Characteristics(Parameters) of an Op-Amp - Definitions, Equivalent Circuit

Characteristics of Op-Amp 

The characteristics of Op-Amp is very important because we can compare the performance of various Op-Amp ICs and select the best suitable Op-Amp for any desired application.

Equivalent Circuit of an Op-Amp 

The characteristics of an Op-Amp are divided into two types namely AC characteristics and DC characteristics. The AC characteristics include the frequency response, frequency compensation, Bandwidth, slew rate, and stability of Op-Amp whereas the DC characteristics include input offset voltage, the input offset current, input bias current, and thermal drift, etc.

Let’s understand each characteristic one by one. 

Open Loop Voltage Gain

Open-loop voltage gain is defined as the differential gain of an Op-Amp in the open-loop mode of operation. Note that open-loop mode is the mode of operation without any feedback. The open-loop gain is very high. It is denoted by A_V.

Input Offset Voltage

The input offset voltage is defined as a small differential voltage that is applied at the input of the Op-Amp to make the output voltage zero. It is denoted by V_ios.

Ideally, the output voltage of an Op-Amp should be zero for a zero-input voltage. But practically, due to unbalancing in the differential input stage of the Op-Amp, the output is not zero. So, we have to apply a small differential voltage at the input stage of the Op-Amp to make the output voltage zero.

The value of input offset voltage is temperature-dependent and it is about a few mV ranges. 

Output Offset Voltage

The output voltage produced due to input offset voltage is called output offset voltage. It is denoted by V_oos. 

Input Offset Current

Input Offset Current is defined as the algebraic difference between current flowing into the inverting and non-inverting terminals of the Op-Amp. It is denoted by I_ios.

Mathematically, it can be expressed as 

Input Bias Current

Input Bias Current is defined as the average value of the current flowing into the inverting and non-inverting terminals of the Op-Amp. As shown in the figure, the current flowing into non-inverting input is I_B2 and the current flowing into inverting input is I_B1.

Hence, the input bias current is given by

The value of input bias current is temperature-dependent and it should be as low as possible. 

Differential Input Resistance

Differential Input Resistance is defined as the equivalent resistance which can be measured at either the inverting or non-inverting input terminal of the Op-Amp with the other terminal connected to the ground. It is also called the input resistance of the Op-Amp. It is denoted by R_i.

The value of differential input resistance should be as high as possible. It is about a few MΩ for the Op-Amps if a transistor is used as an input and about to GΩ for the Op-Amps if FET is used as an input.

Output Resistance

Output Resistance is defined as the resistance which can be measured at the output terminal of the Op-Amp with input short-circuited.

The value of the output resistance should be as small as possible because it improves the output voltage regulation and current sourcing capacity.

CMRR

CMRR stands for Common Mode Rejection Ratio is defined as the ratio of differential gain to common-mode gain. It is denoted by ƿ.

Mathematically CMRR can be expressed as,  

In decibels, CMRR can be expressed as,

CMRR is the ability of the Op-Amp to reject the common-mode signal successfully. Hence, it is the figure of merit of the Op-Amp.

Ideally, the value of CMRR should be infinite and practically it should be as high as possible. Higher the value of CMRR means better the ability of Op-Amp to reject the noise. 

PSRR

PSSR stands for Power Supply Rejection Ratio is defined as the change in input offset voltage of an Op-Amp due to the change in the supply voltage of an Op-Amp. It is also known as the Supply Voltage Rejection Ratio (SVRR) or Power Supply Sensitivity (PSS).

Mathematically PSRR can be expressed as,  

Ideally, the value of PSRR should be zero and practically it should be as small as possible.

Slew Rate

The Slew Rate of an Op-Amp is defined as the maximum rate of change of output voltage per unit time. It is expressed in Volts/microseconds.

Mathematically PSRR can be expressed as,  

Bandwidth

The bandwidth of the Op-Amp is the range of frequencies over which all the signal frequencies are amplified equally.

The bandwidth of an Op-Amp should be capable of amplifying the signals from 0 Hz frequency. It should be as large as possible. 

Gain Bandwidth Product

Gain Bandwidth Product is defined as the bandwidth of Op-Amp when the voltage gain is unity (1). It is also called unity-gain bandwidth or small signal bandwidth.