Till now I studied voltage feedback opamps and I know how circuits involving them are solved. What is difference between current feedback and voltage feedback amplifier? What are steps in analysis of current feedback opamps?


Current-Feedback Amplifiers (CFA) have a high-impedance non-inverting and a low-impedance inverting input. Hence, for negative feedback the feedback signal into the inverting terminal is a current (input resistance less than 50 ohms). For this reason, the feedback impedance (between output and inv. input) must not fall below a certain minimum value which is specified by the manufacturer (typically, some kohms).

Under these conditions, all the classical gain formulas (known from the voltage opamp) are applicable. But note that because of the lower impedance limit it is not allowed to use a capacitor between output and inv. input.

The most important advantage - in comparison to voltage-feedback amplifiers - is a fantastic slew rate (several thousands of volts per µsec). This is because CFA`s must not be unity-gain compensated. They are stable for all gain values if the feedback impedance meets the above mentioned restrictions.

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