Operational amplifiers, or op-amp circuits, are high-gain DC amplifiers that we use for the amplification of signals in analog circuits. They were invented in the 1940s and quickly became widespread because of their good performance and low cost (they were about 10 times cheaper than alternatives at the time).
Today, we find op amps in almost every piece of electronics; from power supplies to radio receivers to computer sound cards. These are the integrated circuits that are usually built with the help of transistors and resistors.
Before going into detail about operational amplifiers, we must know the basics of transistors and their types. Bipolar junction transistors and Field effect transistors are the important ones. Read about BJT Modes and BJT configurations here.
In this article, we’ll go over how op-amp circuits work and why you should consider adding one to your next circuit design!
What are Operational Amplifiers or Op-Amp Circuits?
An operational amplifier or op-amp in short wording is a type of transistor that can amplify signals. This amplifier has two inputs and one output. Inputs act to invert or non-invert the input signal which is a voltage signal.
The two inputs bear a positive which refers to a “non-Inverting” and a negative sign refers to an “inverting input”. These inputs have a high impedance as compared to the output of the operational amplifier.
We can call these two major types of op amps inverting and non-inverting amplifiers. Comparators normally use Inverting operational amplifiers and protecting against input spikes we use non-inverting operational amplifiers. Which we can also refer to as buffers in the circuit.
Classification of Operational Amplifiers
There are four ways in which we can classify the operational amplifiers or op-amp circuits.
The Voltage amplifier circuits have voltages as input and voltages as output.
The Current amplifier circuits have current as input and current as output.
The Transconductance Amplifier circuits have voltage as the input and current as the output.
The Tranresistance amplifier circuits have current as the input and voltage as the output.
Mostly we find the operational amplifiers which amplify the voltage level in the circuit.
How do Op-Amp Circuits work?
An op amp is a semiconductor chip that amplifies and converts signal voltage levels. The difference between inverting and non-inverting amplifiers are the input configuration, where inverting amplification means that the signal is inverted before it is amplified.
When we apply the Inverting op amps are typically used as voltage comparators to compare two voltages or to compare a voltage with a reference voltage. Non-inverting op amps are typically used as differential amplifiers, which is when you use two inputs to determine the difference between them.
Circuit Diagram of Op-Amp Circuits
A typical op-amp circuit looks like below.
As we can see in the above figure, it has two inputs inverting “-” and non-inverting “+” both having different voltages. The impedance between these inputs is Zin. This input impedance is very high to reduce the source loading. This is the impedance between the positive and negative terminal. This impedance does not only include the resistance but also includes the capacitance.
The “A” in the above figure is the gain of the amplifier. This is negative in the case of inverting op-amp configuration. As we can see in the above figure, no feedback connects. Therefore we can say that the gain in this scenario is an open-loop gain. An op amp has a very large open loop gain typically in the range of 10000.
“Zout” is the output impedance which in ideal scenarios should be equivalent to zero but in actual conditions, it has a small value in running circuits.
Inverting Operational Amplifiers
The Inverting topology of op amps works in a way that the current flowing into inputs of op amp must be zero and the output ensures that both inputs have the same voltages or the difference(V+=V-) is zero. let’s have a look at the below figure.
As we can see in the figure, the inverting terminal connects through input and the positive terminal connects with the ground. We denote the gain of this amplifier circuit as -R2/R1 and the output voltage will equal -R2/R1 x Vin. The negative sign is due to the virtual ground and the output of this op amp is out of phase with the input.
Non-Inverting Operational Amplifiers
Non-inverting amplifier circuits follow the same current and voltage rules of op-amp circuits as illustrated above. The major difference is we apply the input signal on non-inverting input. Have a look at the below figure.
From the above figure, we can say the output voltage of this op-amp is equal to Vout= Vin+ Vin(R2/R1). We can write the gain of this amplifier as Vout/Vin= 1+ R2/R1, so the gain is never less than 1.
The output voltage is in phase with the input voltage. The common collector configuration and the common base configuration are the non-inverting configurations of BJT (Bipolar Junction Transistor). Read about BJT configurations and BJT modes by clicking here.
Voltage Follower Operational Amplifier
Unlike inverting and non-inverting operational amplifier circuits, the voltage follower has an output equal to the input and there are no feedback resistances. Whatever changes occur on the input side, the output has the same effect.
Op-amp circuits characteristics like high input impedance and low impedance at output make it a good buffer circuit. In this circuit Vou= Vin, further, have a look at the below figure.
Voltage Comparator Operational Amplifier
The voltage comparator circuit compares both inputs and the output depends on whichever input is higher. We apply a reference voltage on non-inverting input and then the circuit compares the applied one with this reference, if the input signal is higher than the reference then the output equals the +Vcc. If the applied voltage is less than the reference, output equals -Vcc. Let’s have a look at the below figure.
Tips for Choosing the Right Operational Amplifier
Op-amp circuits are very important components when you are designing a circuit because it can be used to amplify or attenuate signals. The first thing to consider when choosing your op amp is whether you need an inverting amplifier or a non-inverting one. An inverting amplifier will take the opposite input and amplify it, whereas a non-inverting amplifier will take the same input and amplify it.
A second thing to consider is what type of op-amp would be best for your project. There are two types of commonly used op amps: bipolar and unipolar. Bipolar op-amp circuits can switch between operating as either aninverting amplifier or non-inverting op-amp circuits, while unipolar ones only work as one of the two kinds.
Most commonly used op amp integrated circuits include the LM324, MC33078, LM324 and LM837 are mostly used operational amplifiers. We can replace these IC’s in any circuit wherever required as a substitute. Lear about LM324 datasheet and key features here to broaden your understanding of the operational amplifier circuits.
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