Transcript Operational Amplifiers - Georgia Institute of Technology

Operational
Amplifiers
David Lomax
Azeem Meruani
Gautam Jadhav
What is an Op-Amp

Low cost integrating circuit consisting of:
 Transistors
 Resistors
 Capacitors
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
Inverting Input Terminal
Non-Inverting Input Terminal
-V
IC Circuit
What do they really look like?
Ideal Op-Amps
V-
IVout
V+

Infinite input impedance


I+ = I - = 0
Infinite gain


I+
V+ = V-
Zero output impedance
 Output
voltage is independent of output current
Inverting Amplifier
RF
iin
R
C
Vin
Vout
RF

Vin
R
iout
Non-Inverting Amplifier
RF
iin
R
C
Vin
Vout
RF
 1
Vin
R
iout
Summing Circuits
• Used to add analog signals
• Voltage averaging function into
summing function
Calculate closed loop gain for each input
ACL1 
Vo  Vin  ACLn
Vo  V1 
Rf
R1
If all resistors are equal in value:
 V2 
 Rf
Rf
R2
R1
 V3 
ACL1 
 Rf
R2
Rf
R3
Vo  V1  V2  V3 
ACL1 
 Rf
R3
Difference Circuit
• Used to subtract analog
signals
• Output signal is proportional
to difference between two
inputs
Vout
If all resistors are equal:
V 2 R3  R1 R4 V1R3


( R4  R2 ) R1
R1
Vout  V2  V1
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:
1
f 
2  R  C
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
V1 is Vref
V2 is Vin
• 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
OR Gate
If U1 or/and U2 = 5V,
U3 = 5V
If U2 and U1 = 0V,
U3 = 0V
Offset Comparator
If
U2 
R2
R1  R2
.U1
U3 = 0V
If
U2 
5.R1  U1.R2
R1  R2
U3 = 5V
Real Vs Ideal Op Amp
Parameters
Input
Impedance
Output
Impedance
Voltage Gain
Common Mode
voltage
Ideal
Typical
∞
106Ω
0Ω
100-1000Ω
∞
105 - 109
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 ?