The typical VTC of a realistic nMOS inverter is shown in Figure below.
- The general shape of the VTC is qualitatively similar to that of the ideal inverter transfer characteristic.
- For very low input voltage levels, the output voltage Vout is equal to the high value of VOH (output high voltage).
- In this case, the driver nMOS transistor is in cut-off, and hence, does not conduct any current. Consequently, the voltage drop across the load device is very small in magnitude, and the output voltage level is high.
- As the input voltage V increases, the driver transistor starts conducting a certain drain current, and the output voltage eventually starts to decrease.
Two critical voltage points are identified on this curve, where the slope of the Vout(Vin) characteristic or gain of the curve becomes equal to -1, i.e.,
These points are VIL (input low voltage) and VIH (input high voltage). Both of these voltages play significant roles in determining the noise margins of the inverter circuit.
- As the input voltage is further increased, the output voltage continues to drop and reaches a value of VOL (output low voltage) when the input voltage is equal to VOH.
- Point where Vin = Vout, is the threshold voltage of inverter Vth.
- Thus, a total of five critical voltages, VOL, VOH, VIL, VIH, and Vth, characterize the DC input-output voltage behavior of the inverter circuit.
- VOH: Maximum output voltage when the output level is logic " 1"
- VOL Minimum output voltage when the output level is logic "0"
- VIL: Maximum input voltage which can be interpreted as logic "0"
- VIH: Minimum input voltage which can be interpreted as logic " 1"