Amplifying DC milivolts to volts

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coinmaster

Joined Dec 24, 2015
502
Hello, I am trying to make a variable resistor based on a changing load impedance on output of an amplifier. I am a novice to electronics so I cannot do this alone unfortunately.

I have come to the conclusion that measuring the RMS of the AC signal on the output is probably the best way to determine a changing impedance.

This is what I made so far using an LTC1966
LTC1966_TA09 (1)-1.png
The difference between a 300 ohm impedance and a 500 ohm impedance is about 2mv on the IC. So I want to use a Jfet to act as a resistor that will allow an adjustable voltage between 75-78ish volts depending on the output of the IC.

I'm totally out of my league on this so help would be appreciated.
 

Papabravo

Joined Feb 24, 2006
12,770
I don't know from your description exactly what your expectations are. What is it that a "variable resistor based on a changing load impedance" is supposed to do? Secondly, why would you want to do such a thing?
 

GopherT

Joined Nov 23, 2012
8,012
Hello, I am trying to make a variable resistor based on a changing load impedance on output of an amplifier. I am a novice to electronics so I cannot do this alone unfortunately.

I have come to the conclusion that measuring the RMS of the AC signal on the output is probably the best way to determine a changing impedance.

This is what I made so far using an LTC1966
View attachment 99077
The difference between a 300 ohm impedance and a 500 ohm impedance is about 2mv on the IC. So I want to use a Jfet to act as a resistor that will allow an adjustable voltage between 75-78ish volts depending on the output of the IC.

I'm totally out of my league on this so help would be appreciated.

How does this thread differ from your existing thread?

http://forum.allaboutcircuits.com/threads/sensing-load-impedance.119756/#post-953891
 

Thread Starter

coinmaster

Joined Dec 24, 2015
502
I don't know from your description exactly what your expectations are. What is it that a "variable resistor based on a changing load impedance" is supposed to do? Secondly, why would you want to do such a thing?
I want a Jfet to alter it's voltage based on the load impedance. So if the load is 300ohms, the Jfet will act as a resistor, allowing 75volts to flow, if the load is 500 ohms, it will allow 78 volts to flow, something along those lines.

I'm just trying to alter the operating point of an amplifier based on load impedance.
 
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