TTL vs. CMOS

Discussion in 'General Electronics Chat' started by GTeclips, Aug 21, 2013.

  1. GTeclips

    Thread Starter Member

    Feb 18, 2012
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    Hello.

    I've been working with circuits for about a year now, and have learned literally nothing. I'm still just about as knowledgeable as when I started (I would take classes, but my school doesn't offer any). Anyways, the two big types of ICs I see are transistor-transistor logic and complementary metal–oxide–semiconductor.

    I understand the difference between them physically to a degree, but what I'm concerned with is which better suites my situation. I simply want to tinker around with OR and NOT gates and just experiment and learn more. Would TTL or CMOS be better for this application? Can I use both types in the same circuit? Does it even really matter?

    Thank you for putting up with me!
     
    Last edited: Aug 21, 2013
  2. Potato Pudding

    Well-Known Member

    Jun 11, 2010
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    Use TTL for learning.

    As long as you can keep them supplied with a regulated 5 Volts you will be good.

    CMOS you will want to have an IC tester (usually also works as a universal programmer and likely to be sold as such.) CMOS can blow up their static sensitive gates if you so much as scratch your head before picking up the IC.
     
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  3. WBahn

    Moderator

    Mar 31, 2012
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    In general, no, you cannot mix them together because their drive capabilities and requirements are not well matched. With some care, you can interface them together and there are families with them, such as 74HCTxx, that are specifically designed to be mixed (HCT - High-speed CMOS with TTL-comptatible I.O).

    Which is best for tinking depends on what kind of tinkering you are doing. Fpr stuff that is more logic-focused, I think CMOS is probably the better choice. For stuff where you are driving a lot of non-logic stuff, TTL has some advantages. But you can do either with either without too much heartache.
     
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  4. WBahn

    Moderator

    Mar 31, 2012
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    Modern CMOS gates are pretty much as tolerant to poor handling as TTL. You've got t get pretty radical to get past the ESD protection structures on most CMOS packages,
     
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  5. GTeclips

    Thread Starter Member

    Feb 18, 2012
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    Thanks for the aid guys!

    I'm mostly interested in the logical aspects of circuitry.
     
  6. WBahn

    Moderator

    Mar 31, 2012
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    The reason I say that CMOS is probably the better choice is because it is so much lower power than TTL typically is and is more tolerant to variations in supply voltage. So you can make larger designs for the same power supply. Then, it generally has much higher fanout, especially for slower stuff, than TTL which means you can drive a lot more gates with a single output thatn you can with TTL.
     
  7. MrChips

    Moderator

    Oct 2, 2009
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    Shucks, you've only just scratched the surface. There are about a dozen different types in the TTL family. And about as many in CMOS family. Which of 24 to choose?

    You don't mix TTL and CMOS because the current/voltage characteristics are different.

    But wait, CMOS HCT family is compatible with TTL. That's what the T in HCT stands for.

    Confused? Not surprising.

    Stick with one family and stay out of trouble.

    Get a 5V regulated power supply and play around with 74LSxx chips.

    Get a 9V battery and play around with 4xxx CMOS chips.

    Either case, you can't go wrong.
     
  8. crutschow

    Expert

    Mar 14, 2008
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    In that case you may find the old, low-frequency CD4000 series of CMOS logic circuits useful. The have a wide, uncritical operating voltage range of 5V to 15V. They are slow (about 1MHz maximum) but speed is not important if you just are using them to learn. Also since they are an old CMOS process they have a relatively thick gate oxide and thus are quite tolerant of static charges. (But you should still ground yourself before handling them.)

    One of the important things to remember, when using CMOS, is that all unused inputs must be connected to ground or V+, as appropriate for the input. A common error for newbies using CMOS is to leave them floating. Floating CMOS inputs have an undefined state which will give erratic results in a circuit. That holds true for used inputs also. You must always have the input connected to something that defines the state as either high or low. For example, if you use a SPST switch such as a push-button to generate a high state, then you must add a resistor to ground to generate the low state when the switch is open (it can be a large resistor since the CMOS input impedance is very high, similar to a small capacitor).

    Have fun!
     
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