Voltage Controlled Resistor for AC Signal ??

[sgugan]

Hi ,
I need a VCR for AC signal attenuation ( ON resistace less than 1 ohm). is there any ready made part available? if not how to made it? How to use JFET for AC Signal?

 

[FvM]

Did you already review the previous threads at edaboard related to JFET as variable resistor? Some are listed in the "Similar Threads" below, but there are more. What did you learn from the discussion, your post doesn't suggest any impact of it?

The lowest rds,on I'm aware of is achieved by U290, which is presently manufactured by Interfet.

You should tell other relevant application parameters besides on-resistance.

 

[poorchava]

I'm currently designing a circuit involving voltage controlled resistance. So far i've made simulations which proove, that this method is possible, but involves lots of auxiliary components.

Currently I'm going to experiment with photoresistors (i've seen some optocouplers with photoresistor output). Photoresistors may proove to be much easier to control, since control voltage isn't connected in any way to signal line. Problems may concern instability (someone pointed that out on some forums which i read) and converting control voltage to current (one should drive LED's with current, not voltage)

 

[sgugan]

Did you already review the previous threads at edaboard related to JFET as variable resistor? Some are listed in the "Similar Threads" below, but there are more. What did you learn from the discussion, your post doesn't suggest any impact of it?

I am aware of FET as a VCR for DC. Since FET has reverse diode between source and drain how can we use for AC signal. This is what my direct question. through review of old threads unable to get solution for this problem.
initially i was trying with LDR and found NSL32 photo resistive opto coupler but it has ON resistance 500 ohm.i don't know the availability of low resistance LDRs.
My spec is.
resistance range: 1 to 500 ohms
current rating : 1 to20 mA
Voltage : 100mV to 2V

 

[Orson Cart]

There are some devices out there based on a LED and a photo sensitive resistor (LDR, cadmium sulphide I think) in a package (4 pin), opto-resistor, they go from a few M-ohms down to about 50 ohm - very linear resistance - unlike a fet - you can also make it your self from the two elements - a bright red led works well) Regards, Orson Cart

here's one that works down to 1.5 ohm: http://www.qsl.net/wa1ion/vactrol/vactrol.pdf

and a list of available: Silonex Inc.: Products: Audiohm Optocouplers

 

[FvM]

JFET don't have a substrate diode. The said U290 seems basically suited, a parallel circuit of several devices will be necessary to achieve 1 ohm, because the specified rdson range is 1 - 3 ohm, see the datasheet. 2V should be acceptable with standard linearization circuit depending on your linearity requirements.

There's a previous thread of the OP about LDR: https://www.edaboard.com/threads/201254/#post848217

 

[sgugan]

Thanks for your valuable replies..
i try with U290 and VTL5C3.

 

[FvM]

here's one that works down to 1.5 ohm

Looking sharp at the datasheet suggests, that the minimum resistance is 1.5 kohm rather than 1.5 ohm.

 

[Orson Cart]

Where it says 40mA for 1.5 ohm, page one, 5ohm @ 10mA,

the graph does seem to show that at 50 degC after 24 hrs, ~1k5 ohm is the best available for 40mA.

 

[FvM]

Where it says 40mA for 1.5 ohm, page one, 5ohm @ 10mA

it says 1 mA for 30 kohm one line above. I also didn't realize this at first sight, chasing for the spectacular low resistance. But it's a trivial typo, obviously. The datasheet also states for the other part VTL5C4 features a very low “on” resistance, which is actually 75 ohm typ.