On an inverting op amp, how does Rf and Ri form a voltage divider? Where Rf is a feedback resistor and Ri is an input resistor to the inverting input of an op amp. The non-inverting input is grounded.

 

Originally posted by abd2005@Nov 14 2005, 11:27 PM
On an inverting op amp, how does Rf and Ri form a voltage divider? Where Rf is a feedback resistor and Ri is an input resistor to the inverting input of an op amp. The non-inverting input is grounded.

[post=11651]Quoted post[/post]​

In both configurations, inverting and non inverting, Rin and Rf always both are a voltage divider between output and output.

If Rin is grounded you read that as 0V (I think your problem is THIS). The other end, obviously, is Vout.

 

In a basic inverting op-amp circuit as you describe, the 'voltage divider' is between the signal input at Ri & the op-amp output at Rf.

The op-amp will act to change it's output voltage so the middle point of the divider (inverting input) is always at the same voltage as it's none-inverting input (i.e. 0V or Ground in your example).

e.g if Ri is 1K and Rf is 2.5K, a -1V input would cause the opamp output to settle at 2.5V, a -1.5V input would give 3.75V out etc.

The inverting input of the op-amp in this configuration is sometimes called a Virtual Earth. If you put a meter or scope on it, it's at 0V regardless of the signal through the circuit, a long as everything is in the working range of the op-amp.

The op-amp circuit Gain is Rf / Ri
If Ri is larger than Rf you will get a fractional gain.

 

Originally posted by rjenkins@Nov 15 2005, 04:16 AM

The op-amp circuit Gain is Rf / Ri.

[post=11653]Quoted post[/post]​

atferrari and rjenkins have both provided very good information regarding the operation of an inverting opamp.

The only correction I would make is in the inverting opamp circuit gain expression that rjenkins has indicated. I am pretty sure he meant to write "the opamp circuit gain is -Rf / Ri". It is the occurrence of the negative sign in the gain expression that accounts for the inversion that takes place in the inverting opamp.

hgmjr