Subject :- VLSI Design

Topic :- MOSFET Inverters

Difficulty - Medium

• Figure below shows the generalized circuit structure of an nMOS inverter. The input voltage of the inverter circuit is also the gate-to-source voltage of the nMOS transistor (Vin = VGS), while the output voltage of the circuit is equal to the drain-to-source voltage (Vout= VDS).
• The source and the substrate terminals of the nMOS transistor, also called the driver transistor, are connected to ground potential; hence, the source-to-substrate voltage is VSB = 0.

• One terminal of the load device is connected to the drain of the n-channel MOSFET, while the other terminal is connected to VDD, the power supply voltage.

  

• Applying Kirchhoff s Current Law (KCL) to this simple circuit, we see that the load current is always equal to the nMOS drain current.

  

• The voltage transfer characteristic describing Vout as a function of Vin under DC conditions can then be found by analytically solving above equation for various input voltage values. 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"