Basics Of Operational Amplifier

# Basics Of Operational Amplifier

Operational Amplifier, also called as an Op-Amp, is an integrated circuit, which can be used to perform various linear, non-linear, and mathematical operations. An op-amp is aÂ direct coupled high gain amplifier. You can operate op-amp both with AC and DC signals. This chapter discusses the characteristics and types of op-amps.

## Construction of Operational Amplifier

An op-amp consists of differential amplifier(s), a level translator and an output stage. A differential amplifier is present at the input stage of an op-amp and hence an op-amp consists ofÂ two input terminals. One of those terminals is called as theÂ inverting terminalÂ and the other one is called as theÂ non-inverting terminal. The terminals are named based on the phase relationship between their respective inputs and outputs.

### Characteristics of Operational Amplifier

The important characteristics or parameters of an operational amplifier are as follows âˆ’

• Open loop voltage gain
• Output offset voltage
• Common Mode Rejection Ratio
• Slew Rate

This section discusses these characteristics in detail as given below âˆ’

### Open loop voltage gain

The open loop voltage gain of an op-amp is its differential gain without any feedback path.

Mathematically, the open loop voltage gain of an op-amp is represented as âˆ’

Av=v0v1âˆ’v2Av=v0v1âˆ’v2

### Output offset voltage

The voltage present at the output of an op-amp when its differential input voltage is zero is called asÂ output offset voltage.

### Common Mode Rejection Ratio

Common Mode Rejection Ratio (CMRR) of an op-amp is defined as the ratio of the closed loop differential gain,Â AdAdÂ and the common mode gain,Â AcAc.

Mathematically, CMRR can be represented as âˆ’

Note that the common mode gain,Â AcAcÂ of an op-amp is the ratio of the common mode output voltage and the common mode input voltage.

### Slew Rate

Slew rate of an op-amp is defined as the maximum rate of change of the output voltage due to a step input voltage.

Mathematically, slew rate (SR) can be represented as âˆ’

SR=MaximumofdV0dtSR=MaximumofdV0dt

Where,Â V0V0Â is the output voltage. In general, slew rate is measured in eitherÂ V/Î¼SecV/Î¼SecÂ orÂ V/mSecV/mSec.

## Types of Operational Amplifiers

An op-amp is represented with a triangle symbol having two inputs and one output.

Op-amps are of two types:Â Ideal Op-AmpÂ andÂ Practical Op-Amp.

They are discussed in detail as given below âˆ’

### Ideal Op-Amp

An ideal op-amp exists only in theory, and does not exist practically. TheÂ equivalent circuitÂ of an ideal op-amp is shown in the figure given below âˆ’

AnÂ ideal op-ampÂ exhibits the following characteristics âˆ’

• Open loop voltage gaineÂ Av=âˆžAv=âˆž
• If (the differential) input voltageÂ Vi=0VVi=0V, then the output voltage will beÂ V0=0VV0=0V
• Bandwidth isÂ infinity. It means, an ideal op-amp will amplify the signals of any frequency without any attenuation.
• Common Mode Rejection RatioÂ (CMRR)Â isÂ infinity.
• Slew RateÂ (SR)Â isÂ infinity. It means, the ideal op-amp will produce a change in the output instantly in response to an input step voltage.

### Practical Op-Amp

Practically, op-amps are not ideal and deviate from their ideal characteristics because of some imperfections during manufacturing. TheÂ equivalent circuitÂ of a practical op-amp is shown in the following figure âˆ’

AÂ practical op-ampÂ exhibits the following characteristics âˆ’

• Input impedance,Â ZiZiÂ in the order ofÂ Mega ohms.
• Output impedance,Â Z0Z0Â in the order ofÂ few ohms..
• Open loop voltage gain,Â AvAvÂ will beÂ high.

When you choose a practical op-amp, you should check whether it satisfies the following conditions âˆ’

• Input impedance,Â ZiZiÂ should be as high as possible.
• Output impedance,Â Z0Z0Â should be as low as possible.
• Open loop voltage gain,Â AvAvÂ should be as high as possible.
• Output offset voltage should be as low as possible.
• The operating Bandwidth should be as high as possible.
• CMRR should be as high as possible.
• Slew rate should be as high as possible.

NoteÂ âˆ’ IC 741 op-amp is the most popular and practical op-amp.