Pontentiometer with transistor

[asdrojas]

Hello I need to make a VCR (voltaje controlled resistor) with transistors becouse I have to work with high power (beetween 8W-40W). It has to have 4 terminals, 2 for the output resistance, and another 2 for the control. I have been working with some designs with transistors, resistors and a potentiometer (to be replaced by a VCR) that works like a high power variable resistor. The rank of the desing must be 100 ohms to 2k omhs and my desing is far to reach that. I don't have useful documentation. I would like that somebody tell me how to find info or show me some similar desing.
Thanks.

 

[dick_freebird]

You need to get a few things figured out beforehand.

Is the current unipolar, or reversing? That will constrain
device selection and topology. A ground-referred resistor
is the easy case. A truly independent one, is a challenge.
I've made cheesy active loads as you describe but they
were really 3-terminal.

How linear? Across what voltage range? Linearly resistive
through what frequency? These will tell whether you need
an active feedback or a simple control scheme, whether
you have a prayer of operating a FET in linear region to
suit.

What about (say) back-to-back PhotoMOS with the gate
photocells at the center of the butterfly, and a replica
pair in the feedback network of a control loop? That'd be
kinda neat.

But if you have to use "transistors" alone, you'd better have
a nice constrained application-range and attributes.

 

[asdrojas]

Hello, the current is monophasic AC, ground independent, without bias, and it hasn't to be very linear. It has to had feedback, for this I going to use a VCR but now I'm focussing in making a just variable resistor. I'd like that somebody give me some idea or info with something similar.