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Old 5th July 2013, 06:44 PM   #1
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Default Importance of open loop gain and bandwidth of a class AB

Just now getting into designing class AB amps with global feedback, and I was wondering what the importance of open loop gain and bandwidth with regards to performance in the audio range?

Obviously, open loop gain will affect your DC offset, but other than that, what are the considerations? And for a good step response (in the audio range), what is a good 0db frequency to design around? 100k? 1MHz?
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Old 5th July 2013, 07:02 PM   #2
jcx is online now jcx  United States
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Putzeys, The F-word - or, why there is no such thing as too much feedback

http://www.linearaudio.net/images/on.../volume1bp.pdf

may be dynamic link - poke around Jann's Linear Audio site Online Articles


Cordell's, Self's, both audio power design books are mostly showing the high feedback case

Last edited by jcx; 5th July 2013 at 07:05 PM.
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Old 5th July 2013, 07:04 PM   #3
Bensen is offline Bensen  Belgium
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I think it depends on your design goals. Some want alot of zero's behind the comma concerning THD, they will use as much open loop gain as they can achieve without making the amp unstable.

Personally, my target is an open loop gain between 40 and 50, this gives me around 20 to 30db feedback. This gives normally a nice FFT whit very small or none higher harmonics. The open loop bandwidth will then be around 4 to 6khz.

In my previous amp, I had a jumper to increase the open loop bandwidth to over 20khz. I could clearly hear the difference with or without the jumper, but could not tell which sounded better.
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Old 5th July 2013, 10:02 PM   #4
Boscoe is offline Boscoe  United Kingdom
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Bensen you must be smoking something!
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Old 6th July 2013, 08:05 AM   #5
Bensen is offline Bensen  Belgium
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Nope, please clearify yourself. The above is my personal experience. Remark, I am not an EE and learned all by myself.

My last build: post 2837
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Old 6th July 2013, 09:31 AM   #6
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by Bensen View Post
....................... my target is an open loop gain between 40 and 50, this gives me around 20 to 30db feedback. ..................
Maybe he is referring to this?
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Old 6th July 2013, 11:35 AM   #7
Boscoe is offline Boscoe  United Kingdom
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And open loop bandwidth at 6KHz!
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Old 6th July 2013, 04:10 PM   #8
Bensen is offline Bensen  Belgium
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I don't see your point? For example, the famous JC1 has an open loop bandwidth of approx 4khz.





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Old 6th July 2013, 04:29 PM   #9
Boscoe is offline Boscoe  United Kingdom
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Because it wouldn't work as an audio amplifier. You need a open loop bandwidth of MHz for it to work well in the audio spectrum.
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Old 6th July 2013, 04:41 PM   #10
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Some authors here are using the phrase "open loop bandwidth" to mean the unity gain crossover frequency (which is usually 1 to 15 MHz for power amps).

Other authors are using the phrase "open loop bandwidth" to mean the -3dB frequency of the open loop Bode plot, i.e. the location of the first pole (which can be between 10 Hz and 40 kHz).

Perhaps the two different meanings cause confusion. One way to resolve the confusion is for the Original Poster "Fusion916" to draw a diagram and to ask specific questions about the diagram.
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