I've been trying to get my head around transistors again. I watched this video:
In that he said that whilst there was a 0.7V drop between base and emitter, because of the PN junction, there was "almost no" drop between collector and emitter because the PNP junction "cancelled it out". In fact he measured around a 0.1V drop between collector and emitter due to manufacturing issues.
Meanwhile, on this page (Transistors 101):
http://sunburst.usd.edu/~schieber/psyc770/transistors101.html
He says that:
2.) If the transistor is on, the Collector voltage is 1.6 volts higher than the Emitter voltage.
Both are using a similar circuit.
So they can't be both right, eh?
So I set up a test:
The LED had a forward voltage of 1.71 V.
The transistor had a Vbe of 0.872
The theoretical current through R1 would be:
(5 - 0.872) / 4700 = 0.000878 (0.878 mA)
Measured was 0.857 mA, so that is reasonably close.
The theoretical current through R2 would be:
(12 - 1.71) / 560 = 0.018375 (18.38 mA)
Measured was 18.57 mA, so that is pretty close.
I measured 12.06 V at the +12 V point, and 10.28 at point C, giving a voltage drop of 1.78 V in circuit.
I measured 60 mV at point D, so it would appear that the collector/emitter voltage drop was closer to 0.1V than 1.6V.
I measured 19.20 mA at point E, so that would be roughly the collector/emitter current plus the base/emitter current.
Does all that sound right? And does that make this quote incorrect?
If the transistor is on, the Collector voltage is 1.6 volts higher than the Emitter voltage.