In this configuration of 2 NPNs, how is V_OUT following V_IN? Ignoring recombination current:
- Assuming conventional current, current flows from Node_A to Q1 collector and Q2 base, but is somehow replenished by a voltage source that we can't see.
- Assuming electron flow, current from Q1 collector and Q2 base are drained in Node_A which is a constant voltage source or ground that we can't see.
Now if we were to assume recombination current, we have an exception. But even after reading through some Recombination Current articles, I still can't see why this would be behaving like this.
Can somebody give an explanation?
I'm sure this isn't Reverse Mode, as V_IN causes Q1 to be forward biased and the emitter of Q2 is pulled to ground.
Dave answered that Q1 is acting like a dual diode. So I thought of testing if it was with this simplification:
I've actually had to ramp up the resistor to ground to 100k from the previous 10k to obtain a full swing. The collector still follows the emitter. Perhaps there's resistance at the emitter, so there's feedback in the collector? I checked the Gummel-Poon model and it seems to agree with this assertion. Just adding this little bit.
Something that I want to correct is that the the base-emitter of Q1 forms a closed loop of voltage source V2. The base of Q2 (on the first figure) and 100k R2 (of the second) only taps onto this. So it's following.