.....I know gain of inv amp is -Rf/R1 , But if ckt is going to complete through the path vin-R1-Rf-Rload-Gnd then when op-amp is coming in picture. Because if we will use transistor instead of opamp then current will flow thr transistor so accordingly in the opamp ckt current should flow through opamp then & then only we can say that this is inverting amplifier using opamp and we will use formula of opamp.....I am eagrly waiting for your kind reply....
I think, your error is that you think in
current terms only. Don't forget that the opamp is considered as a
voltage amplifier (best example: FET input stage with no input current).
Start with your assumption that the current goes through the chain R1-Rf-Rload-Gnd and realize that this current develops a voltage at the node between R1 and Rf.
This voltage enters the opamp input and causes an opamp output voltage, which causes a special effect called "negative feedback".
Now, you have - as I have mentioned earlier -
two voltages affecting the whole circuit. In steady-state condition these two voltages determine the current through the resistors thereby creating a so called "virtual ground" potential at the inverting input. For a complete understanding of this effect it is necessary to study the feedback principle.
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Here comes one possible "visualization" of the feedback effect - perhaps it helps:
For steady-state conditions there must be an equilibrium as described as follows:
Both voltages (Vin and Vout) create a very small voltage Vinv at the inverting opamp input terminal that must satisfy the amplifier condition Vout=Ao*Vinv. (Ao=open-loop gain).
Example: Ao=1E6 and Vinv=1E-6 volts.
Now you can write down two current equations:
I=(Vin-Vout)/(R1+Rf) and I=(Vinv-Vout)/Rf
Equating both expressions and setting Vinv<<Vout leads to
Vout/Vin=-Rf/R1.
Note: This simple result is valid for an idealized opamp only (Ao approaching infinity, which means: Vinv=0)