Darlington Pair's Current Direction

[BlackMelon]

Hello,

I do a simulation on the attached schematic. The input is 12-0V square wave with 50% duty cycle at f=8kHz. I want to know the Q1's collector current waveform, so I connect R3 = 1 ohm, and measure the voltage across it. As you can see, when the input is 12V, V+ - V- = 450mV. This means there is a current from the left pin of R3 to the right pin.

My question:
When Vin =12V, does this mean the current of Q1 flow from emitter to collector? If so, why does the NPN transistor carry such a current?

Thank You
BlackMelon

 

[BobK]

No, it is from the base to the collector. Your 10Ω base resistor is ridiculously low, and this circuit is not operating the way a darlington normally would, it is operating like 2 diodes. There is plenty of current to saturate Q2, so it's collector will be at about 0.2V. This make a current path of 12V signal -> 10Ω resistor -> base of Q1 -> collector of Q1 -> 1Ω resistor -> collector of Q2.

Try using a 1MΩ resistor as the base resistor and you will see how a darlington normally operates.

Bob