Subject :- VLSI Design

Topic :- MOS Circuit Design Styles

Difficulty - Medium

• A 2:1 multiplexer is shown in Figure below. This gate selects either input A or B on the basis of the value of the control signal 'C'. When control signal C is logic low the output is equal to the input A and when control signal C is logic high the output is equal to the input B.

• A 2 : 1 multiplexer can be implemented using transmission gates. Figure below shows the connection diagram of the 2 : 1 multiplexer using transmission gates.

  

• When the control signal C is high then the upper transmission gate is ON and it passes A through it so that output = A.

• When the control signal C is low then the upper transmission gate turns OFF and it will not allow A to pass through it, at the same time the lower transmission gate is 'ON' and it allows B to pass through it so the output = B.

Advantages of using transmission gate logic.

• A CMOS transmission gate can be constructed by parallel combination of nMOS and pMOS transistors, with complementary gate signals.
• The main advantage of CMOS transmission gate compared to nMOS transmission gate is to allow the input signal to be transmitted to the output without the threshold voltage attenuation.
• It allows full rail transition i.e. ratioless logic
• The equivalent resistance is relatively constant during transition.
• Some gates are efficient implemented using transmission gate.