# TTL NAND gate (multi-emitter transistor)?

#### abdulwahab.hajar

Joined Jun 14, 2016
93
Hey everyone, hope you are all doing well

I have this university project, where I need to build a binary adder which basically adds two 3-bit numbers, now I know the approach is to use half-adders and full-adders but I need to build these from their corresponding components.

when studying the way gates are designed, in order to take more than one input we use a multi-emitter transistor which makes sense. However, I can't seem to find a multi-emitter transistor to use in order to build my circuit, are they not made in To-92 packaging or some other packaging which can be fit on a breadboard?
If they are produced, could you perhaps suggest a multi-emitter BJT, or what they are termed because I can't find any online?

Thank you!

#### LesJones

Joined Jan 8, 2017
2,726
I don't think you will be able to obtain a multi emitter transistor. Are you sure that you are not allowed to build it using simpleTTL nand and nor gates ? It would be much easier than using transistors.

Les.

#### abdulwahab.hajar

Joined Jun 14, 2016
93
I don't think you will be able to obtain a multi emitter transistor. Are you sure that you are not allowed to build it using simpleTTL nand and nor gates ? It would be much easier than using transistors.

Les.
Yeah it's a digital electronics course, we have to use basic components such as transistors

#### abdulwahab.hajar

Joined Jun 14, 2016
93
You will have to build a multiple emitter circuit.

What you are looking for is a binary summer, not adder.

What's the difference between a summer and an adder?
The way I see it, I'll make 2 full adders and 1 half adder.
The half adder sums the last digits of the binary number.
The 1st full adder sums the middle digits of the binary numbers in addition to the carry out from half adder.
The 2nd full adder sums the first digits of the binary numbers, in addition to the carry out from the 1st full adder.

Do you think there is perhaps a better way?

#### Papabravo

Joined Feb 24, 2006
14,655
What's the difference between a summer and an adder?
The way I see it, I'll make 2 full adders and 1 half adder.
The half adder sums the last digits of the binary number.
The 1st full adder sums the middle digits of the binary numbers in addition to the carry out from half adder.
The 2nd full adder sums the first digits of the binary numbers, in addition to the carry out from the 1st full adder.

Do you think there is perhaps a better way?
Better according to what criteria? There are multiple ways to accomplish a task, but you need to specify a way to compare the solutions.

Checkout Mead and Conway for information on Propagate-Kill carry logic.

#### eetech00

Joined Jun 8, 2013
2,034
Hey everyone, hope you are all doing well

I have this university project, where I need to build a binary adder which basically adds two 3-bit numbers, now I know the approach is to use half-adders and full-adders but I need to build these from their corresponding components.

when studying the way gates are designed, in order to take more than one input we use a multi-emitter transistor which makes sense. However, I can't seem to find a multi-emitter transistor to use in order to build my circuit, are they not made in To-92 packaging or some other packaging which can be fit on a breadboard?
If they are produced, could you perhaps suggest a multi-emitter BJT, or what they are termed because I can't find any online?

Thank you!
This type of BJT configuration is used in integrated circuits.
But see below.

#### ebp

Joined Feb 8, 2018
2,332
The multi-emitter transistor was used in TTL. TTL is dead, or at best moribund, primarily because advances in CMOS have produced gates that are faster, consume vastly less static power and are smaller. TTL required resistors which use a lot of area on and IC die. It also had inherent asymmetries that result in asymmetric drive capability and hence propagation delays and slew rates.

The precursor, of sorts, to TTL was diode transistor logic (DTL). The "inherent" logic function of a single multi-emitter transistor is a negative NOR gate, which by DeMorgan's theorem is an AND gate - if any input is LOW, the output is LOW, only if all inputs are HIGH will the output be HIGH. This is easily implemented with diodes. There is nothing in terms of logic function to be gained by trying to use multiple transistors to emulate a multi-emitter transistor (which, as others have said, does not exist as a discrete device).

One place where difficulties can arise in making gates out of discrete components is that a change of logic function can mean a change in whether inputs and outputs are current sinking or current sourcing. TTL inputs are always current sourcing and output are always current sinking, so the work well together. A diode AND gate has current source inputs, while an OR gate has current sinking inputs. This isn't necessarily unworkable, but must be considered.

Actually building a multi-bit full adder with ripple carry using discrete devices is a big project. It's pretty complex just with IC gates. It is even bigger if you implement carry lookahead.

#### ebp

Joined Feb 8, 2018
2,332
This type of BJT configuration is used in integrated circuits.
But see below.
Your circuit is nothing like a multi-emitter transistor as used for gates. With a multi-emitter transistor the inputs are the emitters and the output is the collector. The base is tied to the positive rail through a resistor. It is more or less a common base circuit with multiple inputs via multiple emitters. Your circuit is just a pair of emitter followers with a common input via an unnecessary resistor.

#### ArakelTheDragon

Joined Nov 18, 2016
1,350
There is no difference between a summer and an adder, "summer" is the old term, which should be used at least according to my opinion.

#### eetech00

Joined Jun 8, 2013
2,034
Your circuit is nothing like a multi-emitter transistor as used for gates. With a multi-emitter transistor the inputs are the emitters and the output is the collector. The base is tied to the positive rail through a resistor. It is more or less a common base circuit with multiple inputs via multiple emitters. Your circuit is just a pair of emitter followers with a common input via an unnecessary resistor.
Yes...I know that...but I didn’t want to give the whole solution away since it is a university school project..

eT

#### ebp

Joined Feb 8, 2018
2,332
There is no difference between a summer and an adder, "summer" is the old term, which should be used at least according to my opinion.
Just stop!
In electronics a summing circuit is invariably an analog circuit and an adder is invariably digital.

#### Audioguru

Joined Dec 20, 2007
11,249
You cannot find multi-emitter transistors to make your own antique TTL gates then use even older RTL or DTL gate schematics shown in Google on Wikipedia. I used RTL gates in about 1965, DTL gates in about 1968, TTL gates in about 1972 and Cmos gates in about 1975 until today.

#### MrAl

Joined Jun 17, 2014
7,849
Hi,

That's my take too but i would first heavily consider the context.
Normally, a summer is a device used to sum two analog signals while an adder is a device used to sum two digital signals.
As i said though, be careful to consider the context because in the English language the act of summing is much the same as the act of adding.

#### ebp

Joined Feb 8, 2018
2,332
Struth!
With a digital adder you add the addend to the augend and get Ta Da! a sum. And a carry.

Of course it was abundantly clear form the original post that the TS is interested in digital adders.

Mouser has over 1000 LS283 4-bit adders with fast carry output IN STOCK! I wouldn'ta thunk it.

---
When the serpents were sent forth to multiply, the adders used logarithms.

#### abdulwahab.hajar

Joined Jun 14, 2016
93
Thank you all, I found a workaround using RTL indeed... although CMOS really is something worth considering!
I have found a solution nevertheless, is there a certain way to mark this thread as answered? or do I just leave it like this?

#### WBahn

Joined Mar 31, 2012
26,277
I have found a solution nevertheless, is there a certain way to mark this thread as answered? or do I just leave it like this?
Just leave it like it is; it will soon drop off the first page of threads and die a natural death. Occasionally someone will come along and add something worthwhile to a thread (and, yes, just as likely add something not worthwhile), so we leave them open unless there's a specific reason to close it.