I found an older thread of yours, in which RFDx offered a very simple
XNOR gate. Later Zero999
added amplifiers on its inputs. I assume this is where you got the idea from.
So, step by step. First, let’s look at RFDx’s version. I’ve redrawn it in a different layout to make the behavior clearer. I also used switches (to +5V and GND) as inputs, because this way the behavior is clearer:
Interactive version in Falstad’s simulator (BJT XNOR gate)A single half of that circuit is a
common emitter configuration, which under normal circumstances would act as a inverter. There are two such halves, one for each input. Now the trick: they aren’t attached to GND, but the opposite input. This way if one input is high and the other is low, it’s a normal inverter. But if the other input is also high, V
BE is 0 V and nothing happens. You may remove one of the transistors to just study the behavior of the other one.
Zero999 added amplfiers to the input. They are inverters, but in this particular case it doesn’t affect the result of the logic operation:
![Resistance in a 1 bit Adder/Subtractor](https://www.eevblog.com/forum/index.php?action=dlattach;topic=432559.0;attach=2296407;image)
Thanks to this amplification the gate isn’t dependent on how much current inputs can source and sink. Since the inverters already have 10 kΩ impedance to +5 V, the resistors at the bases of central transistors can be removed.
Now, you did two things. First you added a resistor at the emitter of one of the input amplifiers. Then took output from that resistor. Effectively this forms a
voltage follower, except that it’s choked by all the circuitry above. This is not a matter of any resistances, but the signal being the value of otherwise “sewage” current.
![Smiley :)](https://www.eevblog.com/forum/Smileys/default/smiley.gif)
Hope that helps.
An important note, though. RFDx’s design is powered from 5 V. This voltage is crucial here, because half of the time base-emitter junctions of these transistor are reverse biased. And those have very low breakdown voltage: popular ones have maximum of -6 V.