highest frequency for jfet in triode(ohmic) mode

Discussion in 'General Electronics Chat' started by Don8, Aug 13, 2014.

  1. Don8

    Thread Starter Member

    Jul 22, 2014
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    I wanted to use a jfet as a voltage controlled resistor. This application needs to be used into the RF spectrum @2.5 GHz. Will it work? I find a lot of information on jfets, and frequency but I can't determine if a small signal @3GHz <50mv can be applied to a jfet without any parasitic capacitive/inductive effects taking over.
     
  2. crutschow

    Expert

    Mar 14, 2008
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    That value of the parasitics depends upon the design of the JFET. At 2.5GHz just about any device has parasitic inductance/capacitance that will affect the circuit response so you will have to take that into account in the circuit design.
     
  3. Don8

    Thread Starter Member

    Jul 22, 2014
    31
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    Do you know of any jfets that are designed to minimize the parasitics? I've been looking, but I haven't seen any mention of time or frequency on a VCR data sheet.

    Perhaps, I should get a wide bandwidth jfet and just use that instead?
     
  4. Papabravo

    Expert

    Feb 24, 2006
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    The only way to know for sure is to get a network analyzer and characterize some devices. Can you get access to one?
     
  5. Don8

    Thread Starter Member

    Jul 22, 2014
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    1
    I'd like to just find a VCR that has SOMETHING about frequency or time on it's datasheet, or get a good part number from someone, but if it's down to trial and error I guess I understand.

    How about an estimate of success? is it even likely that any jfet will be able to operate in the triode mode at 2.5Ghz. Has such a time ever been done?
     
  6. crutschow

    Expert

    Mar 14, 2008
    13,019
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    Here's some references to microwave FETs. Note that GaAs MESFETs are higher frequency type devices than standard silicon JFETs and are commonly used as microwave amplifiers. Here's one manufacturer of such devices.
     
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