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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
An op-amp is designed to work with GLOBAL feedback from output to input, likely encompassing more than one gain stage.
Of course you can design a single-stage amplifier without applying any feedback. It will depend upon the largest input signal you want to handle and the characteristic of the amplifying device. Bipolar transistors have higher gain but start distorting at smaller input voltages compared to JFETs, which have lower gain. If the input signal is large enough to create more distortion than you want then degeneration (local feedback) can be added to effectively lower the gain and increase the signal handling. There will always be some parasitic feedback due to capacitive coupling between output and input and the transconductance of any amplifying device will be degenerated by the resistance of emitter/source/cathode connection. But these will be negligible except at very high frequencies and very high currents.
It is worth noting that adding feedback, either globally or locally, will increase noise. Any amplifying device has an inherent dynamic range - the difference between the largest signal it can handle before distorting and its noise floor. Feedback can never improve that range, only move it around - if you add feedback to improve signal handling the noise floor will raise up too.
Global feedback improves linearity by reducing the input voltage seen by the first amplifying device to very small levels.
The stability of feedback amplifiers (and control systems in general) is a subject that books could be written about, and indeed have. Many times over!
Follow Ups:
I was not trying to be a pain...but it gets under my skin when people talk about zero feedback amps, and not realize what an unstable and un-reproducible that design is :). Zero global feedback is a common practice in industry.
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
G'day all, this is an interesting thread! Doing quite a lot of DIY stuff myself (including moving magnet cartridge phono stages) for many years now, I've come to regard op amps very highly indeed, something that I probably would not have said ten or so years ago!Op amps can have 'issues', but as with all circuit designs, it takes a very good design and implementation to show op amps at their best. At the moment my phono stage of listening choice is a simple single op amp based full feedback (kit) design with a few unremarkable component replacements and it sounds very nice indeed with nothing to complain about at all.
It delivers around 40 db of low distortion and (very) low noise RIAA equalisation voltage gain and it sounds great, which is precisely what it is supposed to do. Regards, Felix.
Edits: 08/28/14
For me it was witnessing what a GSP ERA Gold V phono stage could do. That has to be more than 10 years.
My monoblocks use very high power opAmps, a Blue Circle design of about a decade ago or more (same devices used in their longstanding series 200 power amps).
Never trust an Atom, they Make Up everything!
just sayin', not makin' it up either.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: 08/28/14 08/30/14
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