Explain transfer Characteristics for NMOS inverter showing different regions. What is the effect of variation in (W/L) ratio .

Subject: Basic VLSI Design

Topic: MOSFET Inverters

Difficulty: Medium

bvlsi(46) • 2.7k  views

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"
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