The GATE Grind

GATE 2014 EC (EC4) – Question 47

Analog Circuits · BJT and MOSFET Amplifiers · 2 marks · Multiple choice

The common-collector amplifier shown is in forward active operation; bias circuitry is omitted. I_C is collector current, V_BE base–emitter voltage, V_T thermal voltage, g_m transconductance and r_o output resistance. Which condition ensures nearly constant small-signal voltage gain over a wide range of R_E?

Diagram for GATE 2014 EC (EC4) question 47
  1. $g_mR_E\ll1$
  2. $I_CR_E\gg V_T$
  3. $g_mr_o\gg1$
  4. $V_{BE}\gg V_T$

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Correct answer: (B) $I_CR_E\gg V_T$

Explanation

Ignoring small corrections, an emitter follower has Av≈gmRE/(1+gmRE). Its gain is nearly constant near1when gmRE≫1. Since gm=IC/VT, this condition is **ICRE≫VT,B**. Large intrinsic gain gmroalone does not ensure sufficiently large emitter loading resistance.