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Transconductance (gm) Explained

 

Transconductance (gm)

Transconductance (gm)

BJT (Bipolar Junction Transistor)

gm = Ic / Vt

Where:
Ic = collector current (A)
Vt ≈ 26 mV at room temperature

Example:

If Ic = 2 mA:
gm = 2 mA / 26 mV ≈ 0.077 S (77 mS)

MOSFET

Formula 1:

gm = 2Id / (Vgs - Vth)

Formula 2:

gm = √(2kId)

Where:
Id = drain current
Vgs = gate-source voltage
Vth = threshold voltage
k = device constant

Summary

  • BJT: gm = Ic / 26mV
  • MOSFET: gm = 2Id / Vov

Units

gm is measured in Siemens (S)

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