When on a small semiconductor wafer which is usually made by silicon that is capable of holding thousands and millions of transistors, capacitors and resistors are called integrated circuits or simply IC. These wafers are so small in size and can function as an amplifier, oscillator, and timer or can be used in memory designing.
A solid-state analog device or linear integrated circuit is used in various electronic applications. In this article we will be discussing one of the most important linear integrated circuits which is an operational amplifier, commonly known as op-amp).
Introduction to Operational amplifier:
• The Operational Amplifier (commonly called op-amp) is a multi-terminal, direct-coupled (an amplifier in which output of one stage is coupled to the input of the next stage in such a way that it allows signal with zero frequency to pass from input to output), high gain amplifier.
• It consists of a number of the differential amplifier and an output.
• The input applied is a dual input and balanced output differential amplifier. The dual input means that one input terminal is operated at the positive voltage and other at corresponding negative voltage.
• The input stage of the operational amplifier provides voltage gain and establishes the input resistance.
• The operational amplifiers can be used to amplify dc as well as the ac input signal.
• The operational amplifiers were initially designed for mathematical computations.
Ideal characteristics of operational amplifier:
• Open-loop voltage gain is infinite.
• Input resistance is infinite.
• Bandwidth is infinite.
• The common-mode rejection ratio is infinite.
• Slew rate is infinite.
• The output resistance is zero.
• Whenever the input is zero the operational amplifier should give output as zero.
• The power supply rejection ratio should be zero.
Pin diagram and schematic symbol:
Schematic symbols are symbolic representations used to save time while evaluating any numerical problem. The figure shown is the simplest and widely used symbol. The symbol has two inputs, one is inverting terminal (shown with negative sign) and the other is non-inverting terminal (shown with a positive sign).
If the signal is applied at the inverting terminal then there is a 180-degree phase shift between input and output signals in amplified form.
If the signal is applied at the non-inverting terminal then there is no phase shift between input and output amplified signal.
Let ‘Vid’ represent the difference between signals then
The gain of the amplifier (A) = Vo/Vid.
Where Vo is the output voltage. Vid is equal to V1-V2. Here V1 is the voltage applied to invert terminal and V2 is applied to non-inverting terminal. By further evaluation Vo= A (V1-V2).
The operational amplifier amplifies the difference between inverting and non-inverting terminal hence it is called a difference amplifier.
Pin diagram of IC 741 operational amplifier:
IC 741 is usually supplied in dual inline packaging. It is small in size and is highly compact. It consists of a total eight pins. The counting starts from the left side of the notch.
Pin 1 is the offset null. Offset means the input voltage that must be applied to bring the output voltage to zero. It ensures that offset is removed from the output.
Pin 2 is the inverting of the input supply.
Pin 3 is the non-inverting input supply.
Pin 4 is supplied.
Pin 5 is offset null.
Pin 6 is an output terminal.
Pin 7 is plus Vcc.
Pin 8 is generally not connected.
Operational amplifier configurations:
There are two types of configurations:
1. Open-loop operational amplifier configuration.
2. Closed-loop operational amplifier configuration.
Open-loop operational amplifier configuration:
If there is no feedback connected between the input and output terminal of an operational amplifier then it is called open-loop operational amplifier configuration.
The drawback of using open-loop operational amplifier:
The distorted amplified signal is obtained in an open-loop configuration. The gain is not constant and it changes with change in temperature. The bandwidth of open-loop configuration is small.
Its application is used in the comparator.
Closed loop operational amplifier configuration:
If there are feedback connections between the input and output terminal of an operational amplifier then it is called closed-loop operational amplifier configuration.
The drawback of open-loop configuration is covered up in a closed loop by using feedback.
When part of the output is applied back to the input then we say that feedback is connected. It is of two types: positive feedback and negative feedback.
In positive feedback the feedback signal and input signal are in phase.
In negative feedback, the feedback signal and input signal are 180 degrees out of phase. It stabilizes the amplifier gain and distortion is reduced. The bandwidth in negative feed is increased and it reduces the effect of temperature on the operational amplifier.
Operational amplifier circuits:
There are two basic configuration of operational amplifier circuit:
1. Inverting amplifier
2. Non-inverting amplifier
By the ideal characteristic of operational amplifier gain is infinite. The ratio of Vo and Vi is equal to zero. Hence V1 and V2 will be equal and V1=0, V2=0 by the concept of virtual ground*. By applying KCL in the circuit shown we get:
*virtual ground: This means the voltage across inverting terminal is equal to the voltage across non- inverting terminal and when V1 and V2 are equal to zero then this is called the concept of virtual ground.
The input voltage as shown in the circuit diagram is applied to the positive or we say noninverting terminal of the operational amplifier.
The gain of non-inverting amplifier is given by:
A= 1+ Rf/Ri
Application of operational amplifier:
1. Buffer amplifier: The gain of buffer amplifier is unity. It is used to isolate the input signal from the load.
2. Comparator :
In the operational amplifier as a comparator, there is the main role of the reference voltage. The circuit is shown in the figure if the input voltage (Vin) is greater than the reference voltage (Vref) the output will be +Vcc. If input voltage (Vin) is less than the reference voltage (Vref) the output will be –Vcc.
3. Integrator :
In an operational amplifier as an integrator, a capacitor is connected as feedback. The advantage of using an integrator is that it has less distortion, Sharpe frequency response and follows linearity.
It is used in a low pass filter circuit, used in a control system or communication system and in analogy to digital converter.