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In a differential amplifier using JFETs, the differential output offset may arise due to which of the following causes?

Difficulty: Medium

Correct Answer: Mismatch between FET parameters, resistance mismatch, and variation in supply voltage

Explanation:


Introduction / Context:
Differential amplifiers are widely used for rejecting common-mode signals and amplifying only differential inputs. In practice, imperfections cause offset voltage even with zero input difference. Understanding these causes is crucial in analog design.


Given Data / Assumptions:

  • Amplifier uses JFET input transistors.
  • Possible causes include device mismatches, resistance mismatches, and supply fluctuations.


Concept / Approach:
Differential offset occurs when the symmetry of the circuit is disturbed. Mismatch between the FET's threshold voltages or transconductance, unequal resistors in load or bias circuits, and variation in supply voltage all contribute to output offset.


Step-by-Step Solution:

Check device mismatch: Idss or Vgs(th) differences shift balance.Check resistance mismatch: unequal load or bias resistors distort symmetry.Supply variation: affects bias currents and can create imbalance.


Verification / Alternative check:

Simulation of a differential pair with matched vs mismatched parameters confirms output offset.


Why Other Options Are Wrong:

Options excluding any of the three causes are incomplete.


Common Pitfalls:

Assuming ideal symmetry; ignoring supply sensitivity.


Final Answer:

Mismatch between FET parameters, resistance mismatch, and variation in supply voltage
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