D Latch using Transmission Gates

  • Thread starter jaus tail
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  • #1
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Summary:
Why is inverter used for D Latch using Transmission Gates?
My book has this diagram:
246326

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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  • #2
berkeman
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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...
 
  • #3
Baluncore
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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.
 
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  • #4
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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.
 
  • #5
Baluncore
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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.
 
  • #6
Baluncore
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  • #7
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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.
 

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