Full Adder in Ltspice

Thread Starter

omkarshinde680

Joined Oct 7, 2020
36

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dl324

Joined Mar 30, 2015
12,826
Welcome to AAC!
clipimage.jpg
The logic seems wrong to me. If all three inputs are LOW, the output is HIGH (at least it would be if the 68k resistor wasn't there); that's wrong.

I didn't read the Instructable article. It's usually not worth the bother.
 

Alec_t

Joined Sep 17, 2013
12,009
Dl324 has told you one way it is wrong.
To implement a full adder I think you'll need an AND function, an OR function and an XOR function, but the circuit posted doesn't seem to provide the latter function. Try googling "Diode logic XOR".
 
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Thread Starter

omkarshinde680

Joined Oct 7, 2020
36
hi o68,
One obvious problem in your LTSpice circuit , is that the PNP is upside down.!

E
I dont think an orientation could make any difference to the circuit as long as its emitter collector and base terminal is connected to the same components mentioned just as described in the instructables
 

Thread Starter

omkarshinde680

Joined Oct 7, 2020
36
hi 680,
Look at this version of the circuit, I have applied a stepped input on A,B,C inputs, just to show the way the analog is summed on the output.
It may help you debug the original circuit.
E
the asc file u sent me i tried running it but i guess u used a different diode which isnt present in my std library. can u pls send me that diode file so that i can import it into my ltspice or which other diode shall i use from the std library which would closely resemble to the results?
 

Thread Starter

omkarshinde680

Joined Oct 7, 2020
36
hi 680,
Look at this version of the circuit, I have applied a stepped input on A,B,C inputs, just to show the way the analog is summed on the output.
It may help you debug the original circuit.
E
i have attached the image of how the desired output of a full adder looks like. I tried using different transistor and i got the output just like yours however why am i not getting it like this? any idea?
 

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ericgibbs

Joined Jan 29, 2010
12,597
hi,
If you don't post your asc file I cannot determine where the plot Nodes are.?
Also use Labels ie: A,B etc.
E

BTW:
When you ran my asc file, did you get the same results as me.??
 

Thread Starter

omkarshinde680

Joined Oct 7, 2020
36
I read enough of the article to see that it's not intended to be a digital circuit. Do you have a practical application for an adder that doesn't give a binary output?
i am required to build a full adder using diodes transistors as the major component in my college project thats why i wanted to get a digital output. I didnt get your last question statement
 

Thread Starter

omkarshinde680

Joined Oct 7, 2020
36
You have an analogue output circuit, but the posted do.png image is for a digital output circuit.
Instead of using that 'destructible' :) circuit you could build a full adder using the LTspice standard digital gates.
actually yes but i am required to build full adder using diodes and transistor as the major components
 

dl324

Joined Mar 30, 2015
12,826
i am required to build a full adder using diodes transistors as the major component in my college project thats why i wanted to get a digital output. I didnt get your last question statement
If you want a digital output, you're going to need more than one transistor.

A half adder takes an XOR and an AND gate. A full adder can be implemented with two half adders and an OR gate.
 
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