Introduction to Operational amplifiers
An amplifier is a device that amplifies the input signal to desired output of required gain. There are so many types of amplifiers are there. These are specified for certain a use depends on the occasions. An operational amplifier is also an amplifier that is used to amplify both alternating and direct current signals at the input. The main building blocks or elements of an operational amplifier are two or more differential amplifier stages. This differential stage is followed by a level transistor and output stages. Here the differential amplifier stage includes set of differential amplifiers.
A differential amplifier is a circuit made up of transistors which is used to amplify the difference between the input signals. Here in differential amplifiers, when same input is given to both input terminals then the output voltage will be zero. A small difference in the input voltages may lead to large gain at the output. These types of differential amplifiers has many advantages; mainly in the field of instrumentation circuits. There are four differential amplifier configurations,
We can construct multistage differential amplifiers for getting desired output gain. For this we can use direct coupling between the successive stages of differential amplifiers. The direct coupling will remove the lower cut off frequency imposed by cooling capacitors. Hence it can amplify both AC as well as Dc signals.
The CMMR can be defined as the common mode rejection ratio. It is the relative sensitivity of an operational amplifier to difference signals as compared to common mode signals. The common mode rejection ratio is one of the important parameter for analyzing the performance of various electronics equipments like amplifiers. As the value of CMMR increase the operational amplifier will be better.
Block diagram representation of operational amplifiers
In general, the operational amplifiers uses a four stage cascaded structure as shown in the figure. The four stages of an operational amplifier are as follows,
Here the first stage or input amplifier uses a dual input balanced output amplifier. The function of this stage is to provide the high gain for the difference signals. It will also reject the common mode signals enter into the amplifier. Thus, this stage suppresses any undesired noise which is common to both input terminals.
The second stage consists of dual input single output differential amplifiers. The function of this stage is also to provide a additional gain to the operational amplifiers. Here there is a direct coupling exists between the first and second stage; this will lead to an output DC level which is well above the ground. Hence in order to reduce this DC level we are using a level shifting stage. This includes an emitter follower element. This stage is also known as a buffer stage. The high input resistance of this stage will help to prevent the loading down the high gain of second stage. The last stage is a push pull amplifier; this will increase the output voltage swing and increases the current supplying capability of operational amplifiers. The resistance of this stage is very low as compared to other stages.
The characteristics of ideal operational amplifiers
Reason for high popularity of operational amplifiers
The operational amplifiers have several advantages in the electronics industry. Operational amplifiers are very common in most of the electronic equipments. The wide popularity of operational amplifiers is mainly due to its performance near to the theoretical values.
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