electronic troubleshooting chapter 8 operational amplifiers
TRANSCRIPT
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Electronic Troubleshooting
Chapter 8Operational Amplifiers
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Operation Amplifiers• Overview
• Original OP-Amps• 1940/50’s Tube circuits• Discrete component semi-conductor circuits followed• I the first monolithic ICs started appearing in the 1960s
• The first was in 1963 • The 741 was released in 1968
• Packaging• Cans• DIPs• Surface mount
Picture from Wikipedia, see
the terms of use on their
site.
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Operation Amplifiers• Overview
• Characteristics• Multistage amplifier
• Coupling Cap• Simplified drawing on
the top • Complementary
Symmetry output» Low output
impedance• Some have FETs on the
input• Bottom - simplified
drawing of LF351» 741 replacement
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Op Amp Basic configuration
• Open Loop • Gain
• Ideal Gain = infinity• Actual = 200k into
millions• Input Impedance
• Ideal = infinity• Real mega ohms
• Output impedance• Ideal = zero• Actual ranges to less
than 1 ohm
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Inverting Amplifier
See formulas on the bottom of page 192 and example on 193
Critical to understanding operation with feedback
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Noninverting Amplifier
See formulas on the middle of page 193 and example on 194
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Voltage Follower
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Amplify AC Signals
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Open Loop Voltage Gain vs Freq
741
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Finding Upper Cut-off Frequency
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Compensate Op Amp
Some very old OP-Amp ICs require external components to prevent high freq oscillations, such
as Fairchild’s 709
Data Sheet: http://www.datasheetcatalog.com/datasheets_pdf/L/M/7/0/LM709.shtml
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Voltage Follower in AC Circuit
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Differential Amplifiers
• Characteristics• Uses ICs instead of discrete components• Gain is based RF and R1 • RA and RB also factor into the operation
• Use 10 and 100K and walk through
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Differential Amplifiers• Characteristics
• Only the difference between signals should be amplified• How well this is accomplished in an actual Op-Amp is measured by the Common Mode
Rejection Ratio - CMRR• Ideally – infinite• Actual is listed in the manufacturers specification sheet
• Common Mode Gain
• Example Problem 8-5 on page 198
geInputVoltaCommonModeV
ageOutputVoltCommonModeV
where
V
VA
cm
cmo
cm
cmocm
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Integrator
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Level Detector• Characteristics
• As shown the circuit is a zero crossing detector• Swap the inputs and its an inverting zero crossing detector
• Detecting other levels besides zero volts • Back to original drawing: add a DC voltage to the inverting
input• You now have a level detector for that voltage• Swap the inputs and you have an inverting detector
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Level Detector• Zero Crossing Non-Inverting
• Zero Crossing Inverting
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Level Detector• Positive Voltage Level Detector Non-Inverting
• Positive Voltage Level Detector Inverting
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Level Detector• Negative Voltage Level Detector Non-Inverting
• Negative Voltage Level Detector Inverting
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Level Detector w/Noise Tolerance• Level Detector Non-Inverting w/Positive feedback
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Level Detector w/Noise Tolerance• Level Detector Non-Inverting w/Positive
feedback• Formulas
• VUT = VRef(1+1/n) - (-Vsat/n)
• VLT = VRef(1+1/n) - (+Vsat/n)
• VCtr = (VUT - VLT)/2 = VRef(1+1/n)
• VH = VUT - VLT = {(+Vsat) - (-Vsat)}/n
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Level Detector w/Noise Tolerance• Level Detector Inverting w/Positive feedback
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Level Detector w/Noise Tolerance• Level Detector Inverting w/Positive feedback
• Formulas
• VUT = VRef(n/{n+1}) + (+Vsat/{n+1})
• VLT = VRef (n/{n+1}) + (-Vsat /{n+1})
• Vctr = (VUT + VLT)/2 = VRef (n/{n+1})
• VH = VUT - VLT = = {(+Vsat) - (-Vsat)/(n+1)}
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LM339 Comparator
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Comparator Squaring Circuit
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Lo-Battery Indicator
Op-Amp is configured as a 1.5V
level detector
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Locating Faults
• IC failures• Almost always from
• Handling• Misuse
• Typical misuse/Handling problems
• Power supply voltages that are too high – Check datasheets
• Power supply connections are reversed
• Simple protection is possible
» Use some diodes
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Locating Faults• IC failures
• Typical misuse/Handling problems• Too large of input voltages
• If max input is below 0.7V» Use diodes» Else use zener diodes
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Locating Faults• IC failures
• Typical misuse/Handling problems• Output shorted
• Small resister sized to prevent the max output current from being exceeded
See page 206
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Other problems
FET problems
Zero Problems
See page 206 for
discussion