Hi,
I've been trying to figure out this circuit and I cannot understand how Q3 and Q4 receive base voltage and how the output of those parallel transistors is high or positive when both inputs are either both 0 or both 1.

 

They don't "receive" base voltage. They are diode-connected transistors. The base and the collector will always be at the same voltage - about 0.7 volts less than the emitter.

 

That makes sense now. Could you explain a little more on how this circuit output is high when negative voltage is applied. Thanks

 

That makes sense now. Could you explain a little more on how this circuit output is high when negative voltage is applied. Thanks

There is a single 6V power supply, so all voltages will be between GND and +6V. there are no negative voltage that can be "applied.
The inputs S1 and S2 can only be high or floating. When they are floating no current flows in R1 or R2. When no current flows through a resistor the voltage is essentially the same on both ends and the collector base junctions of Q1 and Q2 are effectively at ground (negative side of the battery)

 

That cleared up a lot. Thanks for all the information , it helped a lot.

 

Home work ?

 

Good. More questions always welcome.

 

Home work ?

Possibly a bit out of date for homework. I haven't seen a +6V battery used with transistor circuits in a very long time.

 

It is for a hobbyist club.

 

It is for a hobbyist club.

fantastic,
I did not realise such clubs still existed,
as a "kid" my school had such clubs, crystal radios were the normal, the wizards made single transistor radios....

Can I ask where in the world you are,

 

Brooklyn, NY