Transcript Electronic Troubleshooting Chapter 8 Operational Amplifiers

Electronic Troubleshooting
Chapter 8
Operational Amplifiers
Operation Amplifiers
• Overview
• Original OP-Amps
Picture from
Wikipedia, see
the terms of
use on their
site.
• 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
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
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
Inverting Amplifier
Critical to understanding operation with feedback
See formulas on the bottom of page 192 and example on 193
Noninverting Amplifier
See formulas on the middle of page 193 and example on 194
Voltage Follower
Amplify AC Signals
Open Loop Voltage Gain vs Freq
741
Finding Upper Cut-off Frequency
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
Voltage Follower in AC Circuit
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
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
Acm 
Vcmo
Vcm
where
Vcmo  CommonModeOutputVoltage
Vcm  CommonModeInputVoltage
• Example Problem 8-5 on page 198
Integrator
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
Level Detector
• Zero Crossing Non-Inverting
• Zero Crossing Inverting
Level Detector
• Positive Voltage Level Detector Non-Inverting
• Positive Voltage Level Detector Inverting
Level Detector
• Negative Voltage Level Detector Non-Inverting
• Negative Voltage Level Detector Inverting
Level Detector w/Noise Tolerance
• Level Detector Non-Inverting w/Positive feedback
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
Level Detector w/Noise Tolerance
• Level Detector Inverting w/Positive feedback
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)}
LM339 Comparator
Comparator Squaring Circuit
Lo-Battery Indicator
Op-Amp is
configured as a 1.5V
level detector
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
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
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
Other problems
See page
206 for
discussion
FET problems
Zero Problems