A low pass filter can be defined as low frequencies that exist above a cutoff frequency that will be permitted through a filter. A high-pass filter operates below the cutoff frequency permitting higher frequencies to pass through the filter. Low-pass filters are used to smooth out an image, and they can help remove the aliasing effect. A high pass filter can be used to sharpen an image and work to remove noise. The BJT amplifier can act as a filter based on the quantity being amplified by the circuit, so it could be power, voltage, or current being amplified.
An internal capacitor works to form a low-pass filter that has external resistors. The external capacitance with a low frequency has a high impedance making the gain low, and when high, there is a low impedance and high gain. When it is a high frequency, the coupling with the bypass capacitor is a short circuit and cannot affect the response of the amplifier.
Having two cutoff frequencies will cause the higher to decrease and the lower frequencies to increase.
When multi-stage amplifiers are determined, the bandwidth can be determined by the more stages there are, the steeper the high and low pass filters will be. If and when the stages are capacitively coupled, then each capacitor will roll off the chosen frequency.