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In Reply to: RE: You cannot build an amplifier with no feedback posted by Penguin on August 26, 2014 at 19:49:11
The first gain stage of my phono preamp consists of a pair of J-FETs, each running open loop into a linear drain resistance.Where's the feedback?
Mark Kelly
Edits: 08/27/14Follow Ups:
if so there it is :) Called a parallel feedback. If you have none, then there is no feedback, but that makes the FETs running somewhat out of the mostl linear range, ie you have more distortion....some FETS tolerate it more than others. and there is still the feedback internal to the the device, but that is negligible compared to the open loop gain. if you use them in a cascade config then the lower one provides a variable parallel feedback :)dee
;-D
True terror is to wake up one morning and discover that your high school class is running the country.
quote by Kurt Vonnegut
Edits: 08/26/14 08/26/14
The J-FETS are running in balanced mode, there's no AC voltage at the source.
Mark Kelly
in absolute terms the answer is no, because at one frequency or another the model will contain residual resistances between gate, source and drain, and there is the Miller capacitance which kicks in big time at high frequencies...in practical terms at a given frequency band, where the fringe effects can be neglected, you can get to practically zero feedback. To make it work you need to select components very carefully. That may not be a viable option for a production design, so designers use some amount of localized feedback to ease component selection. It is all a compromise, and meeting specs. If you use no feedback your THD maybe somewhere .01% and .1% and your IM might be between 1% and 0.1% depending on the particular sample of a component. which is ok for a DIY project you just keep swapping them till you find one that meets your particular need. Try that when you have to make 10 units a day :)...having a local feedback say 10 db will give you a 3.2 fold reduction in the numbers ...so you can meet specs with any of the components. so you can publish a spec that say you device will be with a THD of 0.03% and IM less then 0.3%.
dee
;-D
True terror is to wake up one morning and discover that your high school class is running the country.
quote by Kurt Vonnegut
Edits: 08/27/14 08/27/14 08/27/14
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