Operational Amplifiers

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Operational Amplifiers Powered By Docstoc
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


     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 impedance
     Output    voltage is independent of output current
Inverting Amplifier
                            RF   iout

         iin   R



                   Vout    RF
                   Vin     R
Non-Inverting Amplifier
                         RF   iout

     iin   R


           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
                                             • Output signal is proportional
                                             to difference between two

                                           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
                      • 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

 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:

                      f 
                          2  R  C
                              R = R2
                              C = C2
High Pass Filter
               • Filters out frequencies below a specified
               • 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              .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
Output                 0Ω     100-1000Ω
Voltage Gain           ∞       105 - 109

Common Mode            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
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
 David G Alciatore & Michael B. Histand,
  Introduction to Mechatronics and
  Measurement Systems
 Electronics book
Questions ?