Overdrive voltage
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Overdrive Voltage is usually abbreviated as Vov, and usually only referred to in the context of MOSFETs (PMOS or NMOS). The overdrive voltage is also related to the gate voltage, the supply voltage, and the threshold voltage of MOSFETs using the following equation: Vov = Vgs - Vt.
The reason why overdrive voltage is important is because of its relationship to Vds, the drain to source voltage (i.e. the drain voltage relative to the source), which is what provides you with the region of operation the MOSFET is under. The table below shows how to use overdrive voltage in a meaningful way to understand what region of operation the MOSFET is under:
| Conditions | Region of Operation | Description |
|---|---|---|
| Vds > Vov; Vgs > Vt | Saturation (CCR) | The MOSFET is delivering a high amount of current, and changing Vds won't do much. |
| Vds < Vov; Vgs > Vt | Triode (Linear) | The MOSFET is delivering current in a linear relationship to the voltage (Vds). |
| Vgs < Vt | Cutoff | The MOSFET is turned off, and should not be delivering any current. |
A more physics-related explanation follows:
In an NMOS transistor, the channel region under zero bias has an abundance of holes (ie, it is p-type silicon). By applying a negative gate bias (Vgs < 0) we attract MORE holes, and this is called accumulation. A positive gate voltage (Vgs > 0) will attract electrons and repel holes, and this is called depletion because we are depleting the number of holes. At a critical voltage called the THRESHOLD VOLTAGE (Vt or Vth) the channel will actually be so depleted or holes and rich in electrons that it will INVERT to being n-type silicon, and this is called the inversion region.
As we increase this voltage, Vgs, beyond Vth, we are said to then be OVERDRIVING the gate by creating a stronger channel, hence the OVERDRIVE VOLTAGE (Called often Vov, Vod, or Von) is defined as (Vgs - Vth)
