Operational amplifier parameters

Explains the parameters of Op-Amp 741 IC.

1. Op-Amp 741 parameters - A.Usha Rani M.Sc. Physics

2. The following are the parameters of Op-Amp Input offset voltage Output offset voltage Input offset current Input bias current CMRR(Common Mode Rejection Ratio) Slew Rate

3. Input offset voltage It is the amount of voltage that must be applied at the input terminals to make the output voltage zero and is denoted by V io . For an ideal Op-Amp, the output voltage must be zero if the voltage at both the input terminals is zero. But in practice a small amount of voltage appears(due to imbalance in the differential amplifier in the Op-Amp ) at the output even if the voltage at both input terminals is zero. To make this output voltage zero, an equal amount of voltage is applied at input terminals which is called input offset voltage.

4. Output offset voltage It is the amount of voltage that appears at the output terminal (due to imbalance in the differential amplifier in the OP-Amp), even if the voltage at both the input terminals is zero. To make this output voltage zero, an equal amount of voltage is applied at input terminal . It is denoted by V oo

5. Input bias current Ideally no current flows into the input terminals of Op-Amp. But in practice a small amount of current flows into the input terminals when power supply is given to an Op-Amp. These currents are called bias currents. It is the average of both the bias currents flowing into the input terminals of Op-Amp. It is denoted by I B

6. Input offset current It is the difference between the two input bias currents flowing into the input terminals of Op-Amp. It is denoted by I io

7. CMRR(Common Mode Rejection Ratio) CMRR gives the ability of the Op-Amp to reject common mode signals. It is defined as the ratio of differential mode gain to common mode gain. CMRR = A d /A c Where A d is gain of the Op-Amp in differential mode and A c is the gain of the Op-Amp in common mode.

8. When two signals of same magnitude are applied at the two inputs of Op-Amp, the gain of the Op-Amp is called common mode gain(A c ). When two signals of different magnitude are applied the two inputs of Op-Amp the gain of the Op-Amp is called differential mode gain(A d ). Since Op-Amp is a differential amplifier, ideally common mode gain should be zero. Hence the ideal value of CMRR for an Op-Amp is ∞.

9. Practically for an Op-Amp CMRR is very large. Hence to express the large value of CMRR, logarithm is used and CMRR is given by 20log(A d /A c )dB.

10. Slew Rate Slew rate is defined as the rate of change of output voltage of an Op-Amp with respect to the change in input voltage . It is defined at unity gain of Op-Amp and is expressed as Volts/micro second . Let a voltage V o Sin ω t be applied to Op-Amp. Then rate of change of output voltage with respect to time is dV o / dt = V o ω Cos ω t Slew rate is given by maximum rate of change of output voltage i.e., S = ( dV o / dt ) max = V o ω = 2 π fV o

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