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op amp

Operational Amplifiers Summary

We can conclude our section and look at Operational Amplifiers with the following summary of the different types of Op-amp circuits and their different configurations discussed throughout this op-amp tutorial section.

Operational Amplifier General Conditions

  • • The Operational Amplifier, or Op-amp as it is most commonly called, can be an ideal amplifier with infinite Gain and Bandwidth when used in the Open-loop mode with typical DC gains of well over 100,000 or 100dB.
  • • The basic Op-amp construction is of a 3-terminal device, 2-inputs and 1-output, (excluding power connections).
  • • An Operational Amplifier operates from either a dual positive ( +V ) and an corresponding negative ( -V ) supply, or they can operate from a single DC supply voltage.
  • • The two main laws associated with the operational amplifier are that it has an infinite input impedance, ( Z =  ) resulting in “No current flowing into either of its two inputs” and zero input offset voltage “V1 = V2“.
  • • An operational amplifier also has zero output impedance, ( Z = 0 ).
  • • Op-amps sense the difference between the voltage signals applied to their two input terminals and then multiply it by some pre-determined Gain, ( A ).
  • • This Gain, ( A ) is often referred to as the amplifiers “Open-loop Gain”.
  • • Closing the open loop by connecting a resistive or reactive component between the output and one input terminal of the op-amp greatly reduces and controls this open-loop gain.
  • • Op-amps can be connected into two basic configurations, Inverting and Non-inverting.

The Two Basic Operational Amplifier Circuits

basic operational amplifier circuits

  • For negative feedback, were the fed-back voltage is in “anti-phase” to the input the overall gain of the amplifier is reduced.
  • For positive feedback, were the fed-back voltage is in “Phase” with the input the overall gain of the amplifier is increased.
  • By connecting the output directly back to the negative input terminal, 100% feedback is achieved resulting in a Voltage Follower (buffer) circuit with a constant gain of 1 (Unity).
  • Changing the fixed feedback resistor (  ) for a Potentiometer, the circuit will have Adjustable Gain.

Operational Amplifier Gain

operational amplifier gain bandwidth product

  • The Open-loop gain called the Gain Bandwidth Product, or (GBP) can be very high and is a measure of how good an amplifier is.
  • Very high GBP makes an operational amplifier circuit unstable as a micro volt input signal causes the output voltage to swing into saturation.
  • By the use of a suitable feedback resistor, (  ) the overall gain of the amplifier can be accurately controlled.

Differential and Summing Amplifiers

differential and summing amplifiers

 
  • By adding more input resistors to either the inverting or non-inverting inputs Voltage Adders or Summers can be made.
  • Voltage follower op-amps can be added to the inputs of Differential amplifiers to produce high impedance Instrumentation amplifiers.
  • The Differential Amplifier produces an output that is proportional to the difference between the 2 input voltages.

Differentiator and Integrator Operational Amplifier Circuits

differentiator and integrator amplifiers

 
  • The Integrator Amplifier produces an output that is the mathematical operation of integration.
  • The Differentiator Amplifier produces an output that is the mathematical operation of differentiation.
  • Both the Integrator and Differentiator Amplifiers have a resistor and capacitor connected across the op-amp and are affected by its RC time constant.
  • In their basic form, Differentiator Amplifiers suffer from instability and noise but additional components can be added to reduce the overall closed-loop gain.

19 Comments

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  • a
    abdiaziiz

    what does mean ‘virtual grounded’

    • A
      Asim Zulfiqar

      It is the property of op-amp to keep the same potential at its input terminal so in non-inverting amplifer when the positive terminal is grounded the op-amp will try to keep the same potential at non-inverting terminal and the potential at non-inverting terminal will be 0v which is not actually connected to ground, hence called virtual ground.

  • m
    marshall mashinya

    buffer as unity gain (0db)and reproduces the input voltage.what possible use is a circuit such as this ,which offers no voltage gain or any other form of signal modification.

  • H
    Herschel

    Bias current compensation?
    Considering an inverting amplifier, I often see a resistor added from the non-inverting input to ground. The suggested value is equal to the parallel value of the other two resistors.
    I only see this in op amps with bipolar transistors on the input. Op amps that have bias currents in nA. Is this resistor just not needed in JFET input op amps with bias currents down into pA range?

  • M
    MicheleD

    Thanks for this clear introductory tutorial on OpAmps. It did, along with some other works, help me get a better understanding of OpAmps. I am left though with two rather problematic hurdles. One is that I still do not have an understanding of the pros and cons of inverting vs. non inverting amplifiers. Under what conditions should one go for one as opposed to going for the other. If you could possibly elaborate on this. The other hurdle is that I planned a strain gage application using a differential amplifier which looks good on paper but which simply does not work. I get no output on Vo. Characteristics are 1K/1M for a gain of 1000. Bridge is 4x1K balanced via a potentiometer to yield 0 volts between v+ and v-. R1=R2=1K, R3=R4=1M. V+ to ground via R4 as per your schematic. When gage is stressed (+4 ohms) I get 2.5v on V+, 2.5 + 4mV on V-. V0 stays at 0 all the time (twitches between + and – 1mV). I used both LM520 and AD822 – same bad results, VCC at +5V, VEE at 0 – common ground). I am getting to believe that amplifying 4mV over 2.5 volts is stretching the capabilities of the OpAmp and that perhaps the solution lies with an instrumentation amplifier, which I have yet to try. If you could possibly comment ?? Thanks, Michèle

  • J
    Jaliya

    Amazing teaching ability.!! Thank you very much for all your tutorials about op amps. I’m studying for an electronic exam and these are the only notes I’m studying about op amps.. Hope they will help me @ the exam hall..

  • s
    sameerkhan

    Wow.
    Thanks first to tutorial.
    Sattapadinga enakku pirayisanam irunthichi

  • M
    M I P

    A very good website for tutorials on OPAMPS

  • E
    ErIn tHoMaS

    YOUR EXPLANATION ON OP AMPS REALLY HELPED ME UNDERSTAND THE CONCEPT. THANKS WAYNE.

  • A
    Aryan

    Dude! you are an electronics ROCKSTAR!!!

  • B
    Brendan

    How can you go around saying that a real component is ideal, with infinite qualities?
    That would have really confused me had I not already known this is impossible. Someone already mentioned you got the Db thing wrong, capitalised the wrong letters, typos…
    I don’t think I can trust your information with that kind of grammar.

    • Wayne Storr

      Hello Brendan, When designing op-amp circuits, or calculating the transfer function of a particular op-amp configuration, we do assume ideal conditions. While an ideal op-amp has infinite input impedance, zero output impedance, infinite open-loop gain or bandwidth, some of these features are very nearly achieveable by practical op-amps, especially the FET types, and as such input bias currents or output offset voltages, etc. are generally ignored in the calculation of an op-amps feedback network or circuit configuration.

      Regarding your other comments, a gain of 100,000 is equal to 100dB as calculated below in reply to DeeBee’s comment. Sometimes typos do creep in so if you find one you are at liberty to let me know and I will correct it. Also if you feel the information provided within these tutorials is incorrect in anyway you are free to use other websites.

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