Question

Consider the following statements regarding a differential amplifier using an FET pair. The differential output offset voltage is due to:

1. Mismatch between FET parameters

2. Difference between the values of resistors used in the circuit even though they are marked nominally equal

3. Variation in the operating voltage of the circuit

Which of the above statements are correct?

1. 1, 2 and 3
2. 2 and 3 only
3. 1 and 3 only
4. 1 and 2 only

Answer 1

Correct Answer - Option 4 : 1 and 2 only

• Real operational amplifiers have some imperfections compared to an ideal model.
• It may have an offset like an analog meter which is not zeroed. The inputs may draw current.
• The offset voltage is the effect of having the output voltage something other than zero volts even though when the two input terminals are shorted.
• Operational amplifiers are differential amplifiers, they are supposed to amplify the difference in voltage between two input connections to have output.
• Hence offset is due to mismatch between FET parameters and difference between the values of resistors used in the circuit even though they are marked nominally equal.