Understanding BJT Operating Regions: Saturation and Cutoff | Lab Homework

[TheTopGun]

Homework Statement

Hey all, I'm midway through a lab on 'BJT for switching', we are suppose to choose values for RB and RC between a certain range so that it operates in saturation region. I have this circuit:
https://dl.dropboxusercontent.com/u/53341615/simulated%20circuit%20step%202.png

DC analysis shows that it is operating in the saturation region (meets the criteria, all good :) ).

When I toggle the switch to the AC signal and view the waveform on the oscilloscope, the transistor is in the saturation region when input is high 5v, however I am unsure what region it is in when the input is low.

Oscilloscope: https://dl.dropboxusercontent.com/u/53341615/Waveform%20view%20step%203.png

Is this now operating in the cutoff region, IE = 0A . I've done DC analyses at 0V and it does show IE to be 0A.

So effectively its switching between saturation and cutoff, every cycle of the AC, which would explain the 180 degree phase shift on the oscilloscope.

Am I understanding this correct ? Any feedback would be great!

Thanks for your time.

 

[NascentOxygen]

It sounds like you have it right.

When there is no base current, the transistor is in cut-off, and this means there is no voltage dropped across Rc.