When you have a network of resistors and capacitors, how do you choose the resistors and their corresponding capacitors to calculate the time constants? For example, in a simple case like this:

How would you calculate the time constant (-3dB point) at the base of the transistor? Is it using just C1 and R2 or C1, R1, and R2? What is the rigorous method for determining this when faced with more complicated arrangements?

Thanks

 

Try to find AC equivalent resistance seen from capacitors terminals. For example for C2 we have Rc + RL (not shown in the diagram).
For C1 we have (R1||R2||hie) + Rsource.