The NAND gate of it is a quad two input type. In plain ways the circuit of this type of TTL operates as follows. The Q1 showed in the figure is a two emitter NPN transistor.
History[ edit ] A real-time clock built of TTL chips around TTL was invented in by James L. Buie of TRWwhich declared it, "particularly suited to the Transistor transistor logic devices ttl essay developing integrated circuit design technology.
Not only did others make compatible TTL parts, but compatible parts were made using many other circuit technologies as well. The most recently introduced family 74Fxx is still sold today[ when? Typically, TTL chips integrate no more than a few hundred transistors each.
Functions within a single package generally range from a few logic gates to a microprocessor bit-slice. TTL also became important because its low cost made digital techniques economically practical for tasks previously done by analog methods. Most computers used TTL-compatible " glue logic " between larger chips well into the s.
Until the advent of programmable logicdiscrete bipolar logic was used to prototype and emulate microarchitectures under development. TTL inputs are the emitters of bipolar transistors. In the case of NAND inputs, the inputs are the emitters of multiple-emitter transistorsfunctionally equivalent to multiple transistors where the bases and collectors are tied together.
Inputs both logical ones.
When all the inputs are held at high voltage, the base—emitter junctions of the multiple-emitter transistor are reverse-biased. This is because the transistor is in reverse-active mode.
An approximately constant current flows from the positive rail, through the resistor and into the base of the multiple emitter transistor. An input logical zero. Note that the base—collector junction of the multiple-emitter transistor and the base—emitter junction of the output transistor are in series between the bottom of the resistor and ground.
If one input voltage becomes zero, the corresponding base—emitter junction of the multiple-emitter transistor is in parallel with these two junctions.
A phenomenon called current steering means that when two voltage-stable elements with different threshold voltages are connected in parallel, the current flows through the path with the smaller threshold voltage.
That is, current flows out of this input and into the zero low voltage source. As a result, no current flows through the base of the output transistor, causing it to stop conducting and the output voltage becomes high logical one.
During the transition the input transistor is briefly in its active region; so it draws a large current away from the base of the output transistor and thus quickly discharges its base. It limits the number of inputs that can be connected the fanout.
Some advantage of the simple output stage is the high voltage level up to VCC of the output logical "1" when the output is not loaded.
A common variation omits the collector resistor of the output transistor, making an open-collector output. This allows the designer to fabricate logic by connecting the open-collector outputs of several logic gates together and providing a single external pull-up resistor.
If any of the logic gates becomes logic low transistor conductingthe combined output will be low. Examples of this type of gate are the  and series.
Open-collector outputs of some gates have a higher maximum voltage, such as 15V for the useful when driving other than TTL loads. TTL with a "totem-pole" output stage[ edit ] Standard TTL NAND with a "totem-pole" output stage, one of four in To solve the problem with the high output resistance of the simple output stage the second schematic adds to this a "totem-pole" " push—pull " output.
It consists of the two n-p-n transistors V3 and V4, the "lifting" diode V5 and the current-limiting resistor R3 see the figure on the right.
It is driven by applying the same current steering idea as above. When V2 is "off", V4 is "off" as well and V3 operates in active region as a voltage follower producing high output voltage logical "1". When V2 is "on", it activates V4, driving low voltage logical "0" to the output.
Again there is a current-steering effect: The second series combination has the higher threshold voltage, so no current flows through it, i. V3 base current is deprived. Transistor V3 turns "off" and it does not impact on the output.TTL Logic Levels.
A majority of systems we use rely on 5 V TTL Logic Levels. TTL is an acronym for Transistor-Transistor Logic. It relies on circuits built from bipolar transistors to achieve switching and maintain logic states.
Transistors are basically fancy-speak for electrically controlled switches. TTL(transistor-transistor logic), a logic family to realize logic gates, can be open collector, totem pole or tri state.
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