Op-Amps having negative feedback



same input.

The noninverting input is grounded.

For finding the gain, let’s assume there is infinite impedance at the input (i.e. between the inverting and non-inverting inputs).

Infinite input impedance implies zero current at the inverting input.

If there is zero current through the input impedance, there is NO voltage drop between the inverting and noninverting inputs.

Thus, the voltage at the inverting input is zero!

- The zero at the inverting input is referred to as virtual ground.

Since there is no current at the inverting input, the current through Ri and the current through Rf are equal:

Iin = If.

The voltage across Ri equals Vin because of virtual ground on the other side of the resistor. Therefore we have that

Iin = Vin/Ri.

Also, the voltage across Rf equals –Vout, because of virtual ground. Therefore:

If = -Vout/Rf

Since If = Iin, we get that:

-Vout/Rf = Vin/Ri

Or, rearranging,

Vout/Vin = -Rf/Ri

So,

Acl(I) = -Rf/Ri

Thus, the closed loop gain is independent of the op-amp’s internal open-loop gain.

The negative feedback stabilizes the voltage gain.

The negative sign indicates inversion.


source

If you want to report any meterial please