I'm learning about transistors so I build the circuit in the image below and plotted the voltage at Vc (collector) as a function of base voltage, marked VBE. The transistor was a 2N3904. I had two power supplies, one supplying the 5v rail and a second that I could adjust from 0 volts upwards (they had a common ground). I noticed that I couldn't get the base voltage to go much above 0.8 volts. If I pushed up the supply voltage to 5 volts I could get the base voltage to about 0.85 volts. I am sure there is a simple explanation but why does the voltage at the base max out at about 0.8v no matter what voltage I apply?
I know that the transistor is saturated by the time I get to 0.8 volts.
Update: I've also now plotted the base voltage as a function of the applied voltage via the 1K resistor.
Having now read the comments and thought more about this, the explanation seems straight forward. We see from the second graph that at a VBE around 0.7 v the curve starts to flatten out, this is of course when the transistor starts to open up and in doing so the BE resistance drops thus holding the VBE voltage constant after that. As others have pointed out this can't go on forever but will eventually start to rise again once the resistance stops dropping.