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ttl gates


TTL is particularly well suited to integrated circuits because the inputs of a gate may all be integrated into a single base region to form a multiple-emitter transistor. Such a highly customized part might increase the cost of a circuit where each transistor is in a separate package, but, by combining several small on-chip components into one larger device, it conversely reduces the cost of implementation on an IC. As with all bipolar logic, a small current must be drawn from a TTL input to ensure proper logic levels. The total current drawn must be within the capacities of the preceding stage, which limits the number of nodes that can be connected (the fanout).

TTL contrasts with the preceding resistor–transistor logic (RTL) and diode–transistor logic (DTL) generations by using transistors not only to amplify the output but also to isolate the inputs. The p-n junction of a diode has considerable capacitance, so changing the logic level of an input connected to a diode, as in DTL, requires considerable time and energy.

As shown in the top schematic at right, the fundamental concept of TTL is to isolate the inputs by using a common-base connection, and amplify the function using a common emitter connection. Note that the base of the output transistor is driven high only by the forward-biased base–collector junction of the input transistor. The second schematic adds to this a “totem-pole output”. When V2 is off (output equals 1), the resistors turn V3 on and V4 off, resulting in a stronger 1 output. When V2 is on, it activates V4, driving 0 to the output. The diode forces the emitter of V3 to ~0.7 V, while V4 base–emitter junction and V2 collector–emitter junction pull its base to a voltage ~0.7, turning it off. By removing pull-up and pull-down resistors from the output stage, this allows the strength of the gate to be increased without proportionally affecting power consumption.

All standardized common TTL circuits operate with a 5-volt power supply. A TTL input signal is defined as “low” when between 0 V and 0.8 V with respect to the ground terminal, and “high” when between 2.2 V and 5 V (precise logic levels vary slightly between sub-types). TTL outputs are typically restricted to narrower limits of between 0 V and 0.4 V for a “low” and between 2.6 V and 5 V for a “high”, providing 0.4V of noise immunity. Standardization of the TTL levels was so ubiquitous that complex circuit boards often contained TTL chips made by many different manufacturers selected for availability and cost, compatibility being assured; two circuit board units off the same assembly line on different successive days or weeks might have a different mix of brands of chips in the same positions on the board; repair was possible with chips manufactured years (sometimes over a decade) later than original components. Within usefully broad limits, logic gates could be treated as ideal Boolean devices without concern for electrical limitations. TTL NOR and OR gates : LOGIC GATES TTL NOR and OR gates. Let’s examine the following TTL circuit and analyze its operation: Transistors Q1 and Q2 are both arranged in the same manner that we’ve seen … … we would have to invert the output logic level with another transistor stage, just like we did with the NAND-to-AND gate example: …
http://www.allaboutcircuits.com/vol_4/chpt_3/6.html

TTL NAND and AND gates : LOGIC GATES In the earlier section on NAND gates, this type of gate was created by taking an AND gate and increasing its complexity by adding an inverter (NOT gate) to the output. However, when we examine this circuit, we see that the NAND function is actually the simplest, most natural mode of operation … REVIEW: A TTL NAND gate can be made by taking a TTL inverter circuit and adding another input. An AND gate may be created by adding an inverter …
http://www.allaboutcircuits.com/vol_4/chpt_3/5.html

TTL stands for Transistor Transistor Logic, which signifies that two transistors are used to drive each output of each chip, one for pulling the output down … For example, consider the SN74LS00 chip; this has 4 nand gates. The inputs of the first nand gate are p1 and p2 , and its output is p3 . The inputs of the second nand gate are p4 and p5 , and its output is p6 . The remaining nand gates are connected to p8 through p13 , and pins 7 and 14 are …
http://www.cs.uiowa.edu/~jones/logicsim/man/node5.html

Two-Input TTL NOR Gate What we can do, however, is use the same technique that we tried in both RTL and DTL, to form a NOR gate. This requires separate inverter transistors just as it did with … The figure to the right shows the schematic diagram of a TTL NOR gate. As you may recall from earlier experiments, it is substantially the same as the RTL and DTL NOR gates we have already explored. The only difference is that this time we are using the TTL input circuitry. …
http://www.play-hookey.com/digital/experiments/ttl_nor2.html

[PDF] TTL logic gates This worksheet and all related files are licensed … File Format: PDF/Adobe Acrobat – Quick View Totem-pole TTL gates usually differ greatly in their maximum source current versus …. provided by the TTL gates, the LED will energize if and only if all …
http://www.ibiblio.org/kuphaldt/socratic/output/ttl.pdf

Digital Logic Gates Electronics Tutorial about TTL and CMOS Digital Logic Gates including Moores Law, AND, OR, NAND, NOR. Standard commercially available Digital Logic Gates are available in two basic forms, TTL which stands for Transistor-Transistor Logic such as the 7400 series … As with the TTL gates, improvements in the circuit design with regards to speed and low power consumption has resulted in the standard 4000 CMOS “CD” family of IC’s being developed. As …
http://www.electronics-tutorials.ws/logic/logic_1.html

Fast CMOS/TTL gates I have been using CMOS gates earlier on. However, I need to use a NAND gate to switch a signal at 30 MHz which 4000 series is unable to do. I have tried normal 7400 TTL which is better than the CMOS but still not very good at 30MHz. Can anyone suggest a fast NAND gate which can do the job? Top … There are also some fast TTL parts such as the 74F FAST series plus the old 74LS TTL family might just cope with 30MHz. …
http://www.theiet.org/forums/forum/messageview.cfm?catid.

Transistor-Transistor Logic (TTL) Transistor -Transistor Logic, or TTL, refers to the technology for designing and fabricating digital integrated circuits that employ logic gates consisting … In the TTL NAND gate of Figure 1, applying a logic ‘1’ input voltage to both emitter inputs of T1 reverse-biases both base-emitter junctions, causing …
http://www.siliconfareast.com/ttl.htm





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