comp.arch.embedded fet for automatic gain control?
Any suggestions for an enhancement mode mosfet to serve as a variable resistence element in an automatic gain control type circuit? Power involved is a bare whisper; worst case voltage is probably 30 volts. There may well be a better remidy than an mosfet, but it looks good at this point.
Hul
Funnily enough I was going through some old log-books the other day and spotted one circuit I used in the mixer console of the dual-deck, microphoine and auxilliary inputs used by DJ's. This was a 1970's era design which was less noisy in operation than some of the currently available Op- Amps. It used dual gated MOSFET's to provide the gain control aspect. Not sure if you would find any of them around now though.
-- ******************************************************************** Paul E. Bennett IEng MIET..... Forth based HIDECS Consultancy............. Mob: +44 (0)7811-639972 Tel: +44 (0)1235-510979 Going Forth Safely ..... EBA. www.electric-boat-association.org.uk.. ********************************************************************
That does ring a bell. The 2 channel aspect with one varying gain probably wouln't fit this application but...
Hul
Paul E Bennett wrote:
For those interested I found a couple of links that might prove useful reading.
-- ******************************************************************** Paul E. Bennett IEng MIET..... Forth based HIDECS Consultancy............. Mob: +44 (0)7811-639972 Tel: +44 (0)1235-510979 Going Forth Safely ..... EBA. www.electric-boat-association.org.uk.. ********************************************************************
Do not use a MOSFET as a variable gain resistor, use a JFET. The best ones are 2N4391 and another one I have used which is practically the same but under another name (same process, was it 4856... no, I don't remember). You can count on it going to as low as 30 Ohm over temperature at
0V G-S; linearity is much better than with other types (still far from perfect, you can get around 1% INL but nothing better without extra measures). Temperature drift is also huge of course, compensating that takes some more extra measures.Dimiter
------------------------------------------------------ Dimiter Popoff, TGI
------------------------------------------------------
I'm think that dual gate MOSFET like BF998 will be suitable.
Thanks for the suggestion, I'll take a look.
Hul
Artem wrote:
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.