Basic Ideas of OP AMP- Basic electronics.

In this article, I will be explaining you some basic points on OP AMP (I.C.) included in Basic Electronics. The full form of OP AMP is operational amplifier. It is included in the syllabus of all technical as well as science students. This article will help you in knowing about what actually OP AMP is?

Operational Amplifier is an important topic for the engineering students as well as the physics students. this article will help you to learning the basic concepts of an IC OP AMP in an easy way. The basic idea of an OP AMP in easy steps are provided here.

  • OP AMP is an example of linear integrated circuit. In 1964, Robert Widlar developed the first IC OP AMP. It is called operational because it can perform basic mathematical operations like addition, subtraction, differentiation, integration. It is used in communication equipment and instruments.
    The basic reason behind the popularity of OP AMP is that the user does not need to know the inner configuration of it consisting of various transistors and resistors. He/She only needs to know the pin configurations to apply in the external circuit with stable and predictable outputs.

  • Circuit Symbol of an OP AMP has five terminals. The terminal d and e are balanced with opposite polarity with respect to ground for biasing. Terminal a is called inverted input terminal because of its opposite polarity with respect to the output. Similarly, terminal b is called the non inverted input terminal because of same polarity with respect to the output. Terminal c denotes the output terminal whose voltage is the difference in voltage of two input terminals with constant of proportionality denoted by A(open loop voltage gain).

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  • An ideal OP AMP has the value of A approaching to infinity for all frequencies. The ideal OP AMP has the following properties-
    1. An infinite voltage.
    2. An infinite input impedance.
    3. Zero output impedance.
    4. An infinite bandwidth

    But, practically OP AMPs are not perfectly balanced and characteristics changing with temperature.

  • Common Mode Rejection Ratio. This term sounds very tricky but actually it is very easy to remember if followed like this-

    Let v1,v2 are the non inverting and inverting voltages with respect to ground.

    Therefore, vd=v1-v2=difference voltage.

    vc=(v1+v2)/2=common-mode signal.

    output voltage(v)=A1v1+A2v2.....1

    so, v1=vc+0.5vd......2


    from 1,2 and 3 we can write, v = Ad*vd + Ac*vc

    In case of ideal amplifier this equation is not valid since Ad is infinitely large while Ac is zero.

    So, common mode rejection ratio was introduced(CMRR) whose value is kept much larger than unity

    Therefore, CMRR=Ad/Ac

  • AC
  • AC equivalent circuit of an OP AMP. Ra=inverting input resistance with respect to ground.
    Rb=non inverting input resistance with respect to ground.
    Ri=resistance between two input terminal.
    All the three resistances being very high, the output voltage is the multiple of difference input signal(vd) and amplifier-gain(A). The gain A is the gain voltage Ad for a very high difference signal. Therefore, in an ideal equivalent circuit Ra,Rb and Ri are infinite, Ro is zero and A is infinite.

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  • The graph shows the open loop characteristics of an OP AMP where Vo is plotted against vd. For a positive D.C. supply, Vo increases to a maximum positive saturation voltage of +V and the remains constant for the further increase in voltage. Similarly, the graph follows a negative saturation voltage of -V for the negative supply of D.C. voltage and then remains constant.


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