Originally Posted by bluejets
I have seen this output transistor configuration before to try to get output inversion, and I think it could be a problem.
Connected as common emitter is fine, i.e. with R12 zero and R11 as 1K.
However, when you make R11 zero and R12 as 1K, it becomes a common collector configuration. In this mode, as the base starts to conduct, it causes the emitter voltage to rise and thus , turn off the transistor again. I think you may have problems getting it to saturate under this mode.
If R11 is 1k and R12 is 0 then the output is inverse to the input, sort of like an amplifier circuit. If R11 is 0 and R12 is 1k then this is an emitter follower and the output signal is in step with the input signal minus the biasing voltage. I'm being very simple about this because you can get into bypass caps and such but, thats for another time.