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NFB is an acronym for negative feedback.

Lets say that we have an amplifier with three stages: input (differential) stage followed by voltage amplification stage and output power stage. And we have a NFB from output of the power stage back to the differential input stage of the amplifier.

  • Will the open-loop gain of an amplifier decrease as the portion of global NFB is increased?

  • Referring to the lower graph: With 100dB of gain, the amplifier has roll-off point of frequency at 500 Hz. If we would decrease the gain of the same amplifier, would we also increase the roll-off point of frequency to about 10k Hz (as marked with green line)?

enter image description here

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    \$\begingroup\$ NFB doesn't change the open loop gain (a fixed property of the op amp circuit) it decreases the overall stage gain AND increases the bandwidth (google gain bandwidth product) \$\endgroup\$ – JIm Dearden Nov 9 '17 at 13:46
  • \$\begingroup\$ @JImDearden If it decreases the overall stage gain then the amplifiers gain as a whole is decreased also. \$\endgroup\$ – Keno Nov 9 '17 at 14:06
  • \$\begingroup\$ For your understanding: In your graph, the open-loop gain is 100 dB - and it remains, of course, unchanged. If you apply negative feedback the new "overall gain" (closed-loo gain) is 60 dB (in your example). And - as you can see - the bandwidth has increased correspondingly. \$\endgroup\$ – LvW Nov 9 '17 at 14:29
  • \$\begingroup\$ @LvW With negative feedback we increase the gain bandwidth? \$\endgroup\$ – Keno Nov 9 '17 at 15:07
  • \$\begingroup\$ @Keno no, the gain x bandwidth product stays constant. NFB lowers the gain of the complete system (here from 100 to 60dB) thus bandwidth increases in proportion, since the product of the two is constant (in first approximation, if the opamp has dominant pole compensation, etc, etc). \$\endgroup\$ – peufeu Nov 9 '17 at 15:17

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