The presence of capacitors in an amplifier gain makes it depends on the frequency. The coupling and decoupling capacitors limit the low-frequency response and small signal parameters of transistors frequency-dependent and parasitic capacitances associated with active devices limit their high-frequency response. Besides an increase in the number of cascaded amplifier stages in turn also limit the response at low and high frequencies.
Figure 3 .1. Amplifier gain versus frequency for a) general amplifier, b) amplifier without coupling and bypass capacitors (amplifier directly coupled).
Figure 3 .1. To show the gain of an amplifier as a function of frequency. Clearly identifies three areas: low frequency, midrange and high frequencies. At low frequencies, the effect of coupling and bypass capacitors is important. A midrange frequencies, these capacitors have an impedance null being able to be replaced by a short circuit. At high frequencies, the frequency limitations of active devices determine the maximum operating frequency of the amplifier. These areas are defined by two parameters: cutoff frequency less than or ƒL and higher switching frequency or ƒH. Both parameters are defined as the frequency at which the amplifier gain drops to 1 / 2 or 0.707 with respect to the amplifier gain at medium frequencies. The bandwidth of the amplifier or bandwidth (BW) is defined as
BW HL = f - f (3 4)
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