# D Latch using Transmission Gates

• jaus tail
In summary, the use of three inverters in this digital integrated circuit serves multiple purposes. The lower inverter acts as an input buffer amplifier, while the upper two-inverter loop provides a positive feedback digital memory. This creates a "sample and hold" circuit for the digital signal. Additionally, by feeding the output of each inverter back to the input of the other, a flip-flop is created which acts as a memory element. The inverters also compensate for any voltage drops, ensuring that the logic level remains high.
jaus tail
TL;DR Summary
Why is inverter used for D Latch using Transmission Gates?
My book has this diagram:

Book: Digital Integrated Circuits by Jan M. Rabaey

I don't understand the purpose of using the three Inverters. It's not mentioned in the book, nor could I find anything on the internet.

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Probably because you generally want to buffer all inputs and outputs (to get high input impedance and low output impedance. The only time I've used an unbuffered inverter is in some crystal oscillator circuits...

jaus tail
jaus tail said:
I don't understand the purpose of using the three Inverters.
Two inversions cancel to make a non-inverting amplifier.

The lower inverter is an input buffer amplifier. Q follows D, through two inverters, while the lower transmission gate is conducting.

The upper two-inverter loop provides a positive feedback digital memory. It holds the last digital output state while the D input is isolated and the upper transmission gate is conducting.

It is a “sample and hold” circuit, applied to a digital signal.

jaus tail and berkeman
But why do I need inverters? If the two inverters are in series, the output is same as input. I might as well not use them.

jaus tail said:
If the two inverters are in series, the output is same as input. I might as well not use them.
With two inverters, if you feed the output of each, back to the input of the other, you have a flip-flop which is a memory element.

jaus tail

jaus tail
Also maybe it can be that inverter also makes up for voltage drop if any. Like inverter will again either pull up to 5 V or 0 V. Like if memory has output as 5 V, then with time it may degrade to 4.5 V and then to 3 V. If it goes below 2.5 V, it may be treated as logic 0.

So with 2 inverters the 4.5 V is again pulled up, and logic remains high.

## 1. What is a D Latch using Transmission Gates?

A D Latch using Transmission Gates is a type of electronic circuit that is used to store and remember a single bit of information. It is commonly used in digital systems to hold data and control signals.

## 2. How does a D Latch using Transmission Gates work?

A D Latch using Transmission Gates consists of two transmission gates, which are electronic switches, and two inverters. The transmission gates are controlled by the input signals and allow the data to pass through when the control signal is high. The inverters ensure that the output remains stable when the control signal is low.

## 3. What are the advantages of using a D Latch using Transmission Gates?

One of the main advantages of using a D Latch using Transmission Gates is its simplicity. It requires fewer components compared to other types of latches, making it more cost-effective and easier to implement in digital systems. It also has a fast response time and consumes less power.

## 4. What are the applications of a D Latch using Transmission Gates?

A D Latch using Transmission Gates is commonly used in digital systems for storing data and control signals. It is also used in memory circuits, data registers, and flip-flops. Additionally, it can be used in microprocessors, communication systems, and other electronic devices.

## 5. What are the limitations of a D Latch using Transmission Gates?

One of the limitations of a D Latch using Transmission Gates is its susceptibility to noise. Since the transmission gates are controlled by electronic signals, any external noise can affect the output. It also has a limited storage capacity, as it can only store a single bit of data.

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