Transistor XOR gate

[gjoo]

I am having trouble understanding this circuit. I do not understand how the bases of Q3 and Q4 receive voltage. Also, how is the output of the 4 transistor configuration high or 1 when both inputs are 1 or 0? Appreciate any help on this circuit.

 

[AnalogKid]

At the start, S1 and S2 are open, Q1-Q4 are off, the base of Q5 is floating so it's off, there is no base current path through Q5 for Q6 so it's off, and the LED is off. For XOR inputs = 0,0, the output is 0.

Note that if there were any Q5 emitter current, it has a two paths to GND, through Q1 and Q2 bases to R1 and R2 to GND.

When S1 closes, Q1 stays off. The Q3 emitter is pulled up above its base, turning it on. S1 current goes around Q4 and into R3, turning on Q5. This turns on Q6 and the LED. The Q76 base current goes through Q5, coming out the emitter, entering the Q2 emitter, going through the Q2 base-emitter junction, coming out the base, and going through R2 to GND.

Follow a similar path for S1 open and S2 closed.

When both switches are closed, both the Q1 and Q2 bases are hard tied to Vcc, turning them off, Now there is no path to GND for the Q6 base current through Q5, So when both switches are closed (XOR inputs equal 1,1), the output is 0.

ak

 

[crutschow]

Do you need a XOR circuit or do you just want to analyze this particular one?

 

[gjoo]

Do you need a XOR circuit or do you just want to analyze this particular one?

I want to analyze this circuit. I'm a little confused on how it works.

 

[gjoo]

At the start, S1 and S2 are open, Q1-Q4 are off, the base of Q5 is floating so it's off, there is no base current path through Q5 for Q6 so it's off, and the LED is off. For XOR inputs = 0,0, the output is 0.

Note that if there were any Q5 emitter current, it has a two paths to GND, through Q1 and Q2 bases to R1 and R2 to GND.

When S1 closes, Q1 stays off. The Q3 emitter is pulled up above its base, turning it on. S1 current goes around Q4 and into R3, turning on Q5. This turns on Q6 and the LED. The Q76 base current goes through Q5, coming out the emitter, entering the Q2 emitter, going through the Q2 base-emitter junction, coming out the base, and going through R2 to GND.

Follow a similar path for S1 open and S2 closed.

When both switches are closed, both the Q1 and Q2 bases are hard tied to Vcc, turning them off, Now there is no path to GND for the Q6 base current through Q5, So when both switches are closed (XOR inputs equal 1,1), the output is 0.

ak

At the start, S1 and S2 are open, Q1-Q4 are off, the base of Q5 is floating so it's off, there is no base current path through Q5 for Q6 so it's off, and the LED is off. For XOR inputs = 0,0, the output is 0.

Note that if there were any Q5 emitter current, it has a two paths to GND, through Q1 and Q2 bases to R1 and R2 to GND.

When S1 closes, Q1 stays off. The Q3 emitter is pulled up above its base, turning it on. S1 current goes around Q4 and into R3, turning on Q5. This turns on Q6 and the LED. The Q76 base current goes through Q5, coming out the emitter, entering the Q2 emitter, going through the Q2 base-emitter junction, coming out the base, and going through R2 to GND.

Follow a similar path for S1 open and S2 closed.

When both switches are closed, both the Q1 and Q2 bases are hard tied to Vcc, turning them off, Now there is no path to GND for the Q6 base current through Q5, So when both switches are closed (XOR inputs equal 1,1), the output is 0.

ak

Thank you for the thorough response. I was mixing up something when I was looking at the circuit, but after reading your response a couple of times , I think I understand it now.