Q:

Lower corner frequency in an emitter follower (Common collector)

I have a doubt about the lower corner frequency of an emitter follower.
I don't rember where, but I have read that it is given by:
\$\omega_L=\frac{1}{R_BC_1}\$ where \$R_B\$ is given by the parallel of the two biasing resistances R1 and R2, and \$C_1\$ is the capacitance before the base of the BJT. This expression should be derivated considering that the input part of the circuit is similar to a high pass filter. 

Could you tell me if I correctly rember this thing?

A:

For most transistor circuits the parallel combination of R1 and R2 act as a single resistor that limits the voltage on the base when frequency is low. This is because the impedance of C1 increases with lowering input frequencies. 
When the impedance of C1 matches the parallel resistance of R1 and R2 this is known as the cut off frequency or 3 dB point.

