# Operational Amplifiers

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```					Operational
Amplifiers
David Lomax
Azeem Meruani
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
I-
V-

Vout
I+
V+

   Infinite input impedance
   I+ = I - = 0
   Infinite gain
   V+ = V-
   Zero output impedance
 Output    voltage is independent of output current
Inverting Amplifier
RF   iout

iin   R

C

Vin

Vout    RF

Vin     R
Non-Inverting Amplifier
RF   iout

iin   R
C

Vin

Vout      RF
 1
Vin       R
Summing Circuits
• Used to add analog signals
• Voltage averaging function into
summing function

Calculate closed loop gain for each input
 Rf                   Rf             Rf
ACL1                 ACL1           ACL1 
R1                 R2              R3

Rf            Rf            Rf
Vo  Vin  ACLn      Vo  V1          V2          V3 
R1            R2            R3

If all resistors are equal in value:     Vo  V1  V2  V3 
Difference Circuit
• Used to subtract analog
signals
• Output signal is proportional
to difference between two
inputs

V 2 R3  R1 R4 V1 R3
Vout                    
( R4  R2 ) R1   R1

If all resistors are equal:   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

R2
If   U2              .U1
R1  R2
U3 = 0V
5.R1  U1.R2
If   U2 
R1  R2
U3 = 5V
Real Vs Ideal Op Amp
Parameters     Ideal        Typical

Input                  ∞         106Ω
Impedance
Output                 0Ω     100-1000Ω
Impedance
Voltage Gain           ∞       105 - 109

Common Mode            0          10-5
voltage
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
 Newark Electronics
 DigiKey
 Jameco
References
 David G Alciatore & Michael B. Histand,
Introduction to Mechatronics and
Measurement Systems
 http://www.elexp.com/t_gain.htm