Virtual ground

This circuit generates an output voltage which is a function of the input current using an operational amplifier. Since almost no current flows into the input of the operational amplifier, the current that flows to the feedback resistor is approximately equal to the input current If ≈ Iin The inverting input of the operational amplifier is at virtual ground so the output voltage (Vo) is Vo = -(If)(Rf) Vo = -(Iin)(Rf) The transfer function of the circuit is Vo / Iin = - Rf which is a transresistance in Ohms (Ω). Rm = - Rf = Vo / Iin Because of the negative sign this circuit can be called an Inverting Transresistance Amplifier. That will help relate this circuit to the inverting voltage amplifier configuration. At the inverting input terminal the inverting voltage amplifier configuration is a transresistance amplifier, the input resistor converts the voltage into current. If the input current in this circuit has a source resistance then, by source transformation, it is equivalent to an op amp inverting voltage amplifier. In some contexts the negative sign can be dissociated from the transresistance and the circuit can be called simply as Transresistance Amplifier.