SBOSAA5B April 2022 – September 2022 OPA2675
A current-feedback op amp such as the OPA2675 can hold an almost constant bandwidth over signal gain settings with the proper adjustment of the external resistor values, which are shown in the Typical Characteristics; the small-signal bandwidth decreases only slightly with increasing gain. These characteristic curves also show that the feedback resistor is changed for each gain setting. The absolute values of RF on the inverting side of the circuit for a current-feedback op-amp can be treated as a frequency response compensation element, whereas the ratios of RF and RG set the signal gain.
Figure 8-2 shows the small-signal frequency response analysis circuit for the OPA2675.
The key elements of this current-feedback op amp model are:
α = buffer gain from the noninverting input to the inverting input
RI = buffer output impedance
IERR = feedback error current signal
Z(s) = frequency-dependent open-loop transimpedance gain from IERR to VO
A current-feedback op amp senses an error current in the inverting node (as opposed to a differential input error voltage for a voltage-feedback op amp) and passes this on to the output through an internal frequency-dependent transimpedance gain. The Typical Characteristics show this open-loop transimpedance response, which is analogous to the open-loop voltage gain curve for a voltage-feedback op amp. Refer to the training videos shown in TI Precision Labs for further understanding on the CFA operating theory.
The values for RF versus gain shown in Figure 8-3 are approximately equal to the values used to generate the Typical Characteristics and give a good starting point for designs where bandwidth optimization is desired.