisionPhoto Diode Pre-Amp

Low DC Errors

Low Ibias , Vos and Drift

Low Noise

High-Speed

+

2

Vs

Vs

AD8065

Isc

Ib

R2


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Noise Gain

+

-

R2

R1

Vout

I

I

VER2

1

RV

VVR

EREROUT

+=

Noise Gain - gain of error signals (VER ) between the inputs Non-Inverting noise gain = Voltage Gain [R2/R1]

Inverting Noise gain = absolute value of the Voltage Gain +1

1R

VI

ER

=

1

21R

R

V

V

ER

OUT

+=


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All Input Error Sources End up

at the Output

Input Referred Errors are multiplied by the Noise Gain

Initial VOS

and VOS

Drift Shift VOUT

from the expected DC level

VOS

drift multiplied by the change in temperature in C

Example: 2mV initial offset + 10 V/C with 100C shift and a

gain of 5 creates 15mV offset at the output. I

Band I

Bdrift with resistance (R

1IIR

2) at the summing node

effectively create an additional VOS

Example: 10 A and R1= R

2= 2k creates 10mV offset

+

-

R2

R1

IB=10 A

IB

Vout

Bosout IRVV *2==


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15


Input Voltage and Current Noise

2 Sources of Voltage and Current Noise

Low frequency Noise

Magnitude Increases as frequency decreases (1/f)

Wideband noise is flat over frequency

Usually Specified in Noise Density [nV/Hz and pA/Hz]

Multiply by the square root of the frequency range to determine the RMS noise The intersection is referred to as the corner frequency

CORNER

FREQUENCYFREQUENCY

Voltage or

Current

Noise

Density


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Common Mode Rejection Ratio (CMRR)

& Power Supply Rejection Ratio (PSRR)

CMRR is a ratio (output to input) of amplifiers ability to reject an equal signal on both ofthe inputs

Similarly, PSRR is a ratio (output to power supply variation) of amplifiers ability to rejectpower supply noise

+

-

4V

-4V

4V

-4V

dB

V

VLOGCMRR

V

VLOGCMRR

in

out

828

60020

20

==

=

4mV

-4mV


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Rail to Rail Amplifiers

Rail-to-rail amplifiers maximize signal swing, either on the input, the output or both.

True Rail-Rail op amps can swing to within a few mV of their power supply rails.

Non rail-to-rail op amps usually require between 1-3 volts of headroom to the supply rail

Analog Devices Rail to Rail Amplifiers

Rail to Rail Output

Fast FETsTM

AD8091/2 Very Low Cost, High-Performance

Rail To Rail Input

AD8031/2 Low Power High-Speed


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Rail to Rail vs. Non Rail to Rail Amplifiers

R-RR-R

In

In

Out

Out

+VS

+VS

-VS

-VS

VIN

VOUT


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Output Swing

Operating Region Decreases with Increased Frequency

Output Power [dBm] = 10log[V2rms

/(RL)] x1mW

Vout

Saturation

IoutShort Circuit

Vout

Iout

Operating Region

Increasing

Frequency


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Low-Power, Application Considerations

Minimize supply voltage circuitry or battery requirements

Reduce cooling requirements

Lower Heat Dissipation Saves Cost and Space

Smaller heat sinks Essential in higher density PCB

Increases system stability and reliability

Example:

System with 5 AD8058

(+/-5V)*(6.5mA/amp max)* (10 amps) = 650mW

Using AD8039

(+/-5V)*(1.7mA/amp max)* (10 amps) = 170mW

W Power savings


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21


Relation Between

Open Loop Gain and Phase

Oscillation will occur when Phase Delay 360 and a Gain >0dB Phase Margin is the phase remaining before oscillation where the

gain curve crosses 0dB

Margin of Less than 30 degrees is too little for safe operation

Open Loop Gain vs Freq..

-20

-10

0

10

20

30

40

50

0.01 0.1 1 10 1001000

Frequency (MHz)

A

OL

(dB)

Degrees

315

180

405

360

270

225

450

Phase margin

Gain

Phase


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Why Phase Margin is Important

-0.5

-0.4

-0.3

-0.2

-0.1

0

0. 1

0. 2

-0.1 0 0. 1 0. 2 0. 3 0.4 0.5

Time [uSe

Volts

-1 2

-1 0

-8

-6

-4

-2

0

2

4

1 1 0 1 0 0 1 0 0 0

F r e q u e n c y

dB

Excessive Peaking in the closed Loop Frequency Response willreduce the phase margin.

In the Time Domain, Low Phase margin causes Ringing

Reducing phase margin further will create sustained ringing oroscillation


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Slew Rate and Large Signal Bandwidth

Slew Rate Determines the Limit for Large Signal Bandwidth

+

-X Y

maxdt

dVSlewRate =

Maximum Change in Voltage

Change in Time

Amplitude

SRBandwidth

=

2 High Slew RateAD8014

Slew Limited


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Distortion

Changes in the output wave form relative to the inputwave form

For pure sign wave in, the output will have some energy atmultiples of the input frequency - harmonics

10.0

-10.0

-115.0

-110.0

-105.0

-100.0

-95.0

-90.0

-85.0

-80.0

-75.0

-70.0

-65.0

-60.0

-55.0

-50.0

-45.0

-40.0

-35.0

-30.0

-25.0

-20.0

-15.0

-10.0

-5.0

0.0

5.0

32.5000E+60.0000E+0 5.0000E+6 10.0000E+6 15.0000E+6 20.0000E+6 25.0000E+6

ththththththddD

0

10

-10

-20

-30

-40

-50

-60

-70

-80

-90

-100

-110

-120

[dB]

5 10 15 20 25 30

Frequency [MHz]

Fundamental

3rd

2nd


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Ideal for Buffering ADC Driver

Other Applications

IF/Baseband Amplifiers Precision Instruments

Baseband and Video Communications Pin Diode Receivers Precision Buffer

Ultra Low-Distortion and Noise Applications

+

Rf

Rg

AD8007Passive

Filter


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Various Distortion Specifications

THD used for Audio and other systems

Total Harmonic Distortion - sum of all distortions at all harmonics

Usually 2nd and 3rd harmonics contribute the mostSFDR - used for communications and other systems

Spurious-Free Dynamic Range in dB

Range between the input signal and largest harmonic


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NEW High Value, Low Price Products

Fast FETsTM

AD8034 and AD8065 The Highest Bandwidth per Dollar among all FET input Amps $1.19 @ 1k (AD8034)

Precision FET (PRA)

Low-Cost High-Performance

AD8091/2

$0.69 @ 1k (AD8091) Auto Zero (PRA)

Fast Speed-Low Power

AD8038 and AD8039 Highest Speed per mA at only $0.85 @ 1k (AD8038)

CMOS (PRA)

Low Distortion, Low Power

AD8007/8 Best Distortion at specified Is at only $1.19 @ 1k (AD8007)


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Packaging Considerations

All of the Amplifiers are available in small packaging Maximizes the density of the board

Refer to the datasheet for particular amplifier package

SOIC

SC70

SOICSewing Needle

SOT23


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To Be Continued

Part II: Various Amplifier Configurations

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