part i: amplifier fundamentals. agenda ideal amplifiers configurations and operation of amplifiers...

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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Why So MANY AMPS???

Lots of Specifications Some are Important for Different Applications

Each Amplifier is Designed to Improve or Optimize One or a Combination 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


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


Standard Configurations+ -

R1

R1

+ -

Non-Inverting

1

21R

R

V

V

IN

OUT

1

2

R

R

V

V

IN

OUT

Inverting

R2

R2

VIN

VOUT

VIN

VOUT


Operation of an “Ideal” Inverting Amplifier

+ -

R2

R1

Vin Vout

I2

I1

11R

VI

in

)(

0

1

2

21

R

RVV

RR

VV

inout

inout

Virtual Ground Because +VIN = -VIN

21 II


Operation of an “Ideal” Non-Inverting Amplifier

+ -

VinVout

I1

R1

R2

V1

1VVin

1

11R

VI

)1(1

21

21

11

R

RVV

RR

VVV

out

out


Gain-bandwidth product

GBW product = Gain x BandWidth

100000

100

1000

10000

10

1

GBP=1,000,000

X GBP=1,000,000

X

AOL

ACL


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

- +


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 [<1]


Bias Current Drift and Offset Voltage Drift

Offset Voltage is affected by the temperature

Drift is Usually in Units of V/ ºC 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/ ºC Often a minimum and maximum I BIAS is Specified over the

Temperature Range of the amp FET amplifiers have the lowest input Bias current


Very Low Bias Current Fast FETs ™ Amplifier Family Applications

Photodiode Isc is linear over 6-9 decades and is usually in the range of pA-A

Sensitivity is determined by amount of Isc multiplied by R2

Minimizing Ib will ensure the highest possible sensitivity of the system

Additionally, maximizing the bandwidth minimizes the effects of Ib

Precision Photo Diode Pre-Amp

– Low DC Errors

– Low Ibias, Vos and Drift

– Low Noise

– High-Speed+

2

Vs

Vs

AD8065

Isc

Ib

R2


Noise Gain

+ -

R2

R1

Vout

I

I

VER2

1RVVV

R

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

121 RR

V

V

ER

OUT


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

gain of 5 creates 15mV offset at the output.

IB and IB drift with resistance (R1II R2) at the summing node effectively create an additional VOS

Example: 10A and R1 = R2 = 2k creates 10mV offset

+ -

R2

R1

IB=10A

IB

Vout

Bosout IRVV *2


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 FREQUENCY

FREQUENCY

Voltage or Current Noise

Density


Common Mode Rejection Ratio (CMRR) & Power Supply Rejection Ratio (PSRR)

CMRR is a ratio (output to input) of amplifier’s ability to reject an equal signal on both of the inputs

Similarly, PSRR is a ratio (output to power supply variation) of amplifier’s ability to reject power supply noise

+

-

4V

-4V

4V

-4V

dBV

VLOGCMRR

V

VLOGCMRR

in

out

828

60020

20

4mV

-4mV


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


Rail to Rail vs. Non Rail to Rail Amplifiers

R-RR-R

In

In

Out

Out

+VS

+VS

-VS

-VS

VIN

VOUT


Output Swing

Operating Region Decreases with Increased Frequency

Output Power [dBm] = 10log[V2rms/(RL)] x1mW

Vout

Saturation

Iout

Short Circuit

Vout

Iout

Operating Region

IncreasingFrequency


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

½ W Power savings


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)

AO

L (

dB

)

Deg

rees

315

180

405

360

270

225

450

Phase margin

Gain

Phase


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 [uSec]

Vol

ts

-12

-10

-8

-6

-4

-2

0

2

4

1 10 100 1000

Frequency MHz

dB

Excessive Peaking in the closed Loop Frequency Response will

reduce the phase margin.

In the Time Domain, Low Phase margin causes Ringing

Reducing phase margin further will create sustained ringing or

oscillation


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 Rate AD8014

Slew Limited


Distortion

Changes in the output wave form relative to the input wave form For pure sign wave in, the output will have some energy at

multiples of the input frequency - harmonics10.0

-120.0

-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

9th8th7th6th5th4th3rd2ndFUND

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


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


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 most

SFDR - used for communications and other systems

Spurious-Free Dynamic Range in dB

Range between the input signal and largest harmonic


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)


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


To Be Continued…

Part II: Various Amplifier Configurations

part i: amplifier fundamentals. agenda ideal amplifiers configurations and operation of amplifiers...

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