Electronic Troubleshooting

Chapter 8Operational Amplifiers

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.

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

See formulas on the bottom of page 192 and example on 193

Critical to understanding operation with feedback

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

• Example Problem 8-5 on page 198

geInputVoltaCommonModeV

ageOutputVoltCommonModeV

where

V

VA

cm

cmo

cm

cmocm

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

FET problems

Zero Problems

See page 206 for

discussion