Objectives
To find the Common Mode Rejection Ratio (CMRR) of an Operational amplifier.
Components
741 IC
Hook up wires
Bread board
Multimeter
CRO
DC Power Supply
Function Generator
Hook up wires
Bread board
Multimeter
CRO
DC Power Supply
Function Generator
Theory
Common Mode Rejection Ratio (CMRR)
When the same input voltage is applied to both input terminal of an operational amplifier, the op-amp is said to be operating in a commin mode configuration since the input voltage applied is common to both the inputs. It is referredto as a common mode voltage VGS as shown in figure 1. A common mode voltage VGS can be ac, dc or a combination of ac and dc.
Ideally an op-amp only differential input voltage no common-mode output voltage VOCM should appear at output. However due to imperfections within an actual op-amp, some common mode voltage VOCM will appear which is very small and often insignificant compared to VCM. Therefore, in practice, the ratio of the output common mode voltage VOCM to the input common-mode voltage VCM, which is called the common mode voltage gain ACM, which is generally much smaller than 1.
When the same input voltage is applied to both input terminal of an operational amplifier, the op-amp is said to be operating in a commin mode configuration since the input voltage applied is common to both the inputs. It is referredto as a common mode voltage VGS as shown in figure 1. A common mode voltage VGS can be ac, dc or a combination of ac and dc.
Ideally an op-amp only differential input voltage no common-mode output voltage VOCM should appear at output. However due to imperfections within an actual op-amp, some common mode voltage VOCM will appear which is very small and often insignificant compared to VCM. Therefore, in practice, the ratio of the output common mode voltage VOCM to the input common-mode voltage VCM, which is called the common mode voltage gain ACM, which is generally much smaller than 1.
ACM = VOCM / VCM
Procedure
1. For measuring Acm connect the the circuit as shown in figure1
2. Give 10 V input to the operational amplifier and measure the help of multimeter.
3. Connect the circuit as shown in figure 2 for measuring Ad
4. Measure the input voltage as well as output voltage and then calculate the value of Ad which should be very close to 1.
5. Calculation of CMRR, take the value of Acm and apply the formula
----------------CMRR = 20 log (Ad/Acm)------------
6. A practical operation amplifier its value should be close to 70 dB.
2. Give 10 V input to the operational amplifier and measure the help of multimeter.
3. Connect the circuit as shown in figure 2 for measuring Ad
4. Measure the input voltage as well as output voltage and then calculate the value of Ad which should be very close to 1.
5. Calculation of CMRR, take the value of Acm and apply the formula
----------------CMRR = 20 log (Ad/Acm)------------
6. A practical operation amplifier its value should be close to 70 dB.
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