Re: JFET gain control "resistor" in Weinbridge oscillator



George Herold wrote:

Hi All, This is my first time posting though I?ve read some of the
discussions. I?ve been having problems with some of the Weinbridge
oscillators that are in our lockin amplifiers. I won?t bore you with
details, but a circuit that was working for years now seems to be
having problems. I used a light bulb as the gain control element.
But I?ve now decided to try moving into the 20th century and use a
JFET as the control element. The circuit I?m modifying comes from J.
Williams at Linear (AN-47). My problem is finding a replacement JFET
for the 2N4338. Ohh I can still find these on the web, but for $4.30
each!!! I found both the PN4119 and J201 as possible replacements and
was looking for any other suggestions. The important parameter is the
on resistance of the FET which should be in the 1k to 3k ohm range.

I suspect I know that app note. I'd just look for fets with similar data.

Beware btw. The time constants of the control voltage of the FET can cause
trouble (inconsistent output).

Graham

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