Compact Logic Gate Options for Efficient Circuit Design

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Discussion Overview

The discussion revolves around the search for compact logic gate options suitable for circuit design, specifically focusing on the availability of NAND, AND, and inverter gates in 8-pin DIP or smaller chips. Participants explore various approaches to using fewer gates, including discrete components and specific integrated circuits.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant expresses a need for logic gates in smaller chip configurations, specifically 8-pin DIP or less, to avoid excess unused gates.
  • Another participant suggests using the 555 timer as an inverter and proposes building gates with diodes for learning purposes.
  • A participant mentions the importance of tying unused inputs to GND or Vcc for stability in logic gate circuits.
  • Discussion includes a reference to the 74AHC1Gxx series as a potential solution for single gate configurations.
  • There are inquiries about creating a simple inverter from an NPN AND gate circuit, leading to suggestions on modifying the circuit.
  • Concerns are raised about the speed of logic gates built from discrete transistors, with questions about the reasons for this slowness.
  • Some participants speculate on the effects of physical size and capacitance of transistors on the speed of discrete versus integrated logic gates.
  • There are differing views on whether discrete transistors can match the speed of integrated counterparts, with some arguing that properly biased discrete transistors can be as fast.

Areas of Agreement / Disagreement

Participants express a mix of agreement and disagreement regarding the feasibility and performance of discrete versus integrated logic gates. The discussion remains unresolved on the specifics of speed comparisons and the implications of using discrete components.

Contextual Notes

Limitations include assumptions about the performance characteristics of discrete versus integrated circuits, as well as the specific configurations and conditions under which these comparisons are made.

Tesladude
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Hi I am learning about logic gates and am having some fun with them but somtimes in my designs I only have use for maybe 1 or 2 gates, this is not convenient sense the chips I use have 4 or 6 gates in one chip, does anyone know where I can find NAND, AND, and inverter gates in 8pin dip or less chips? so I could have 2 and gates or 3 invertes in just an 8pin chip?
 
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8 pin restriction will be difficult, standard was 14 or 16.

The venerable 555 can do logic and is cheap in 8 pin. Tie TRIG and THRESH together and it's an inverter with hysteresis - great for logic signal conditioning. And it's good practice for you learning logic, add diodes to build your desired gates.

does this help?
http://en.wikipedia.org/wiki/List_of_7400_series_integrated_circuits
 
Im not aware of any chips with only 1 or 2 gates in them, it doesn't matter if the chips you are using
have more gates than you need you just don't use the other ones. But one thing you may not know
is that you should tie the unused inputs to the GND or +5V rail via a 1k resistor. This is for stability
reasons. It stops erratic behaviour of the gates you are using

Your other choice, if you are only wanting a couple of gates is to experiment with diodes and transistors
and build gates out of discrete components. It will REALLY teach you about how a gate operates

here's an AND gate...

attachment.php?attachmentid=60694&stc=1&d=1375696885.gif


do some googling for other examples :)

cheers
Dave
 

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Tesladude said:
Hi I am learning about logic gates and am having some fun with them but somtimes in my designs I only have use for maybe 1 or 2 gates, this is not convenient sense the chips I use have 4 or 6 gates in one chip, does anyone know where I can find NAND, AND, and inverter gates in 8pin dip or less chips? so I could have 2 and gates or 3 invertes in just an 8pin chip?

74AHC1Gxx logic gates are singles: http://ics.nxp.com/products/ahc/1g/

:smile:
 
Ya I thought of a similar npn AND gate circuit like that one but what I really need is a simple inverter, any ideas?
 
Keep in mind that if you build your logic gates with discrete transistors, they will be *very slow* gates. Quiz Question -- Why?
 
Ah yes. Leave it to good ol' berkeman to turn it into a quiz. LOL You should have been a teacher. I suspect you have a talent for it. Keep it up.
 
Tesladude said:
Ya I thought of a similar npn AND gate circuit like that one but what I really need is a simple inverter, any ideas?

Sure. Take the AND gate from above, remove the top NPN, add a resistor in the collector terminal of the remaining NPN and move the output from the emitter of the bottom NPN into its collector. How would you size the resistor between the collector of the NPN and the power supply?
 
Averagesupernova said:
Ah yes. Leave it to good ol' berkeman to turn it into a quiz. LOL You should have been a teacher. I suspect you have a talent for it. Keep it up.

:smile:
 
  • #10
berkeman said:
Keep in mind that if you build your logic gates with discrete transistors, they will be *very slow* gates. Quiz Question -- Why?

I think I know why ( not sure) been waiting for tesladude to respond before I spoke

Dave
 
  • #11
Yeah, let's let the young'uns take a shot at it. I was surprised when I first encountered it, but the explanation makes sense.
 
  • #12
berkeman said:
Keep in mind that if you build your logic gates with discrete transistors, they will be *very slow* gates. Quiz Question -- Why?

Perhaps it is the physical size of the transistors. Consequentially there will exist a larger depletion region between the junctions in the transistors (assuming TTL logic, but DTL and RTL will have a similar issue). Is it that the PN junctions may take longer to overcome and begin conducting as compared to their physically smaller counterparts?
 
Last edited:
  • #13
berkeman said:
Keep in mind that if you build your logic gates with discrete transistors, they will be *very slow* gates. Quiz Question -- Why?

"slow" is relative. slow compared to 74xx TTL? slow compared to what?

because i think that "properly biased" (like the old ECL logic was), a discrete transistor will turn on and off as fast as a counterpart that lives on a chip.

the distances between devices eventually come up regarding speed, but i don't think we're there regarding this application. would an inverter made out of a single discrete transistor be all that much slower than an inverter that is 1/6th of a 74LS06?
 
  • #14
rbj said:
"slow" is relative. slow compared to 74xx TTL? slow compared to what?

because i think that "properly biased" (like the old ECL logic was), a discrete transistor will turn on and off as fast as a counterpart that lives on a chip.

the distances between devices eventually come up regarding speed, but i don't think we're there regarding this application. would an inverter made out of a single discrete transistor be all that much slower than an inverter that is 1/6th of a 74LS06?

No, it's *really* slow. :smile:
 
  • #15
sherrellbc said:
Perhaps it is the physical size of the transistors. Consequentially there will exist a larger depletion region between the junctions in the transistors (assuming TTL logic, but DTL and RTL will have a similar issue). Is it that the PN junctions may take longer to overcome and begin conducting as compared to their physically smaller counterparts?

It does have to do with the size of the transistors, specifically the capacitances...
 
  • #16
berkeman said:
It does have to do with the size of the transistors, specifically the capacitances...

you have interelectrode and junction capacitance with chip transistors. and there is some capacitance with the substrate. in fact, I've always thought that monolithic analog crapped out at lower frequencies than discrete analog because of these parasitic capacitance. how is it that MOS transistors on chips are so much faster than a sufficiently biased discrete transistor? is it purely the dimensions of the device?
 
  • #17
Yeah, the typical logic gate has a number of transistors, but their input capacitances are small compared to each single packaged transistor's input capacitance. In many cases it's the Miller capacitance, which makes speed problems even worse.
 

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