Electronic Troubleshooting

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Electronic Troubleshooting Chapter 8 Operational Amplifiers

Operation Amplifiers Overview

• Picture from Wikipedia, see

Original OP-Amps

the terms of use on their • 1940/50’s Tube circuits • Discrete component semi-conductor circuits followed site.

• • 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 100’s of 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 R F and R 1 RA and RB also factor into the operation

• •

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

• • •

Common Mode Gain

A cm

V cmo V cm

where V cmo V cm

 

CommonMode OutputVolt age CommonMode InputVolta ge

Example Problem 8-5 on page 198

Integrator

Level Detector Characteristics

As shown the circuit id a zero crossing signal

• •

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

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

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

See page 206 for discussion Zero Problems

Other problems

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