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with the help of neat block diagram, derive expression for Rjf, Rof, Gmf for voltage series negative feedback amplifier. Give significance of the above mentioned parameters.
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enter image description here

A = vovi gain of ampli without f/b

Af = vovs gain of ampli with f/b

β = vfvo is feedback factor.

voltage gain with negative f/b

vs = vi + vf

but vf = βvo

vs = vi + βvo

"' y Vi = voA

vs = voA + β vo

vs = vo ( 1+aβA)

Af = vovs = A(1+Aβ)

This indicates that in voltage series negative f/b overall voltage gain of ampli decreases by factor ( 1 + Aβ)

i/p Resistance with negative f/b Let Ri = viIi is i/p Resistance

without f/b

Rif = vsIi is i/p Resistance with f/b

Vs = Vi + Vf

Vs = Ii Ri + β Vo

Vs = Ii Ri + β AVi

Vvs = Ii Ri + β AIi Ri

Vs = Ii Ri (1+Aβ)

vsIi = Ri (1+ A β)

Rif = Ri ( 1 + Aβ)

This indicates that in voltage series negative f/b overall voltage gain increases by factor (1 + A β )

o/p Resi with f/b

In order to find o/p Resi with f/b making the i/p VS = 0 and connecting a fictitious voltage source of value vx at the o/p then

Rof = vxIx

Where Ix is current drawn from vx

enter image description here

Ix = voAviRo

But Vi = - vf = - β vo

Ix = vo+AβvoRo

Ro Ix = Vo ( 1 + Aβ)

Rof = voIx = Ro(1+Aβ)

Which indicates that in voltage series negative f/b overall o/p Resi decreases by factor (1+Aβ)

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