VCR - Voltage Controlled Resistor

Thread Starter

MachineHum

Joined Nov 3, 2014
74
I'm looking into applications where I need to control the resistance of an element based on an input voltage... I started by looking at some FET characteristics... and they're not really linear, at all. Once you get into the saturation region, Vgs doesn't even matter and it's just messy.

I hunted around for a while, I found the VCR2N

http://pdf.datasheetcatalog.com/datasheet/vishay/70293.pdf

... and it's not exactly cheap... they can run you up to $10/unit in small volume. But I'm thinking it might fit my application... maybe

I have a few questions, maybe someone may have some experience.

First line - "ac drain-source resistance" so it's not bi-directional? One way resistor?

Later on - "This series of JFET is intended applications where the drain-source voltage is a low level ac signal with no dc comp." now we're getting into impedance, it's actually complex? also this contradicts my first question

Then this:upload_2015-3-20_15-29-1.png

rDSbias - What's this? The resistance shifts as a function of Vgs, Vgsoff (Given a range from -3.5 to -7v), and rds
...So this is trying to tell me that the device is completely non-linear?

Later on i'm given some graphs and some other nice things with an app note explaining voltage divider...

Does anyone either have:

1. More info, or a link to more info, on this part?
2. A better/cheaper solution for a VCR?
 

Thread Starter

MachineHum

Joined Nov 3, 2014
74
Some applications that come into my head right away would be... VCO, VCA, VCFs

More then anything I just want a "goto VCR" in my toolbox

edit: thanks for the link, going over now
 

OBW0549

Joined Mar 2, 2015
3,209
Linear Technology makes a voltage-controlled amplifier, the LT1228:

http://www.linear.com/product/LT1228

One of the application circuits on the datasheet is a voltage-controlled state-variable filter.

THAT Corporation makes very high performance VCA chips, such as this one:

http://thatcorp.com/datashts/THAT_2162_Datasheet.pdf

I don't know any distributors that carry their stuff, though, and their minimum order last time I bought from them was 25 pieces.

For a voltage controlled oscillator, you might look at the LM331 VFC chip:

http://www.ti.com/product/lm331
 

crutschow

Joined Mar 14, 2008
24,397
A bipolar voltage-controlled resistor can be made with a CdS photoresistor controlled by an LED light source, but that's very non-linear also. The two devices would need to be mounted close together in a light-tight package.
 

ian field

Joined Oct 27, 2012
6,539
A bipolar voltage-controlled resistor can be made with a CdS photoresistor controlled by an LED light source, but that's very non-linear also. The two devices would need to be mounted close together in a light-tight package.
Sounds like a need for an op-amp driving a darlington shunt transistor, that would make feedback easy, to tailor the linearity.

The LM13700 flashed through the back of my mind, but I can't put my finger on why I thought of that one - might be worth googling for appnotes and see if anything useful crops up.
 
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