Electronic Troubleshooting

1
Electronic Troubleshooting

• Chapter 8
• Operational Amplifiers

2
Operation Amplifiers

• Overview
• Original OP-Amps
• 1940/50s 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.
3
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

4
Op Amp Basic configuration

• Open Loop
• Gain
• Ideal Gain infinity
• Actual 200k into millions
• Input Impedance
• Ideal infinity
• Real 100s of mega ohms
• Output impedance
• Ideal zero
• Actual ranges to less than 1 ohm

5
Inverting Amplifier
Critical to understanding operation with feedback
See formulas on the bottom of page 192 and
example on 193
6
Noninverting Amplifier
See formulas on the middle of page 193 and
example on 194
7
Voltage Follower
8
Amplify AC Signals
9
Open Loop Voltage Gain vs Freq
741
10
Finding Upper Cut-off Frequency
11
Compensate Op Amp
Some very old OP-Amp ICs require external
components to prevent high freq oscillations,
such as Fairchilds 709
Data Sheet http//www.datasheetcatalog.com/datash
eets_pdf/L/M/7/0/LM709.shtml
12
Voltage Follower in AC Circuit
13
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

14
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

15
Integrator
16
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

17
LM339 Comparator
18
Comparator Squaring Circuit
19
Lo-Battery Indicator
Op-Amp is configured as a 1.5V level detector
20
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

21
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

22
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
23
Other problems
See page 206 for discussion
FET problems
Zero Problems