• At low frequencies the coupling
and bypass capacitors can no longer be replaced
by the short – circuit
approximation because of the increase in reactance of these
elements.
• The frequency – dependent
parameters of the small signal equivalent circuits and
the stray capacitive elements
associated with the active device and the network
will limit the high frequency
response of the system.
• An increase in the number of
stages of a cascaded system will also limit both the
high and low frequency response.
• The horizontal scale of
frequency response curve is a logarithmic scale to permit a
plot extending from the low to the high frequency
• For the RC coupled amplifier,
the drop at low frequencies is due to the increasing
reactance of CC and CE, whereas
its upper frequency limit is determined by either
the parasitic capacitive elements
of the network or the frequency dependence of
the gain of the active device.
• In the frequency response,
there is a band of frequencies in which the magnitude
of the gain is either equal or
relatively close to the midband value.
• To fix the frequency boundaries
of relatively high gain, 0.707AVmid is chosen to
be the gain at the cutoff levels.
• The corresponding frequencies f1
and f2 are generally called corner, cutoff, band,
break, or half – power
frequencies.
• The multiplier 0.707 is chosen
because at this level the output power is half the
midband power output, that is, at
mid frequencies,
•
PO mid = | Vo2| / Ro = | AVmidVi|2
/ RO
• And at the half – power
frequencies,
POHPF = | 0.707 AVmidVi|2 / Ro
= 0.5| AVmid Vi|2 / Ro
• And, POHPF = 0.5 POmid
• The bandwidth of each system is
determined by f2 – f1
• A decibel plot can be obtained
by applying the equation,
(AV / AVmid )dB
= 20 log10 (AV / AVmid)
§ Most amplifiers
introduce a 180° phase shift between input and output signals. At low
frequencies, there is a phase
shift such that Vo lags Vi by an increased angle. At high
frequencies, the phase shift
drops below 180°.
Low – frequency
analysis – Bode plot
In the low frequency region of
the single – stage BJT amplifier, it is the RC combinations
formed by the network capacitors
CC and CE, the network resistive parameters that
determine the cutoff frequencies.
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