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GATE 2014 EC (EC2) – Question 22

Analog Circuits · BJT and MOSFET Amplifiers · 1 mark · Multiple choice

In the differential amplifier shown, common-mode and differential-mode gain magnitudes are A_cm and A_d. If R_E increases, which occurs?

Diagram for GATE 2014 EC (EC2) question 22
  1. A_cm increases
  2. Common-mode rejection ratio increases
  3. A_d increases
  4. Common-mode rejection ratio decreases

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Show answer and explanation

Correct answer: (B) Common-mode rejection ratio increases

Explanation

In the usual differential-pair small-signal model, increasing tail resistance reduces common-mode gain while leaving differential gain approximately unchanged at fixed bias. Thus |Ad/Acm|, the **common-mode rejection ratio,increases,B**. With perfectly matched devices and an exactly differential output, common-mode output is already zero; the official comparison uses the practical or single-ended common-mode-gain interpretation.