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Old 6th July 2013, 04:45 PM   #11
jcx is offline jcx  United States
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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

Last edited by jcx; 6th July 2013 at 04:47 PM.
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Old 6th July 2013, 04:56 PM   #12
Boscoe is offline Boscoe  United Kingdom
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However no datasheet I have ever read or heard in the time I've worked in an audio company have used the -3dB point in open loop bandwidth to describe the open loop characteristics of an amplifier.
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Old 6th July 2013, 05:03 PM   #13
jcx is offline jcx  United States
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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
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