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  1. 1. Off
  2. 2. Triode or Ohmic
  3. 3. Saturation
    1. 3.1 Overdrive voltage

1.  Off

VGS < VTH

2.  Triode or Ohmic

  • VGS > VTH
  • Vgd > VTH or VDS < VGS - VTH
  • When VDS << VGS - VTH the Id vs. VDS curve is nearly linear. The transistor then acts like a resistor where R \approx \frac1{\frac{W}L K_P (V_{GS}-V_{TH}). This is why we prefer the term Ohmic region over triode.

3.  Saturation

  • VGS > VTH
  • VDG < VTH or VDS > VGS - VTH
  • Note: The (1 + λ Vds) term in the saturation region equation is known as the Early Effect. This is what causes the non-ideal slope in the saturation curve.

3.1  Overdrive voltage

Overdrive voltage V_o = V_{ds sat} = V_{gs} - V_{th} \approx \sqrt{ \frac{I_D}{ (W/L) K_P/2} }

This is the minimum amount of voltage that must be dropped across the drain to source. This is very useful in computing how far into saturation a transistor is as well as output voltage swings etc.

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Last edited by DrLock. Originally by DrLock.