Subject :- VLSI Design

Topic :- Technology Trend

Difficulty :- Medium

Analyze DC Characteristics of CMOS Gates by studying an Inverter

• DC Analysis – DC value of a signal in static conditions

• DC Analysis of CMOS Inverter – $V_{in}$, input voltage – $V_{out}$, output voltage – single power supply, $V_{DD}$ – Ground reference – find $V_{out}$ = f($V_{in}$)

• Voltage Transfer Characteristic (VTC) – plot of $V_{out}$ as a function of $V_{in}$ – vary $V_{in}$ from 0 to $V_{DD}$ – find $V_{out}$ at each value of $V_{in}$

• pFET: $V_{Tp}\lt0$
  $\beta_p=k'_p (\frac{W}{L}) _p$

• nFET: $V_{Tn}\gt0$
  $\beta_n=k'_n (\frac{W}{L}) _n$

Voltage Transfer Characteristic (VTC) – plot of $V_{out}$ as a function of $V_{in}$ – vary $V_{in}$ from 0 to $V_{DD}$ – find $V_{out}$ at each value of $V_{in}$
Output High Voltage, $V_{OH}$ – maximum output voltage

• occurs when input is low ($V_{in}$ = 0V)

• pMOS is ON, nMOS is OFF

• pMOS pulls $V_{out}$ to $V_{DD}$ – $V_{OH}$ = $V_{DD}$

• Output Low Voltage, $V_{OL}$ – minimum output voltage

• occurs when input is high ($V_{in} = V_{DD}$)

• pMOS is OFF, nMOS is ON

• nMOS pulls $V_{out}$ to Ground – $V_{OL}$ = 0 V

• Logic Swing – Max swing of output signal

• $V_L = V_{OH} - V_{OL}$

• $V_L = V_{DD}$