184-cmos-logic-gate-circuits

Reading Time: 2 minutes

CMOS logic gate circuits are made in CMOS technology. CMOS is abbreviation of Complementary MOS. MOS is abbreviation of Metal-Oxide-Semiconductor. Basic circuit of CMOS logic is CMOS inverter given in figure bellow.

cmos-gate

N channel and P channel MOSFET that CMOS inverter consists of, have input characteristics given in figure bellow. In here, VTN is threshold voltage of N channel MOSFET, and VTP is threshold voltage of P channel MOSFET.

cmos-input

MOSFET operates in saturation (equivalent to active regime for bipolar transistor) when

cmos_eq01

For N channel MOSFET in CMOS inverter, conditions for saturation are

cmos_eq02

and

cmos_eq03

P channel MOSFET in CMOS inverter is saturated when

cmos_eq04

and

cmos_eq05

Transfer characteristic of CMOS inverter has five segments:

  1. N channel MOSFET cutt-off and P channel MOSFET ohmic
  2. N channel MOSFET saturated and P channel MOSFET ohmic
  3. N channel MOSFET saturated and P channel MOSFET saturated
  4. N channel MOSFET ohmic and P channel MOSFET saturated
  5. N channel MOSFET ohmic and P channel MOSFET cutt-off

Whole transfer characteristic of CMOS inverter gate is given in figure bellow.

cmos-transfer

CMOS Three state inverter is given in figure bellow

cmos-3-state

If input denoted as “H” is with low logic value, Q1 and Q4 are cutt-off and output is with high impedance. If “H” is with high logic value, Q1 and Q4 are ohmic and circuit above behave as regular CMOS logic inverter gate.
Logic NAND circuit in CMOS technology is given in figure bellow.

cmos-nand

Logic NAND circuit in CMOS technology has following combinations in input and output stages:

  1. A=0, B=0, Q1 and Q2 cutt-off. Q3, Q4 ohmic VOUT=VCC
  2. A=0, B=1, Q1 and Q3 cutt-off. Q2, Q4 ohmic VOUT=VCC
  3. A=1, B=0, Q2 and Q4 cutt-off. Q1, Q3 ohmic VOUT=VCC
  4. A=1, B=1, Q3 and Q4 cutt-off. Q1, Q2 ohmic VOUT=0

Logic NOR circuit in CMOS technology is given in figure bellow.

cmos-nor

Logic NOR circuit in CMOS technology has following combinations in input and output stages:

  1. A=0, B=0, Q1 and Q2 cutt-off. Q3, Q4 ohmic VOUT=VCC
  2. A=0, B=1, Q1 and Q3 cutt-off. Q2, Q4 ohmic VOUT=0
  3. A=1, B=0, Q2 and Q4 cutt-off. Q1, Q3 ohmic VOUT=0
  4. A=1, B=1, Q3 and Q4 cutt-off. Q1, Q2 ohmic VOUT=0

External links:

CMOS logic on Wikipedia
CMOS logic on Iroi.seu.edu.cn
CMOS logic on Allaboutcircuits

 

Leave a Reply

Your email address will not be published. Required fields are marked *

*