Complementary Metal oxide semi conductor is termed as CMOS.

CMOS circuit consist of both n-type and p-type MOS devices interconnected to form logic function.

Fig(1) - CMOS Inverter Circuit

• The basic CMOS circuit is the inverter, which consists of one p-channel transistor and one n-channel transistor.
• The source terminal of p-channel is at VDD and source terminal of n-channel device is at group

Operation:

• The p-channel MOS conducts when its VGS (gate to source) voltage is vegative.
• The n-channel MOS conducts when its VGS is positive.

Case-1

When the input is low,

Both the gates of n-channel and p-channel are at zero potential.

The input of the p is at -VDD relative to the source of the p-channel device and at 0V relative to the source of the n-channel device.

Therefore, the p-channel voltage is negative. Hence, it is turned and n-channel is turned off.

Therefore, there is a low impedance path from VDD to output and a very high impedance path from output to ground.

Therefore, output is high.

Case-2

When the input is high,

Both the gates are at VDD.

The p-channel device is turned off as the voltage across it is positive and the n-channel MOS device is turned on as the voltage is positive.

Therefore, there is a high impedance path from VDD to output and a low impedance path from output to ground.

Therefore, output is low.

Transfer Characteristics:

• When the Vin is zero. The output voltage is high = VDD
• When the Vin is high output voltage is low
  Vo/p = zero Volts