How to determine if a CFA or a VFA is better for your design

My amplifier buddy, Xavier Ramus, recently wrote a great blog on current feedback amplifiers that I wanted to elaborate on and add a few more pros and cons.

What’s the basic difference between a current feedback amplifier (CFA) and a voltage feedback amplifier (VFA)?

Simply put, in VFA the positive node Vp tracks the negative node Vn by action of negative feedback. In a CFA, however, the tracking happens by design (as my professor used to say).

CFAs haven’t been around for as long as VFAs and are still less popular compared with their VFA counterparts, but they do offer great benefits when used in the right application.

Some key benefits of CFAs are their high bandwidth, extremely high slew rate and low distortion making them suitable for large transient interface including audio applications. Check out the LME49723 to appreciate the low noise and low distortion.  On the other hand, CFAs typically lack the precision of VFAs and can have large amounts of input bias currents which can result in a higher current noise density. They also have mismatched impedances at their inputs (low input impedance at the negative node) as a buffer is used internally between the inverting and non-inverting inputs.

To circumvent the lack of precision in CFAs, combine them with VFAs in a composite amplifier fashion.  Professor Sergio Franco discusses many of these topologies in chapter 8 of his book, “Design with Operational Amplifiers and Integrated Circuits”.

CFAs are an excellent choice for applications requiring fast current to voltage conversion- think transimpedance in photoconductive modes- driving cables in optical systems which require a high output current and even for active filter design. The OPA695 is a 12-V CFA with a noise floor less 3nV/rtHz and 90mA at its output.

A common mishap associated with CFAs is their use as integrators. Inserting a capacitor in the feedback loop can lead to serious instability. I recommend using a resistor in series with the inverting input leading up to the feedback capacitor. The value of the resistor, however, must be chosen carefully to avoid decreasing the bandwidth unnecessarily.