Which term represents the body-effect transconductance in the MOSFET small-signal model?

• A. gmb
• B. gm
• C. ro
• D. gd

Answer: (A)

Body bias changes the threshold voltage, and in the MOSFET’s small-signal model this effect shows up as a transconductance from body to source. This is represented by gmb, which multiplies the small-signal body-to-source voltage vbs to produce a drain-current change. It sits alongside gm vgs (gate-to-source control) but specifically accounts for how body bias modulates Id. The other terms describe different aspects: gm is gate-to-source transconductance, ro is the output resistance from channel-length modulation, and gd is a gate-to-drain conductance. So the body-effect transconductance is gmb.