Can Op-Amps Create Effective Voltage-Controlled Resistors?

[Jdo300]

Hello All,

I'm trying to determine if there is a way to make a voltage controlled resistor using an Op-Amp (or multiple op-amps). I want to use this VCR to control the gain of another op-amp but wanted to see if there was an altenative approach to using a JFETs or an LED/CDS photocell arrangement.

Thanks,
Jason O

 

[MATLABdude]

A digital potentiometer? Not quite voltage controlled, but if you have a digital control scheme, it works pretty well.

 

[Jdo300]

Yes I do know about the digitally controlled ones. but what I am looking for is something that can be used in an analog circuit using op-amps. I can't help but think that there must be some way of hooking up an op-amp to make it behave like a resistor. since Vdrop = I * R. My thought would be something similar to the combination of a multiplier and an I to V converter circuit but I'm not exactly sure how to approach the problem.

I wouldn't be suprised if there was even an IC out there somewhere that could do this, but the challenge is how would one do this if all you had were a bunch of run-of-the-mill op-amp ICs and some caps, resitors, and diodes to work with?

Thanks,
Jason O

 

[dlgoff]

You can use a field effect transistor (FET) to make a voltage control resistor.
"users.ece.gatech.edu/~lanterma/sdiy/datasheets/transistors/vishay_fet_cvr_an.pdf"[/URL]

 

[Jdo300]

Hey these are cool circuits. Thanks :-). Quick question though, what's the rationale behind using JFETs instead of MOSFETs?

 

[dlgoff]

Well, I think you could use either. Here's a wiki link to FETs:
http://en.wikipedia.org/wiki/MOSFET"

 

[Averagesupernova]

Why not keep this in the same thread? You can use a 1496 as a voltage controlled gain amplifier. There are other methods, but since you were talking about op-amps I decided to throw out the LED/CDS approach since it fit well and you wanted to do it with ONLY op-amps. FETS work ok but for this application I'm not sure you want to mess with them since your signal will have to be offset since FETS need a bias voltage. You can't let the drain voltage get less than the source voltage for instance in an N channel FET. Once you've controlled the signal where you want it you need to AC couple it back to a DC voltage of zero in your application.

 

[Jdo300]

Hi Averagesupernova,

I just started a separate thread about the VCRs since I wanted to focus on that for the moment and didn't want to derail the main topic on the other thread. I am trying to find information about the 1496 that you mentioned. Are you referring to the LT1496? I'm not sure I'm looking at the right part but if so, how do I wire it up as a voltage controlled gain amplifier?

Thanks,
Jason O

 

[Averagesupernova]

A balanced modulator is usually what it is spec'd as. It has many other applications as the data sheet should say.

 

[waht]

Check out the PIN diodes. They are used extensively where variable resistance is required, especially in RF design.

 

[Jdo300]

Thanks, I found the datasheet.

Now, just to confirm my understanding on this, there is no way to make a voltage controlled gain amplifier using regular Op-amp ICs without using a MOSFET/JFET OR some speciaized op-amp with an adjstable gain input?

Thanks,
Jason O

 

[BogdanFloresc]

A few days ago, I developed a schematics for a fast unidirectional voltage controlled resistor (it have a - and a + ), but for now is in tests because I need to find a simple and fast way ( no op amp ) to compensate some offset and to built a better current amplifier/current mirror. The schematics works on this principle : Ohm law is emulated with one multiplier and few voltage/current differential amplifiers (summing and substracting currents). The schematics contains some 25 transistors (no IC because I need very fast response and for now is a test schematics) 8 diodes, some 20 resistors and a compensation capacitor (I try to get rid of the capacitor (or make it very small, because is a miller integrator compensation) by proper scaling of the loop amplification).
The principle is quite simple. A MOS transistor is steered by an amplifier witch measure current passing through transistor and the voltage at the drain and source. Then by some divide/multiply/summ/substraction , the drive is such a way to emulate Ohm law.

The device is built for 12 Volt operation but the voltage maybe from 10 Volt to 30 Volt.
In the 12 Volt configuration, the device need some 3 Volt upper headroom and more than 3 Volt to GND.

If you have a voltage controlled resistor schematics, please write to me at bogdanflorescu@yahoo.com. If you need the schematics for my device, write to me.
Thank you,
Bogdan