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enter image description here

If the purpose of this circuit is to store the value of D in Q, why should I need a !Q? Why don't use a circuit like this instead?:

enter image description here

  • The first diagram is a flipflop. The second one isn't. It's a latch. You can see in the diagram for the flipflop why you need !Q. It's output feeds back into the circuit. – scorpdaddy May 05 '20 at 14:17
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    @scorpdaddy: The first circuit is a latch, too. – Dave Tweed May 05 '20 at 14:18

2 Answers2

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Advantages of the first circuit over the second:

  • 5 gates instead of 6.
  • two gate types (1- and 2-input NAND) instead of three.
  • gates that invert (NAND, NOR) are inherently simpler and faster than gates that don't (AND, OR).1
  • only 2 or 3 levels of gate delay between input and output instead of 3 or 4.

As you can see, there's no disadvantage associated with having the !Q output even if you don't use it.

In real SRAM chips, the transistors are made as small as possible in order to pack as many as possible into a given area. In order to maximize the reliability (internal signal to noise ratio), each cell is connected to a pair of complementary bit lines (effectively both Q and !Q), and a differential amplifier is used to sense the state of the selected bit.

1 For details, see Why are NAND gates used to make AND gates in computers?

Dave Tweed
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  • That NAND and NOR gates are simpler and faster than AND and OR I really didn't know. I think that this is the answer I was looking for. I thought that NAND and NOR gates were both made from AND and OR, but this comes from a Computer Science perspective, not Electrical Engineering, which is the real practice. Thanks man! –  May 05 '20 at 14:41
  • I added a link to a question that explores the details of that. – Dave Tweed May 05 '20 at 14:45
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Logic gates are internally structured in mosfets or transistors, both the OR and AND gates use more mosfets than a NAND gate, as such it ends up as a simpler circuit at that level,

I recommend you follow the image source link as it draws out the internals of the logic gates for you.

enter image description here

Image link and a good resource

Reroute
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