VCR - Voltage Controlled Resistor

Discussion in 'General Electronics Chat' started by MachineHum, Mar 20, 2015.

  1. MachineHum

    Thread Starter Member

    Nov 3, 2014
    70
    4
    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?
     
  2. OBW0549

    Well-Known Member

    Mar 2, 2015
    1,328
    890
  3. MachineHum

    Thread Starter Member

    Nov 3, 2014
    70
    4
    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
     
  4. OBW0549

    Well-Known Member

    Mar 2, 2015
    1,328
    890
    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
     
  5. crutschow

    Expert

    Mar 14, 2008
    13,056
    3,245
    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.
     
  6. ian field

    Distinguished Member

    Oct 27, 2012
    4,415
    784
    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.
     
Loading...