Operational Amplifiers

David Lomax

Azeem Meruani

Gautam Jadhav

What is an Op-Amp

Low cost integrating circuit consisting of:TransistorsResistorsCapacitors

Able to amplify a signal due to an external power supply

Name derives from its use to perform operations on a signal.

Applications of Op-Amps

Simple Amplifiers Summers Comparators Integrators Differentiators Active Filters Analog to Digital Converters

Symbol for an Op-Amp

+V

-V

Inverting Input Terminal

Non-Inverting Input Terminal

IC Circuit

What do they really look like?

Ideal Op-Amps

Infinite input impedance I+ = I- = 0

Infinite gain V+ = V-

Zero output impedanceOutput voltage is independent of output current

Vout

V+

V-

I+

I-

Inverting Amplifier

R

RF

Vin

iin

iout

C

R

R

V

V F

in

out

Non-Inverting Amplifier

R

RF

iin

iout

C

Vin

R

R1

V

V F

in

out

Summing Circuits

• Used to add analog signals

• Voltage averaging function into summing function

Calculate closed loop gain for each input

11 R

RA fCL

21 R

RA

f

CL

31 R

RA

f

CL

CLnino AVV 3

32

21

1 R

RV

R

RV

R

RVV

fff

o

321 VVVVo If all resistors are equal in value:

Difference Circuit• Used to subtract analog signals

• Output signal is proportional to difference between two inputs

12 VVVout If all resistors are equal:

1

31

124

4132

)( R

RV

RRR

RRRVVout

Integrating Circuit• Replace feedback resistor of inverting op-amp with capacitor

• A constant input signal generates a certain rate of change in output voltage

• Smoothes signals over time

Differentiating Circuit• Input resistor of inverting op-amp is replaced with a capacitor

• Signal processing method which accentuates noise over time

• Output signal is scaled derivative of input signal

Filters

Low Pass Filters High Pass Filters Band Pass Filters

Low Pass Filter• Used to filter out signals above a specified frequency

• Example: Noise

Frequency range is governed by:

CRf

2

1

Where

R = R2

C = C2

High Pass Filter• Filters out frequencies below a specified frequency

• Reverse locations of resistors and capacitors in a low pass filter

Band Pass Filter

• Created by combining a high and low pass filter

• Only allows signals within frequency ranges specified by the low and high pass filters to pass

Comparator Circuit

• Determines if one signal is bigger than another

• No negative feedback, infinite gain and circuit saturates

• Saturation: output is most positive or most negative value

V1 is Vref

V2 is Vin

OR Gate

If U1 or/and U2 = 5V,

U3 = 5V

If U2 and U1 = 0V,

U3 = 0V

Offset Comparator

If

U3 = 0V

If

U3 = 5V

1

21

22

.URR

RU

21

2112

RR

.RU5.RU

Real Vs Ideal Op Amp

Parameters Ideal Typical

Input Impedance

∞ 106Ω

Output Impedance

0Ω 100-1000Ω

Voltage Gain ∞ 105 - 109

Common Mode voltage

0 10-5

Non-Ideal Op-Amps

Gain Bandwidth Falloff Frequency Slew Rate (ΔV/ΔT) Rise Time

Gain Bandwidth

Gain Bandwidth Product (GBP)- is the product of the open-loop gain and the bandwidth at that gain.

For practical purposes the actual gain should only be 1/10 to 1/20 of the open loop gain at a given frequency to ensure that the op-amp will operate without distortion.

Open and Closed Loop Response

Important Parameters for Op-Amps

Input Parameters Voltage (Vicm) Offset voltage Bias current Input Impedance

Output Parameters Short circuit current Voltage Swing Open Loop Gain Slew Rate

Where to buy Op-Amps

Newark Electronics Radio Shack DigiKey Jameco

References

David G Alciatore & Michael B. Histand, Introduction to Mechatronics and Measurement Systems

http://www.elexp.com/t_gain.htm http://allaboutelectronics.com Electronics book

Questions ?