[quote author="mediatechnology"]Wow. Amazing what unintended results you stumble upon whilst Googling:
Abstract: In high-speed transmission-line applications, it is important to match the output impedance of a line driver to the line. While this is achieved usually with a resistor, an active impedance synthesis has advantages. This application note describes how to use positive feedback around an op amp to create the desired output impedance. Equations and circuit examples are given for low-noise audio, and video op amps driving 50- to 600-ohm loads.
http://www.maxim-ic.com/appnotes.cfm/an_pk/3390[/quote]
Yup, that topology is pretty similar to classic synthesized current source (check old National applications series), but in that case you're trying to make 1M R look like >> instead of 75 to 600.
In this case the trade off is to use an smaller R (like r/10) and then multiply it back up to trade impedance accuracy for increased signal output. Perhaps a useful trade as PS rails keep dropping (3.3V and falling).
An amp designer I worked with used feedback manipulations to improve the damping factor of SR power amps (patented). He was able to actually deliver negative output impedance but I was afraid that would scare the boys and girls, so we were content to only claim 1000:1 damping factors, while routinely delivering 2-3K:1 at final production testing.
I used a variant on the synthesized current source topology in an old console summing bus structure to make 2K bus resistors look like a few hundred K and so reduce the bus noise gain for improved distortion/phase shift/ etc. I even did one console with a dBx VCA as the actual summing amp since it's noise gain was inconsequential.
JR
Abstract: In high-speed transmission-line applications, it is important to match the output impedance of a line driver to the line. While this is achieved usually with a resistor, an active impedance synthesis has advantages. This application note describes how to use positive feedback around an op amp to create the desired output impedance. Equations and circuit examples are given for low-noise audio, and video op amps driving 50- to 600-ohm loads.
http://www.maxim-ic.com/appnotes.cfm/an_pk/3390[/quote]
Yup, that topology is pretty similar to classic synthesized current source (check old National applications series), but in that case you're trying to make 1M R look like >> instead of 75 to 600.
In this case the trade off is to use an smaller R (like r/10) and then multiply it back up to trade impedance accuracy for increased signal output. Perhaps a useful trade as PS rails keep dropping (3.3V and falling).
An amp designer I worked with used feedback manipulations to improve the damping factor of SR power amps (patented). He was able to actually deliver negative output impedance but I was afraid that would scare the boys and girls, so we were content to only claim 1000:1 damping factors, while routinely delivering 2-3K:1 at final production testing.
I used a variant on the synthesized current source topology in an old console summing bus structure to make 2K bus resistors look like a few hundred K and so reduce the bus noise gain for improved distortion/phase shift/ etc. I even did one console with a dBx VCA as the actual summing amp since it's noise gain was inconsequential.
JR