written 3.0 years ago by |
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
Ix = vo−AviRo
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)