TRANSMISSION GATE LOGIC

Summary

In this tutorial, the concept of Transmission Gate Logic (TG) is introduced as an efficient alternative to CMOS logic. TG, a bidirectional switch using both N-MOS and P-MOS transistors in parallel, offers advantages like reduced transistor count, no threshold voltage loss, and constant equivalent resistance during data transfer. The video explains its working principle and demonstrates practical implementations such as 2-to-1 and 4-to-1 multiplexers. TG logic is shown to simplify digital circuit designs, making it an essential tool for reducing complexity and improving efficiency in logic gate implementations.

Takeaways

• 😀 Transmission Gate Logic (TG) is a bidirectional switch using both N-MOS and P-MOS transistors connected in parallel.
• 😀 TG logic reduces the number of transistors required compared to CMOS logic, improving efficiency.
• 😀 N-MOS transistors provide a strong response for logic 0, while P-MOS transistors provide a strong response for logic 1.
• 😀 Transmission gates eliminate threshold voltage loss, ensuring that the entire input voltage range is transmitted without degradation.
• 😀 When control signal C is 1, both N-MOS and P-MOS transistors are activated, allowing the input signal to pass through the gate.
• 😀 When control signal C is 0, both transistors are off, resulting in a high impedance state where no signal is transmitted.
• 😀 The symbol for a transmission gate consists of two arrowheads, representing the N-MOS and P-MOS transistors, with control signals controlling the operation.
• 😀 A 2-to-1 multiplexer can be efficiently implemented using TG logic with just 6 transistors, compared to the 10 transistors required for CMOS logic.
• 😀 The output equation for a 2-to-1 multiplexer is: Output = (S' * A) + (S * B).
• 😀 A 4-to-1 multiplexer can be constructed using 4 transmission gates and control signals A and B to select the appropriate input based on a given truth table.

Q & A

What is the primary advantage of using transmission gate logic over CMOS logic?

The primary advantage of using transmission gate logic is that it requires fewer transistors compared to CMOS logic. This makes it more efficient in terms of transistor usage and simplifies circuit design.

What type of transistors are used in a transmission gate and why?

A transmission gate uses both NMOS and PMOS transistors connected in parallel. This combination allows for ideal responses for both logic 0 and logic 1, as NMOS transistors provide strong logic 0 responses and PMOS transistors provide strong logic 1 responses.

How does the control signal 'C' affect the operation of the transmission gate?

The control signal 'C' controls the transmission of signals through the transmission gate. When 'C' is 1, both NMOS and PMOS transistors are switched on, allowing the input signal at point A to appear at point B. When 'C' is 0, both transistors are turned off, and the transmission gate behaves as an open circuit, preventing signal transmission.

What is the significance of the control signals C and C bar in the transmission gate?

Control signals 'C' and 'C bar' work together to activate the NMOS and PMOS transistors. When C is 1, the NMOS transistor is activated, and when C bar (the complement of C) is 1, the PMOS transistor is activated. This ensures both transistors are on when C is 1, allowing the transmission of the input signal.

What happens when both control signals (C and C bar) are 0?

When both control signals are 0, the NMOS and PMOS transistors are turned off, and the transmission gate becomes an open circuit, blocking the signal from passing through the gate.

How is the truth table for a transmission gate structured?

The truth table for a transmission gate shows the behavior of the gate depending on the control signal 'C'. When C is 1, the input signal is passed to the output. When C is 0, the transmission gate is in a high-impedance state, and no signal passes through the gate.

What are the advantages of transmission gate logic compared to CMOS logic?

The advantages of transmission gate logic include reduced transistor count, no threshold voltage loss (allowing full voltage range transmission), constant equivalent resistance during signal transmission, and the ability to implement some logic gates more efficiently.

What is a 2-to-1 multiplexer, and how does it work?

A 2-to-1 multiplexer has two inputs, one output, and a single select line. Based on the value of the select line, one of the two inputs is passed to the output. If the select line is 0, the first input is selected; if the select line is 1, the second input is selected.

How is a 2-to-1 multiplexer implemented using transmission gate logic?

A 2-to-1 multiplexer using transmission gate logic is implemented by using two transmission gates. Each input is connected to a transmission gate, and the select line controls which input is passed to the output. The logic for the select line determines the signal flow through the gates.

What are the benefits of using transmission gates in multiplexers compared to CMOS logic?

Using transmission gates in multiplexers reduces the number of transistors required for the implementation, as it uses fewer components compared to CMOS logic. Additionally, transmission gates provide better efficiency and do not suffer from threshold voltage losses, resulting in cleaner signal transmission.