High-pass filters are circuits that allow relatively high frequency signals to be transmitted through the input to the output while attenuating relatively low frequency signals. Conversely, low-pass filters are circuits that allow relatively low frequency signals to be transmitted through the input to the output while attenuating relatively high frequency signals. Common-emitter BJTs have both characteristics of low pass and high pass filters.
Coupling capacitors and bypass capacitors (external capacitors) do affect input and output characteristics of amplifier circuits. For example, both voltage gain and phase shift of amplifier circuits can be affected by these capacitors. We can see from the equation for capacitive reactance Xc = 1/(2 pi * f * c) that frequency is inversely proportional to capacitive reactance. Therefore, low frequencies cause high capacitive reactance values and vice versa. High capacitive reactance cause less current to flow and therefore reduce voltage gain (i.e. the voltage gain of a common- collector amplifier). In the common-collector amplifier there is an ideal phase shift of 180°, which is a direct result of the coupling capacitors in the circuit. Also, the bypass capacitor in a CE configuration blocks DC but allows AC to pass and thereby further stabilizing the circuit. Internal transistor capacitance has an adverse effect as input signal frequencies increase causing less than ideal characteristics such as phase shift and reduction in voltage gain.
The bandwidth is the range of frequencies in which an amplifier is designed to operate. This range falls between the dominant upper and lower critical frequencies. The following equation expresses this bandwidth:
- Having two cut off frequencies that are the same in an amplifier causes different outcomes depending on whether or not the critical frequencies are lower or upper critical frequencies. If they are dominant lower critical frequencies, the overall dominant lower critical frequency is increased by a factor of 1/ sqrt[(2^1/n) -1] as shown by the equation f’cl(dom=fcl(dom)/sqrt[(2^1/n) -1], where n is equal to the number of stages of the amplifier. If they are dominant upper critical frequencies. the overall dominant upper critical frequency is reduced by a factor of sqrt[(2^1/n) -1], as shown by the equation f’cu(dom)=fcu(dom)sqrt[(2^1/n) -1].
Floyd, Thomas L. Electronic Devices (Conventional Current Version). Available from: ECPI, (10th Edition). Pearson Education (US), 2017.