Operational Amplifiers

Why Are Operational Amplifiers Widely Used?
        This lesson is the first lesson on operational amplifiers, or op-amps as they are often called.
        Operational amplifiers are widely used in signal processing circuits, control circuits, and instrumentation.  Of all analog integrated circuits, the operational amplifier is the analog integrated circuit which has the most sales and is the most widely used in the widest variety of electronic circuits.  If you are an electrical engineer, you will probably encounter more operational amplifiers than any other integrated circuit device.  It's an important component for electrical engineers who design circuits using them and to all other kinds of engineers who use measurement and control circuits that contain operational amplifiers.

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Where do you find Op-Amps?         Operational amplifiers are used in many places including:

• In strain gage circuitry to measure deformations in structures like bridges, airplane wings and I-beams in buildings.
• In temperature measurement circuitry for boilers and in high altitude aircraft in a cold environment.
• In control circuits for aircraft, people movers in airports, subways and in many different production operations.

        The operational amplifier is a versatile component that can do many things in measurement, signal processing and control.  That versatility is the largest reason that you find so many operational amplifiers being used!

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Your Goals         Your goal for this lesson should be:

  Given an operational amplifier you want to use,

  Be able to connect the positive and negative power supplies correctly.
  Be able to locate the inverting and non-inverting input terminals.

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The 741 - A Typical Operational Amplifer         This is a computer photograph of an operational amplifier.  It's often referred to as an operational amplifier.  We've called this module "The World of Op-Amps" because these creatures can be found almost everywhere you find electronic circuits.  Here is a typical operational amplifier on a circuit board.
 
 


        This particular operational amplifier is an integrated circuit.  Most operational amplifiers today are integrated circuits, even power operational amplifiers that can be used to drive small motors.  Of course the actual size is smaller than the picture above!
        Look carefully at the operational amplifier.  It's important to notice that there is a notch (sometimes a circular depression) on one end (the "top" of the chip in the picture) of the operational amplifier.  The pin shown below the notch is pin 1, and the one above is pin 8.  They're numbered counter clockwise around the chip.    Here is a drawing of the pinout
Above is a drawing of the pin-out.

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What is an Op-Amp Really?         An operational amplifier is a high gain, differential, voltage amplifier.

• It is a voltage amplifier.  The input is a voltage and the output is a voltage.
• The gain is high.  Typically, the gain is over 100,000
• It is a differential amplifier.  It actually amplifies the difference between two voltages.

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When Did Operational Amplifiers Arrive On The Scene?         Operational amplifiers have been around since the late '40s or early '50s.  There have been a number of influential and interesting characters that worked with operational amplifiers including George Philbrick and Bob Widlar.

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Using Operational Amplifiers         If you want to use an operational amplifier, you will need to know several things.

• You will need to know how to power up a chip.  In other words, you need to know how to get the power supply connected so that you can use the chip.  If you don't get the power supply connected the chip will not operate as an amplifier.
• We'll spend a little time looking at those connections.
• You will need to learn about circuits that use operational amplifiers.  That's the topic of the next lesson in this series.

        Examine the pinout for the chip.  Normally, the chip will either be inserted in a circuit board, or wired into a printed circuit board.  In either case, to power the chip you need to make two connections:

• The positive voltage from the supply to pin 7 on the chip.
• The negative voltage from the supply to pin 4 on the chip.
• If you are used to using logic chips where the power goes to the corners, that's not quite what you have here.  It's different and be sure you get it right.

        Every operational amplifier will have some power supply connections.  Often they are not shown on the circuit digram for a circuit you are designing.  When necessary they are shown as below.  Here, Vcc is the positive supply voltage.  Usually,  Vcc  is 12 to 15 volts.  A negative supply is usually needed also, although there are operational amplifers on the martket that operate well just from a positive supply.  The negative supply,  -Vcc, is usually in the range from -12 to -15 volts.  When two supplies are used, they are the same voltage and of opposite sign in almost all cases.         Often an integrated cirtcuit chip has a single operational amplifer.  The symbol for such an operational amplifier is shown below.  There are three voltages associated with this symbol.  Each voltage here is measured with respect to ground.

• V_out  is the output voltage,
• V₊  is the non-inverting input voltage,
• V_-  is the inverting input voltage.

The chip diagram shows how an operational amplifier is connected internally in a typical 741 integrated circuit chip.  Notice that the voltages noted above are the actual connections you will need to make to use the amplifier.  They are not part of the power supply circuitry which is separate.         Remember that the operational amplifier is a high gain, differential voltage amplifier.  In this case the output voltage is a very large multiple of the input voltage.  The multiplying factor is called the gain.  For a 741 operational amplifier, the gain is at least 100,000 and can be more than a million (1,000,000).  That's an important fact you'll need to remember as you put the 741 into a circuit.