Electronic – Closed loop bandwidth vs open loop bandwidth


Is closed loop bandwidth always greater than open loop bandwidth ?
If not on what factors does it depend ?
When can it be greater?

Best Answer

Here is the open-loop bandwidth of a certain op-amp shown in red: -

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The blue line is when certain closed-loop components are applied to the op-amp.

Bandwidth is normally measured at the 3dB point of the frequency response and in the case of an op-amp (open-loop) this will be at 24Hz in the diagram.

If closed loop components were present, the gain would be reduced to (say) 20dB (blue line) but the bandwidth would increase to 1MHz.

The above example is for simple resistors "closing" the loop with negative feedback and the resulting bandwidth (3dB point) is always greater.

However, if an op-amp filter circuit was required that cut-off frequencies above 10Hz, the filter would have a bandwidth of 10Hz. In this example the closed-loop bandwidth is less than the open-loop bandwidth.

Does this help?