PSN-L Email List Message

Subject: Re: line driver question
From: John Popelish jpopelish@........
Date: Wed, 15 Mar 2006 10:09:37 -0500

Geoffrey wrote:
>> It will not improve the accuracy of the follower.  But, if this opamp 
>> drives a feedback current, how 'bout configuring it as a voltage 
>> controlled current source, so that wiring resistance will have little 
>> effect on the accuracy?
> Are you talking about something like they use in industry
> and telephone called a 20 milliamp loop ?

Something like, yes, in that it is a circuit that regulates current, 
rather than voltage.  However, the teletype 20 mA loop had only two 
output values, 0 and 20 mA, and was often only crudely regulated 
(sometimes just with a big series resistor and a high voltage power 

Voltage negative feedback (feedback from the output that is 
proportional to the output voltage) tends to lower the output 
impedance of an amplifier.  That is, it tends to make the output 
voltage almost independent of the load current.

But if the negative feedback is proportional to the load current, 
instead of output voltage, it tends to raise the output impedance, and 
make it almost independent of the voltage.  Add resistance (wiring) in 
series with the load,m and the output voltage increases to compensate 
and keep the current as intended.  This works well only at frequencies 
below those where the wiring acts as a capacitive load that rolls off 
the high frequencies, so there are limits to how fast the current at 
the far end of a wire can be regulated.  But at 40 feet of length, it 
is probably high enough.

Whether or not a current regulator is a better idea than a voltage 
regulator output, in this application, depends on details I don't have 
in front of me.

If accurate voltage at a distance is needed, a feedback pair of wires 
that act as long volt meter leads that return the voltage feedback 
from the load are another possibility for improving accuracy.

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