2015-08-21
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In this range, the effect of the coupling capacitor cannot be ignored. The
reactance of the capacitance in parallel with the load will, however, be very
high at low frequencies and its effect can be ignored. The equivalent circuit
can be drawn as
If we represent C 1 and R L by an impedance Z, then we can immediately write
the current gain as
The low frequency gain can be expressed in terms of the mid-band gain by
dividing top and bottom lines of the above expression by (1 + G o R L):
Remember
It is usual to express the term
as w1, the reason for which you will discover shortly.
Thus, the low frequency current gain can be written as:
The significance of w 1
We have shown that at a frequency of w1, the current gain falls from its midband
value by a factor of √2. As power P = I 2 R, then at frequency w1 the
power gain will be half of its mid-band value.
The bandwidth of an amplifier is usually defined as the frequency range
between the half-power points. Thus, f1=ω1/2π represents the lower halfpower
frequency of the amplifier.
