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How are these schematics different from the traditional BJT logic gates that we learn in high school? Schematics:AND GATE and OR GATE.

Russell McMahon
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Nilanjan
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    We don't know what "traditional BJT logic gates" you learned in high school. – Ignacio Vazquez-Abrams Jan 01 '15 at 10:30
  • He's probably used to transistors with only one emitter... as TI make the transistors themselves they can play tricks you or I can't with our reel of BC108s... For the Schottky transistors you can add your own Schottky diode from B to C. see http://electronics.stackexchange.com/questions/55073/schottky-transistor-not-sure-i-understand-it/55090#55090 –  Jan 01 '15 at 10:37
  • Externally & logically, no difference. Internally, as Brian says. The Schottky gate is essentially a DTL gate internally masquerading as a TTL gate overall. – Russell McMahon Jan 01 '15 at 12:51
  • @Russell: I don't accept that the Schottky is DTL in the normal sense : the Schottky diode is not part of the logic (remove it and the device still functions : just slows down as the transistors saturate). Perhaps yoru understanding is different? –  Jan 01 '15 at 13:22
  • @Rusell and Brian: before you turn this into an essay on the Schottky gate, please beware that OP question is extremely vague. I personally have no idea what he wants to hear. Also only the LS and S versions of those chips use Schottky gates. And he hasn't put those in the title, but only the plain BJT ones. He might be bewildered by the multiple stages or who knows what else... like the multiple emitter transistor on the input, etc. – Fizz Jan 01 '15 at 16:42
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    @BrianDrummond - My bad - I meant LSTTL and said STTL in error. See diagram on page two of datasheet or in example which has magically appeared in the question (and which might instead have appeared in an answer if it were but possible. – Russell McMahon Jan 01 '15 at 23:26
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    This is a good example of a question whose progress has been impeded by being on hold. A reasonable question but with low detail. OP is not a one question hit-and-run poster - they have been around for a while and their English is OK. It's just a matter of getting more detail. This CAN be done in comments but it's very painful compared to starting an answer (or several) which grow with the information base. This is how MANY questions develop. I would rather that original was complete but most aren't. Singling out SOME for extra beating & difficulty makes no sense. – Russell McMahon Jan 01 '15 at 23:34
  • A multiple emitter transistor as seen in eg the TTL and STTL gates in http://i.stack.imgur.com/tVRGw.jpg can be approximate by taking multiple 3 terminal transistors and tying all collectors and all bases together and using the emitters as seen in these examples. (A high value pullup resistor at each input would pull them properly high - just as happens with TTL gates. – Russell McMahon Jan 01 '15 at 23:38
  • @Nilanjan When you say, "the traditional BJT logic gates that we learned in high school", can you post the schematics of what you are referring to? Its a little hard to make comparisons when we don't know what we are comparing to. Perhaps you studied either RTL or DTL? Because the circuit for the AND gate in the datasheet you linked to is pretty standard, with a NAND gate followed by an inverter followed by a totem-pole output. – tcrosley Jan 02 '15 at 03:26
  • The circuit diagram i am reffering to is the TTL standard, where an AND gate had two transistors in series. I myself dont know which answer i wanna hear, its just that i am really confused. All i know that these "standard" logic corcuits in those chips are performing the same logical function as the old DTL or RTL but their electronic characteristics are different. So i wanna understand these electronic differences. – Nilanjan Jan 03 '15 at 08:01

1 Answers1

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How are these schematics different from the traditional BJT logic gates that we learn in high school?

More detail will help us answer better but, typical BJT based gates probably use "Diode-Transistor Logic" as used in early DTL gates. In this system multiple diodes are used as inputs to allow input signals to influence the BJT without interacting with the other inputs.

TTL and later gates tend to use input transistors with multiple emitters - one per input. However, an example of a DTL gate lurking inside a TTL type "LS TTL" gate can be seen in the middle diagram below. While the left hand TTL example and the right hand STTL (Schottky TTL) use multiple emitters as inputs, the LSTTL (low power Schottky TTL) uses 2 diodes to the base of an internal BJT.

See data sheet, page 2 for larger version.

The LSTTL gate is essentially a "DTL gate" with a single transistor at the input and two diodes and a resistor used to perform the logic function.

The TTL and STTL gates use multiple emitters on the input transistor to achieve multiple inputs. None of the BJT's available in high-school had multiple emitters. The dioes here serve the same function.

enter image description here

You can get an approximation to an N emitter transistor by using N BJTs, joining all the N bases, joining all the N collectors and using the N emitters in the same way as the multiple emitters on the multiple-emitter transistor.

Russell McMahon
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  • Interesting! I wonder why the LSTTL version diverges from the pattern used in the others? –  Jan 02 '15 at 11:52
  • @BrianDrummond - Yes. Interesting. Same for [7400](http://www.ti.com/lit/ds/symlink/sn74ls00.pdf), and for [7403](http://www.ti.com/lit/ds/symlink/sn74ls03.pdf), major differences between the 3 for [7402](http://www.ti.com/lit/ds/symlink/sn74ls02.pdf), the [7404](http://www.ti.com/lit/ds/symlink/sn74ls04.pdf) is about unbelievable, the [74LS14](http://www.ti.com/lit/ds/symlink/sn74ls14.pdf) looks like Escher designed it. – Russell McMahon Jan 02 '15 at 14:10