The overdrive voltage is the amount by which the gate-to-source voltage exceeds the Threshold voltage:
where is the applied gate-to-source voltage and the threshold. It is the single number that says how hard the MOSFET is turned on. At the channel is just barely forming and no current flows; the larger , the more charge in the channel and the more current the device can carry.
Why everything is written in terms of
The channel charge is set by how far the gate voltage is above threshold, not by on its own — the threshold part of is “used up” just creating the channel. So is the physically meaningful drive, and it appears directly in every key MOSFET relation:
- The square-law saturation current: , where is the MOSFET transconductance parameter.
- The boundary between triode and saturation: pinch-off happens exactly when (see Channel pinch-off).
- The MOSFET transconductance: .
So is simultaneously the drive knob, the saturation boundary, and the small-signal gain parameter.
The gain-versus-headroom trade-off
Look at . At a fixed bias current , smaller gives larger , hence more voltage gain (a Common-source amplifier has ). So you might want as small as possible.
But there are two costs. First, the device needs to stay in saturation; the output cannot swing below without the transistor leaving the amplifying region and clipping. Second, the small-signal approximation requires the input swing (see MOSFET transconductance); a tiny leaves almost no linear range before square-law distortion sets in. So a small buys gain at the price of output headroom and linearity. Choosing is the central practical decision in single-stage MOSFET design — a typical analogue value is a few hundred millivolts.