Current Feedback Amplifiers (CFB)

We often misunderstand current feedback amplifiers, although they have been available for many years!

Current Feedback (CFB) operational amplifiers (Op-Amps) have been around for more than 30 years. They were designed for extreme high-speed performance, which Voltage Feedback (VFB) amplifiers could not accomplish at that time. 

CFB amplifiers have one major advantage over VFBs, they maintain their bandwidth over a wide range of signal gain. Note that VFB amplifiers are gain-bandwidth dependent, meaning their bandwidth decreases with increasing signal gain.

In practice, CFB amplifiers are commonly used in high-speed applications while VFB amplifiers are preferably used in precision applications.

Now see the figures below (click image to enlarge):

A VFB amplifier has two symmetrical, high-impedance inputs. The fact that the negative input is high-impedance makes the feedback network, driven by the output voltage VO, operate in Voltage-Source mode.

Here the series source impedance of this voltage source is the parallel circuit of RF and RG. The output of this voltage source is connected to the inverting input, providing the voltage potential, vn, at this pin.

The voltage potential at the non-inverting input, vp, is identical to the signal input voltage VI. Thus, the difference between the two input potentials is an error voltage, ve, that is amplified to generate VO.

However, unlike the VFB amplifier, the CFB amplifier has asymmetric inputs. Internally the non-inverting input connects using a unity-gain buffer to the inverting input. Thus, the non-inverting input exhibits the high impedance of the buffer input, while the inverting input presents the low impedance of the buffer output to the feedback network.

This low input impedance makes the feedback network operate in Current-Source mode. The parallel source impedance of this current source again is the parallel circuit of RF and RG.

During normal operation, the input voltage VI drives a current, ip, into the non-inverting input, and the output of the feedback current source drives a current, in, into the inverting input. The difference between the two input currents is the error current, ie. This error current is driven into an internal high-impedance stage, which results in the output voltage, VO.

To summarize, the major difference between a VFB and a CFB amplifier is the type of input error signal generating the output voltage. A VFB op-amp uses an error voltage while a CFB op-amp uses an error current.

⇲ REFERENCE: RENESAS Application Note AN1993 Rev.0.00, May 31, 2018

OK, current feedback op-amps can be successfully used in a variety of applications. More to come later!