# Linear Integrated Circuit Questions and Answers – Common Mode Configuration and Rejection Ratio

This set of Linear Integrated Circuit Multiple Choice Questions & Answers (MCQs) focuses on “Common Mode Configuration and Rejection Ratio”.

1. When does the op-amp said to operate in common-mode configuration?
a) When the input voltage are equal
b) When input voltage is equal to the output voltage
c) When same voltage is applied to both input terminal of an op-amp
d) When different voltage is applied to both input and terminal of an op-amp

Explanation: When the same input voltage is applied to both input terminal of an op-amp, the op-amp is said to be operating in an common mode configuration.

2. Common mode voltage gain of an op-amp is generally
a) >1
b) =1
c) <1
d) None of the mentioned

Explanation: The amplitude of common mode output voltage is very small and often insignificant compared to common-mode input voltage. Therefore, the common mode voltage gain is generally much smaller than 1.

3. Find the op-amp common mode configuration
a)

b)

c)

d)

Explanation: The mentioned circuit has the applied voltage common to both the inputs.

4. Calculate the common-mode voltage gain for the circuit

a) 30*10-3
b) 17*10-3
c) 51*10-3
d) 2.4*10-3

Explanation: The common mode voltage gain is given as ACM = VOCM/VCM = 12µv/5kΩ =2.4*10-3.

5. Define the common-mode rejection ratio (CMRR) of op-amp?
c) CMRR=VOCM/ACM

Explanation: CMRR is defined as the ratio of the differential gain to the common mode gain, that is CMRR=AD/ACM.

6. Determine the common mode output voltage .Given CMRR=65db

a) 2.81mV
b) 281.17mV
c) 28.11mV
d) 0.281mV

Explanation: AD= RF/ R1 = 30kΩ/3kΩ = 10.
WKT, CMRR =( AD* VCM)/ VOCM= 20logCMRR = 65db = 10(65/20) = 1778.28
=> Common mode output voltage, VOCM= ( AD* VCM)/CMRR = (10*5mV)/1778.28= 281.17mV.

7. Find the correct equation for common-mode rejection ratio
b) CMRR = 20log(Vio/ VCM)
c) CMRR = (AD* VCM)/ VOCM
d) All of the mentioned

Explanation: Generally, the CMRR value is very large and usually specified in decibels (db). Also, the CMRR value can establish relationship with common mode output voltage.

8. When an op-amp exhibits poor common mode rejection?
a) Small common mode output voltage
b) Large common mode output voltage
c) Negligible common mode output voltage
d) None of the mentioned

Explanation: A large common mode output voltage for a given common mode input voltage indicates a large degree of imbalance between the two input terminals or poor CMRR.

9. Higher value of common mode rejection ratio can be reached
a) By reducing the common mode voltage
b) By decreasing the differential gain
c) By reducing the common mode input voltage
d) All of the mentioned

Explanation: CMRR =(AD* VCM)/ VOCM, this equation indicates that the higher the value of CMRR, the smaller will be the amplitude of the output common-mode voltage.

Sanfoundry Global Education & Learning Series – Linear Integrated Circuits.

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