The lower cut-off frequency (ωL) is the frequency at which the output voltage drops by 3 dB from its mid-frequency value. It is given by:
ωL = 1/C_{in} * R_{in}
Where:
C_{in} is the input capacitance of the transistor
R_{in} is the input resistance of the transistor
- Upper cut-off frequency (ωH):
The upper cut-off frequency (ωH) is the frequency at which the output voltage drops by 3 dB from its mid-frequency value. It is given by:
ωH = β/C_{out} * R_{out}
Where:
β is the forward current gain of the transistor
C_{out} is the output capacitance of the transistor
R_{out} is the output resistance of the transistor
The frequency response of a common emitter amplifier can be plotted on a Bode plot, which is a graph of the output voltage (in dB) versus frequency (in Hz). The Bode plot of a common emitter amplifier will have a slope of -20 dB/decade below the lower cut-off frequency and a slope of -6 dB/decade above the upper cut-off frequency.
The frequency response of a common emitter amplifier can be used to design amplifiers with specific bandwidth requirements. For example, if an amplifier is required to amplify a signal with a bandwidth of 10 Hz to 10 kHz, then the lower cut-off frequency of the amplifier should be less than 10 Hz and the upper cut-off frequency should be greater than 10 kHz.