How do I calculate the input bias current?

a. What is the calculated input bias current?





b. What is the measured input offset voltage?





c. What is the calculated Common Mode Rejection Ratio (CMRR)?





How do I calculate these values if I only know the output voltage, V+, and the resistances?|||You can't calculate these parameters. They are tested by the opamp manufacturer, and guaranteed to be within the limits on the data sheet.





You can measure them, but it takes sophisticated equipment and a good knowledge of opamps.





Usually you just take the values on the data sheet.





.|||Normally you will be calculating the effects of bias current, CMRR etc, using the specified values from a data sheet. The first step is to get the data sheet by a search e.g. data sheet 741. This will be a PDF file.





The bias current flows into or out of an input. Calculate the voltage due to bias current using ohms law and any resistance in the external circuit of each input. Often the bias current offset (difference between the two input bias currents) is more important because if the two bias currents were the same, and the resistors in each input the same, the offset voltage due to bias current cancels.





Input offset can be measured with the amplifier configured for a high gain. WIth no input the output should be zero too. To separate the effects of current and voltage offsets the external resistors on the amplifier input are made a very low resistance to minimise current offsets.. Divide the output offset by the gain.





The Common Mode Rejection Ratio is the ability to reject common voltages (common = the same) on each input. The amplifier should only amplify the difference between two inputs. Lets say the input is shorted, so both inputs are at 0 Volts. The output should be zero except for offsets. Now change the input from 0 volts to 1V for example (but still shorted). The difference between the inputs is still zero, so the output should not change. Lets say it changed by 1mV. The input changed 1V, the output change 1mV, so the voltage ratio is 1000, or 120dB.





The link below shows a test procedure for these purposes (using an oscilloscope). However the principles are similar for static DC tests.