When feedback is used around an operational amplifier, the closed
loop gain of the circuit is determined by a ratio involving the input and feedback impedances
used. If
the closed loop gain called
for by the feedback configuration
is greater
than the open loop gain available from
the
operational amplifier
for
any particular frequency, closed loop gain will be limited to the open loop gain value. Thus a plot of the closed
loop gain of a X100 (40db) amplifier
using the amplifier of figure 33 would appear as in figure
.
Closed Loop Gain of a X100 (40 dB)
Inverting Amplifier
stability
stability
As indicated above,
the
closed loop amplifier
circuit cannot supply
more gain than is available
from the operational amplifier itself, so at high frequencies, the closed loop
Bode plot intersects and follows the open
loop gain curve. The intersection point between the closed and open
loop curves is important because the angle between the two curves -
or,
more precisely, the “rate of
closure” since the curves aren’t actually straight lines -
determines whether the closed
loop amplifier,
differentiator, etc., being designed will be stable. Principle:
If the rate of closure
between the open and closed loop sections of the Bode plot is greater than 40 db per
decade the system is likely to be unstable. Bode plots may be varied almost at will to insure stability or
to provide some tailor made frequency response
characteristic.
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