I have a basic question seeming quite simple: An op amp with an open loop gain A, and a feedback resistor Rf, are connected as a current-to-voltage converter which can be found in many common applications. If the input impedance of the op amp is very large and can be regarded ideally "infinite" (as the case of FET-input op amp), the input resistance of the current-to-voltage converter can be estimated as Rf/A (Why?). With this presumption, can I estimate the voltage at the virtual ground pin as I*Rf/A (I denotes the input signal current)? How can I estimate the maximum feedback resistance Rf which the op amp can load? Does that value depend on the open loop gain A? If yes, please tell me the way or where I can find a definite answer. Thanks. Jiaqi

- posted
18 years ago