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Analog Electronics Differential Amplifiers

For a BJT differential pair at room temperature with total tail current IT, the small-signal differential transconductance from differential input voltage to single-ended collector-current change is approximately?

AIT/(4VT)
BIT/VT
C4IT/VT
DVT/IT

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Correct Answer: A. IT/(4VT)

Explanation: Each transistor carries IT/2 at balance, giving gm=IT/(2VT). For differential excitation, the single-ended collector-current change is gm(vd/2), so the effective differential transconductance is IT/(4VT).

Reference: Standard analog electronics: differential transconductance

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