Part I: Amplifier Fundamentals

Agenda Ideal Amplifiers Configurations and Operation of Amplifiers Common Amplifier Source Errors Understanding Amplifier Specification

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? VIN + - G X Y VOUT 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

Standard Configurations VIN + - Non-Inverting R2 R1 VOUT R2 Inverting VIN R1 - + VOUT

Operation of an “Ideal” Inverting Amplifier Virtual Ground Because +VIN = -VIN I2 R2 I1 Vin Vout - + R1

Operation of an “Ideal” Non-Inverting Amplifier Vin Vout + - R2 V1 I1 R1

Gain-bandwidth product GBW product = Gain x BandWidth AOL 100000 GBP=1,000,000 10000 GBP=1,000,000 X 1000 ACL X 100 10 1

Nothing is ideal, friends.. - + REAL Real Characteristics Finite open loop gain Offset voltage Input bias & offset currents Finite bandwidth And, these amplifiers are not free…

Input Error Sources - A + Ideal Input Impedance (ZIN) Output Impedance - + Ideal Input Impedance (ZIN) Output Impedance (ZOUT) - A + Input Offset Current (Ios) Input Bias Current (Ib) Offset Voltage (Vos) VOS – The difference in voltage between the inputs [~mV] IB – The Current into the Inputs [~pA to mA] IOS – The difference between the + IB and – IB [~IB /10] ZIN – Input Impedance [MW to GW] ZOUT – Output Impedance [<1W]

Bias Current Drift and Offset Voltage Drift Offset Voltage is affected by the temperature Drift is Usually in Units of mV/ 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 + R2 Low DC Errors Low Ibias, Vos and Drift Low Noise High-Speed Isc Ib AD8065 Precision Photo Diode Pre-Amp Photodiode Isc is linear over 6-9 decades and is usually in the range of pA-mA 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

Noise Gain 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 I R2 I Vout - + VER R1

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 + 10mV/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: 10mA and R1 = R2 = 2k creates 10mV offset IB R2 IB=10mA Vout - + R1

Input Voltage and Current Noise CORNER FREQUENCY FREQUENCY Voltage or Current Noise Density 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

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 + - 4mV -4mV 4V -4V

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 VIN +VS Out -VS In VOUT In +VS R-R Out -VS

Output Swing Operating Region Decreases with Increased Frequency Vout Saturation Increasing Frequency Vout Operating Region Iout Short Circuit Iout Operating Region Decreases with Increased Frequency Output Power [dBm] = 10log[V2rms/(RL)] x1mW

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 10 20 30 40 50 0.01 0.1 1 100 1000 Frequency (MHz) AOL (dB) Degrees 315 180 405 360 270 225 450 Phase margin Gain Phase

Why Phase Margin is Important 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 Maximum Change in Voltage Change in Time Slew Rate Determines the Limit for Large Signal Bandwidth High Slew Rate AD8014 + X Y - 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 - harmonics 10 -10 -20 -30 -40 -50 -60 -70 -80 -90 -100 -110 -120 [dB] 5 15 20 25 30 Frequency [MHz] Fundamental 3rd 2nd

Ultra Low-Distortion and Noise Applications Ideal for Buffering ADC Driver Other Applications IF/Baseband Amplifiers Precision Instruments Baseband and Video Communications Pin Diode Receivers Precision Buffer + Rf Rg AD8007 Passive 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 mSOIC Sewing Needle SOIC SC70 SOT23

To Be Continued… Part II: Various Amplifier Configurations