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

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Are you referring to the 3dB point at which the bandwidth is deemed to be defined or just any amount of bandwidth as long as the gain is above unity? –  Andy aka May 26 '13 at 18:37
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2 Answers

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The question is a bit vague, but here goes.

Bandwidth is conventionally defined as the frequency at which the response of the system is reduced by 3dB (half the power) from whatever its maximum response is.

Most real devices, when operated open-loop, have a low-pass characteristic that causes their response to fall off with increasing frequency. They can have very high response at low frequencies, so their inherent bandwidth is relatively narrow.

If you use negative feedback to reduce the maximum response (closed-loop), then the frequency at which the response drops by half from that new maximum value (because of the device characteristics) is going to be higher.

So yes, for most devices, closed-loop bandwidth is greater than open-loop bandwidth.

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Thanks of the replies.I wanted to ask about the bandwidth of a closed loop buck converter, but wasn't sure how to frame the question.The open loop current transfer function Gii(s) of the buck converter has a bandwidth upto the double pole at wo ? whereas the closed loop transfer function has a much higher bandwidth,as a result the step response for the closed loop transfer function is much faster as compared to open loop.I know there exists a relation between bandwidth and rise time. But what caused the closed loop bandwidth to increase ? –  user22348 May 28 '13 at 3:00
 
You asked a very generic question, so I gave a very generic answer. If you have a question about a specific system, you should edit the information into your question, with a diagram if possible, and then you can get more detailed answers. –  Dave Tweed May 28 '13 at 11:40
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Here is the open-loop bandwidth of a certain op-amp shown in red: -

enter image description here

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?

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About the last paragraph: By "op-amp filter circuit" you mean to a filter located at the closing loop or between the output of the op-amp and GND? Moreover, if the filter cut-off frequency is above 11Hz, wouldn't it make a bandwidth of exactly (24-11)=13Hz ? –  Dor Jun 25 '13 at 20:00
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protected by W5VO♦ May 26 '13 at 19:29

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