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Old 26th January 2011, 07:56 PM   #1
knutn is offline knutn  Norway
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Default The cable as part of the feedback loop

The idea is to move the feedback resistor to the other end of the cable, as shown in the figure. I know this has been tried for power amplifiers, but I was wandering if it was worth a try for preamplifiers connected to power amplifiers. As preamplifier, I am intending to use a transconductance amplifier, with high output impedance. This reduces the risk of instability due to capacitive loading. So, have someone tried this? Or, is it just nothing to gain with this solution?
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Old 26th January 2011, 11:03 PM   #2
jcx is offline jcx  United States
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the cable's parasitic capacitances cut bandwidth, and/or destabilize the loop
and make cable dielectric errors and microphonics bigger problems than the "traditional" approach

if you really want "current signaling" then you need a low Z transimpedance amp at the input of the power amp

remote "Kelvin Sensing" is used in some applications but the drive is usually close to a V source so cable C can be largely ignored, this best compensates for the series R drop of the cable resistance - not a problem for line level signals

putting a balanced receiver with high common mode rejection at the amp input is a more certain path to improvement - use shielded twisted pair, shield DC connected at preamp only
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Old 27th January 2011, 07:31 AM   #3
knutn is offline knutn  Norway
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Quote:
Originally Posted by jcx View Post
the cable's parasitic capacitances cut bandwidth, and/or destabilize the loop
and make cable dielectric errors and microphonics bigger problems than the "traditional" approach

if you really want "current signaling" then you need a low Z transimpedance amp at the input of the power amp

remote "Kelvin Sensing" is used in some applications but the drive is usually close to a V source so cable C can be largely ignored, this best compensates for the series R drop of the cable resistance - not a problem for line level signals

putting a balanced receiver with high common mode rejection at the amp input is a more certain path to improvement - use shielded twisted pair, shield DC connected at preamp only
I intend to use an amplfier as attached, this should imply that the stability should not be worsened, although the bandwidth could be lowered (but here we are talking about very high frequencies). But how could the cable in the feedback loop worsen cable dielectric errors and microphonics? I would think the concept would be an improvement due to feedback theory.

I have to agree with you, when it come to noise suppression, a balanced connection is better.
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Old 27th January 2011, 10:03 AM   #4
AndrewT is offline AndrewT  Scotland
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3 stage LIN topology - NFB tappings?

includes a schematic showing feedback going to source and using source to determine gain of the receiver.
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Old 27th January 2011, 12:11 PM   #5
knutn is offline knutn  Norway
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Yes I can see that the topology is similar, but with one important exception: I use an amplifier, where the output is common source, giving a high output impedance. So how would this affect the performance.
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Old 28th January 2011, 02:33 PM   #6
jcx is offline jcx  United States
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you really need a model of the cable to put some numbers on the effects
I dislike the idea that the loop gain GBW product depends on the type and length of the attached cable (load C)
you need some clear limits on cable parameters to design this at all - and I would use those cable paramters to calculate the performance expected in "conventional" approaches to see if you're really able to improve anything

Last edited by jcx; 28th January 2011 at 02:38 PM.
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Old 28th January 2011, 02:42 PM   #7
WuYit is offline WuYit  Sweden
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Hei,
cable in the feedback loop is a bad idea (see jcxs post). How long is the cable? How much voltage gain and power do you think you need?
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Old 28th January 2011, 05:38 PM   #8
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Hei

What particular parasitic feature of a cable do you want to minimize?
When it comes to stability it actually makes the things worse, not better.

skl!
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Old 30th January 2011, 01:27 PM   #9
knutn is offline knutn  Norway
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Quote:
Originally Posted by jcx View Post
you really need a model of the cable to put some numbers on the effects
I dislike the idea that the loop gain GBW product depends on the type and length of the attached cable (load C)
you need some clear limits on cable parameters to design this at all - and I would use those cable paramters to calculate the performance expected in "conventional" approaches to see if you're really able to improve anything
I have tried to simulate a lumped model a coaxial cable. I have used the same cable length from the preamplifier to the power amplifier and back. When it comes to distortion, the advantage in relation to the traditional approach is only marginal. The stability is, however, much worse; the phase margin is about non-existing even at 100kHz.

The conclusion? Do not do this. A better approach would probably be to use non-feedback approach, where the transmitter is high-impedance and the receiver is low-impedance?
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