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TTL Logic Gates
25 questions By Tony R. Kuphaldt
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Question 13 of 25
Totem-pole TTL gates usually differ greatly in their maximum source current versus maximum sink current (IOH versus IOL). Identify which current rating is usually greater, and also explain why this is.
Reveal answerIOL is typically much greater than IOH for a TTL gate with totem-pole output circuitry. The reason for this should be obvious from inspection of the internal circuitry.
Notes:It should be noted that totem-pole TTL gates can actually source far more current than what is advertised without sustaining damage. The severe limitation on sourcing current is more a function of staying within the permitted output voltage margins for TTL than it is a function of chip heating. Thus, you may generally use a totem-pole TTL gate to source 20 mA to an LED without harm, though the “high” state output voltage (when the LED is lit) will be significantly below the acceptable threshold for a TTL gate input.
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Question 14 of 25
In high-speed digital circuits, a very important logic gate parameter is propagation delay: the delay time between a change-of-state on a gate’s input and the corresponding change-of-state on that gate’s output. Consult a manufacturer’s datasheet for any TTL logic gate and report the typical propagation delay times published there.
Also, explain what causes propagation delay in logic gates. Why isn’t the change in output state instantaneous when an input changes states?
Reveal answerI’ll leave the research of specific propagation time delay figures up to you! The reason propagation delay exists is because transistors cannot turn on and turn off instantaneously. In bipolar transistors, this is due to the time required to establish minority carrier flow within the base layer of the transistor (to turn it on), and to “sweep out” those minority charge carriers out of the base (to turn it off).
Follow-up questions: What difference is there between high-to-low output transitions versus low-to-high output transitions for the gate you researched? Which transition is faster?
Notes:I purposely omitted answers for this question, not only because I want students to do the research on their own, but also because it makes it more interesting when students consult different datasheets and derive different answers (for different logic “families”)!
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Question 15 of 25
Logic gates are limited in the number of gate inputs which one output can reliably drive. This limit is referred to as fan-out:
Explain why this limit exists. What is it about the construction of TTL logic gates that inherently limits the number of TTL inputs that any one TTL output can drive? What might happen if this limit is exceeded?
Locate a datasheet for a TTL gate and research its fan-out limit. Note: this number will vary with the particular type of TTL referenced (L, LS, H, AS, ALS, etc.).
Reveal answerA fan-out limit for TTL exists because TTL outputs have to sink current from TTL inputs in the “low” state, and their current-sinking ability is limited by the output transistor in the driving gate. If this fan-out limit is exceeded, the voltage level at the driven gate inputs may rise above the lower compliance limit.
Notes:For the relatively simple digital circuits that beginning students build, fan-out is rarely a problem. More likely is that students will try to drive a load that is too “heavy,” causing the same voltage level problem.
