Operational Amplifiers(Op Amps)

Adnan PandjouHunter Moore

Tyler Randolph

Agenda

• Background• Ideal Op Amp• Inverting• Non-inverting• Integrating • Differential• Summing• Applications• Conclusion

Background• Originally designed to perform mathematical

operations in 1940 using vacuum technology• The first integrated op-amp to become widely

available was produced in the late 1960’s by Fairchild

• Wide variety of applications, low cost, and easily manufactured

Ideal Op AmpIdeal Op-Amp Typical Op-Amp

Input Resistance infinity 106 (bipolar)109 - 1012 (FET)

Input Current 0 10-12 – 10-8 A

Output Resistance 0 100 – 1000

Operational Gain infinity 105 - 109

Common Mode Gain 0 10-5

Bandwidth infinity Attenuates and phases at high frequencies (depends on slew

rate)

Temperature independent Bandwidth and gain

InvertingOp Amp

Non-inverting

Integrating

Input voltage is integrated by using a capacitor

Inverting op-amp feed back resistor replaced with a capacitor

Smoothes out signals and helps to remove offset Used for PID controllers

Differential

If all of the resistors are equal, the differential op-amp becomes a difference amplifier

If R4=R3 and R2=R1, then it becomes amplified difference op-ampVout=(V2-V1)R3/R1

Vout=V1-V2

Summing

€

Vo = − V1 +V2 +V3( )If all resistors are equal

Summing amplifiers combine signals by adding directly or scaling and then adding

Audio mixers sum several signals with equal gainsDigital-to-analog converters use different resistors to give a weighted sumLED’s use summing amps to apply a DC off set to AC voltage to keep it in its linear operating range

Applications

• Low Pass Filters• High Pass Filters• Offset Comparator• Data acquisition

Low Pass Filter

€

fc = 12π R1R2C1C2

•Used to filter frequencies above fc•Second order•Active

High Pass Filter

€

fc = 12π R1R2C1C2

•Used to filter frequencies below fc•Second order•Active•C2 and R2 switched

Offset Comparator

If

Output = 0V

If

Output = 5V

1

21

22

.URR

RU

+≤

21

2112

RR

.RU5.RU

+

+≥

•Good for setting thresholds

Offset ComparatorExample

• Setting thresholds for IR detector

R1R2

Op-amp

IR detector

Data acquisition• Signal amplification• Example (Lab 2)

Amplification of strain gage signal

Example Data acquisition

Differential Circuit( )

1

31

124

4132

)( RRV

RRRRRRV

Vout −+

+=

Where to get Op-ampsfor Free

• Companies give free sampleswww.national.com

Where to get Op-ampsfor Free

•5 (max 5 of each kind) samples per week

Design tools

Design tools

Design tools

Design tools

Parts List

Design tools

Design simulation

Conclusion

• Wide range of application• Lots of recourses• Look at other previous student

presentations

Questions?

Appendix

InvertingAssumptions Infinite gain (amplification factor) Large internal resistance Derivation

021=+=a ai iii(Large internal resistance)

21ii=

ABout ABoutVVKV VVKV −=−= )(

K = Infinite gain, therefore 0===ABABVVVV

From 21ii=,

€

Vout−VARf

=VA −VinRin

VA1Rin

− 1Rf

⎛ ⎝ ⎜ ⎜

⎞ ⎠ ⎟ ⎟=−VoutRf

−VinRin

−VoutRf=VinRin

i1

i2

ia

VB

VA

€

VoutVin

= −R2

R1

Non-InvertingAssumptions Infinite gain (amplification factor) Derivation

K = Infinite gain, therefore

AinVV=

€

Vout−0R2 +R1

=VA −0R1

VoutR2 +R1

=VinR1

€

Vout=K(Vin−VA)VoutK =Vin−VA

VA

€

Vout =Vin 1+ R2

R1

⎛ ⎝ ⎜

⎞ ⎠ ⎟