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Amplification = Output/Input

Then the amplification of the feedback amplifier should be Vo/(Vs +- Vf)

Here,

Vf = feedback signal

Vo = Output signal

Vi = Input Signal and Vi = Vs +- Vf

Vs = Source signal

I have used V instead of phi

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    \$\begingroup\$ Without explaining your symbols you cannot expect a substantial answer. \$\endgroup\$ – LvW Nov 20 '18 at 10:12
  • \$\begingroup\$ Define Vf by drawing a schematic and using words. Not much to ask. \$\endgroup\$ – Andy aka Nov 20 '18 at 10:33
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Vf is nothing but fraction of output voltage (Vo) that is fed back to the input for various purposes(to increase stability for example) Well let me take op amp example Think of the outer box in picture as black box and u don't know what is inside of it. So for you what u have in hand or what u can measure is Vin and Vout Amplification=Vout/Vin Vf does it's job internally and u don't consider it while writing the gain formula or amplification formulaenter image description here

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  • \$\begingroup\$ STP-regarding stability: Don`t you agree that negative feedback will DECREASE the stability against self-oscillation? \$\endgroup\$ – LvW Nov 20 '18 at 11:37
  • \$\begingroup\$ Negative feedback increases stability. For producing oscillations positive feedback is given \$\endgroup\$ – STP Nov 21 '18 at 11:22
  • \$\begingroup\$ STP_ with all respect: Before making such (wrong) statements here in the forum you should convince yourself about the correctness of your understanding. Consult a relevant textbook - and you will understand the consequences of negative feedback. In most cases, even an evaluation of the BODE diagram (loop gain reduction) will be helpful. Negative feedback stabilizes the DC operating point only and - at the same time - reduces the phase margin. Negative feedback always turns into positive feedback for rising frequencies... \$\endgroup\$ – LvW Nov 21 '18 at 13:37
  • \$\begingroup\$ Sorry - in the 4th line above I wrote "loop gain reduction" which should read "loop gain increase". \$\endgroup\$ – LvW Nov 21 '18 at 13:49
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Think of the whole block diagram as an amplifier wherein \$\Phi_\mathrm{s}\$ is the input and \$\Phi_\mathrm{o}\$ is the output. This is similar to a multi-stage amplifier. multi-stage The total gain is the output of the second amplifier divided by the input of the first amplifier, which is equal to the product of the gains.

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