Q. Frequency response of amplifiers?

All amplifiers exhibit variations of performance as the signal frequency is changed. The frequency response of an amplifier may be defined as the functional dependence of output amplitude and phase upon frequency, for all frequencies. Invariably there is a maximum frequency above which amplification does not occur. Depending on the design of the circuit, there may also be a lower frequency limit below which amplification disappears. The range of frequencies over which significant amplification of themagnitude is obtained is known as the passband, which is bounded by the upper cutoff frequency and the lower cutoff frequency (if one exists).Dc-coupled amplifiers, in which the stages are coupled together for all frequencies down to zero, have no low-frequency limit. On the other hand, circuits containing coupling capacitors that couple stages together for ac while isolating them for dc are limited in their low-frequency response. The high-frequency deterioration of the voltage gain is due to the effect of capacitances that are internal to the transistor.

Figure shows a typical frequency response of a voltage amplifier.While most often the magnitude of the gain is discussed (because it defines the frequency range of useful gain), the phase response becomes important for transient calculations. The midband region is the range of frequencies where gain is nearly constant. Amplifiers are normally considered to operate in this useful midband region. The bandwidth of an amplifier is customarily defined as the band between two frequencies, denoted by ωH and ωL, which corresponds to the gain falling to 3 dB below the midband constant gain. Thus,

W_3dB = ω_H - ω_L

which is shown in Figure. When both ωL and ωH have significant values, the amplifier is known as a bandpass amplifier. A narrow-bandpass amplifier is one in which W_3dB is small relative to the center frequency of the midband region. In a low-pass unit (dc amplifier) there is no low 3-dB frequency.