Selecting the Feedback Resistor for a Current-Feedback Amplifier

In a current-feedback amplifier circuit, the feedback resistor’s absolute value matters to frequency response and stability. Preserving only the resistor ratio can change the behavior. Review the manufacturer’s recommended values before scaling the network or configuring a voltage follower.

Use the device’s recommended network

The AD8000 data sheet explains the role of feedback resistance in stability and bandwidth and provides recommended circuit values. It also discusses sensitivity to stray capacitance at the inverting input.

Record the amplifier, supply voltage, intended gain and load. Select the feedback and input components from the applicable guidance or an analysis supported by the manufacturer. Do not replace the feedback resistor with a short simply because a voltage-feedback follower often uses that connection.

Distinguish ratio from absolute value

Two resistor pairs can produce the same nominal closed-loop gain while presenting different feedback conditions. For illustration, a pair of 500-ohm resistors and a pair of 1-kilohm resistors have the same ratio, but that does not make them interchangeable around a current-feedback device.

When changing gain, determine which resistor the manufacturer’s method holds fixed and how the other value is selected. Review noise, output loading and power dissipation along with bandwidth. Avoid optimizing one parameter while silently changing the stability assumption.

Include layout and filtering

Keep the feedback path and inverting-input region consistent with the data-sheet layout guidance. Identify capacitance from pads, traces, test points and connected devices. A large observation pad near the summing node can alter behavior even when the schematic values remain correct.

Review any proposed feedback capacitor carefully. A filtering approach commonly used with one amplifier architecture may not be suitable for another. Use a configuration explicitly supported by the selected device’s documentation or obtain application guidance for the required response.

Test the intended range

Measure frequency response and small-signal step behavior at the actual gain and load. Check large-signal operation separately where required. Include relevant supply and temperature conditions, and document probe loading so the test does not obscure or create peaking.

Retain the chosen resistor values, their source and measured response with the schematic. Recheck substitutions that change package, resistor values or routing, even if nominal gain is identical. The resulting design record should explain why the absolute feedback network is appropriate for the selected amplifier, rather than treating it as a generic ratio that can be scaled freely whenever power, noise or component availability becomes a concern. Preserve the tested board revision with the component-value record.