I have the following circuit 
[![enter image description here](https://i.stack.imgur.com/uVPsg.png)](https://i.stack.imgur.com/uVPsg.png)

I have to determine the zones where the BJT transistor is working according to the input voltage (which as you can see is a triangular wave with 4V amplitude). Now here's the thing: I know that if ui is negative that the BJT is on the open-circuit zone. ´
The problem comes when we have ui positive. We know for sure that emmiter-base junction is direct biased. The collector-base junction can however be direct or inverse biased. Do you have any idea how can I determine the values of ui?