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?
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?
Putzeys, The F-word - or, why there is no such thing as too much feedback
http://www.linearaudio.net/images/onlinearticlesPDF/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
http://www.linearaudio.net/images/onlinearticlesPDF/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
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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.
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.
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
http://www.diyaudio.com/forums/solid-state/96192-post-your-solid-state-pics-here-284.html
Greetz
My last build: post 2837
http://www.diyaudio.com/forums/solid-state/96192-post-your-solid-state-pics-here-284.html
Greetz
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.
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.
read the Putzeys article - or any decent treatment of feedback in amplifiers - "open loop" corner frequency isn't a good design criteria
GBW product, maximizing feedback loop gain over "working band" are feedback amp goals
maximizing available feedback requires running the amp as fast as it remains stable - so faster output device, high bias that keeps ft of the output Q up are good
today audio power amplifiers with faster, possibly paralleled BJT or properly driven MOSFET can work with feedback loop unity gain intercept of a few MHz - allowing up to 40 dB excess loop gain at 20 kHz
push up "VAS" quality, gain with "beta enhancement" and you can apply 2nd order compensation for another ~10x global loop gain at 20 kHz
GBW product, maximizing feedback loop gain over "working band" are feedback amp goals
maximizing available feedback requires running the amp as fast as it remains stable - so faster output device, high bias that keeps ft of the output Q up are good
today audio power amplifiers with faster, possibly paralleled BJT or properly driven MOSFET can work with feedback loop unity gain intercept of a few MHz - allowing up to 40 dB excess loop gain at 20 kHz
push up "VAS" quality, gain with "beta enhancement" and you can apply 2nd order compensation for another ~10x global loop gain at 20 kHz
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very popular for the Otala camp - he put forth an incorrect model, prescription for avoiding TIM - using the fc of the open loop gain
it is common for them to crow about "open loop gain flat to beyond 20 kHz" as per the Guru's recommendation
the Putzeys article is clear about Otala's error in analysis, prescription - high gain, "low open loop bandwidth" can have TIM reduced by appropriate means
it is common for them to crow about "open loop gain flat to beyond 20 kHz" as per the Guru's recommendation
the Putzeys article is clear about Otala's error in analysis, prescription - high gain, "low open loop bandwidth" can have TIM reduced by appropriate means
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