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Friday, February 15, 2013

Closed Loop Gain




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  frothe 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

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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