Re: JFET gain control "resistor" in Weinbridge oscillator
- From: Eeyore <rabbitsfriendsandrelations@xxxxxxxxxxx>
- Date: Sun, 07 Sep 2008 07:39:16 +0100
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'm SURE there's a similar Nat Semi app note but if you can find it, look
at Nat Semi's LB (linear brief) 16 dating from 1971 by Robert C Dobkin. No
fets at all !
http://www.national.com/ms/LB/LB-16.pdf
Keeping those old data books and application guides pays sometimes.
Graham
.
- References:
- JFET gain control "resistor" in Weinbridge oscillator
- From: George Herold
- JFET gain control "resistor" in Weinbridge oscillator
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