The voltage feedback (VF) operational amplifier (op amp) is the most common type of op amp. The less well known current feedback (CF) op amp has been commercially available for about 20 years, but many designers are still uncertain about how to use them. Terminology is a confusing factor for many people. The CF op amp is a trans impedance op amp and so has a different vocabulary associated with it. This report attempts to show that there are more similarities than differences between CF and VF op amps when considering basic circuit operation.
The ideal VF op amp model is a powerful tool that aids in understanding basic VF op amp operation. There is also an ideal model for the CF op amp. Figure (a) shows the VF ideal model and Figure (b) shows the CF ideal model.
Figure:
In a VF op amp,
Vo = a x Ve (1)
Where Ve = Vp – Vn is called the error voltage and a is the open loop voltage gain of the amplifier.
In a CF op amp,
Vo = ie x Zt (2)
Where ie is called the error current and Zt is the open loop trans impedance gain of the amplifier. An amplifier where the output is a voltage that depends on the input current is called a trans impedance amplifier because the transfer function equates to an impedance i.e.,
Vo/ie = Zt
Source: James Karki / Texas Instruments
