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Old 8th January 2016, 01:47 AM   #1
rlaury is offline rlaury  United States
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Join Date: Sep 2005
Default Multi Stage amplifier bandwidth.

With 2 stages cascaded, should the second stage bandwidth be wider than the first?
Some where I read the second stage should be wider by some percentage so that -3dB
points don't coincide at the same frequency to prevent excessive phase shift.
But for the life of me I can't find out the equation.
Anyone know what rules are??
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Old 8th January 2016, 07:17 AM   #2
Koonw is offline Koonw  Malaysia
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To me if "coincide" means no difference (in phase shift or gain bandwidth product).
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Old 8th January 2016, 07:25 AM   #3
rlaury is offline rlaury  United States
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I was referring to the frequency. I want to have a single frequency roll off.
Sorry for the confuzion. :-)
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Old 8th January 2016, 07:29 AM   #4
Koonw is offline Koonw  Malaysia
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Quote:
Originally Posted by rlaury View Post
I was referring to the frequency. I want to have a single frequency roll off.
Sorry for the confuzion. :-)
No problem. If same freq. roll off it appears the phase shift is doubled (twice as worst) the result is same with my simulator.
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Old 8th January 2016, 08:20 AM   #5
DF96 is offline DF96  England
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With two stages cascaded the total phase shift is the sum of the individual phase shifts whichever order you put them. To get an early rolloff like a first order filter from two stages you need them to have quite different rolloffs, but the order does not matter.

However, there may be a reason to put the narrower bandwidth one first. This has nothing to do with phase shift but because it is generally a good idea to eliminate unwanted signals as early as possible in an amplification chain. Once a signal has gone it can't create any more trouble, and later stages usually have higher amplitudes so problems can get worse.
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