Custom Search

More than 10,000 searchable pages indexed.

Greetings: There is so much good stuff on RF Cafe that there is no way to list or link to all of it here. Please use the Search box or the Site Map to find what you want - there is a good chance I have it. Thanks!

•−•  ••−•    −•−•  •−  ••−•  •
RF Cafe Morse Code >Hear It<
Job Board
About RF Cafe™
Copyright 1999-2015

Switching Negative Supply line using Positive control - RF Cafe Forums

Because of the high maintenance needed to monitor and filter spammers from the RF Cafe Forums, I decided that it would be best to just archive the pages to make all the good information posted in the past available for review. It is unfortunate that the scumbags of the world ruin an otherwise useful venue for people wanting to exchanged useful ideas and views. It seems that the more formal social media like Facebook pretty much dominate this kind of venue anymore anyway, so if you would like to post something on RF Cafe's Facebook page, please do.

Below are all of the forum threads, including all the responses to the original posts.

Post subject: Switching Negative Supply line using Positive control Posted: Mon Aug 27, 2007 4:52 pm


Joined: Fri Jan 26, 2007 2:40 am
Posts: 14
OK, I've got this problem where I have to turn be able to control power to sections of my board during TX or RX mode. For example lets say I've got a +5V supply that runs to some Op Amps. Some are used in TX mode and some are used in RX mode but that they aren't on all at the same time. We've used a control line (coming from a PIC, 0 to 3 V) to drive a NMOS Fet which in turn drives current thru a gate resistor of a PMOS device to either open or short the PMOS to connect the +5V line to the Op Amps. We repeat this circuit for TX and RX.

That works fine for a +5V but what about the -5V? The first problem is the control line, which is too high a voltage to switch a Fet. I obviously have to level shift but I can't seem to get any fet to drive completely ON since I also have the problem in that I don't have access to a voltage lower than -5V.

I've tried to run a search on the net for this but I keep coming up with "Switching power supplies" or some such. Any idea what this kind of problem is called? That might help my search.




Post subject: Posted: Tue Sep 11, 2007 5:14 pm


Joined: Fri Feb 02, 2007 5:22 pm
Posts: 5
Location: Overland Park, KS
How about 3V uC drives gate of open collector inverter or buffer.

Buffer drives gate of PMOS. Drain of PMOS is at 5V. Gate of PMOS pulled to drain via 1k.

Source of PMOS tied to gate of NMOS. Source of NMOS tied to -5V. Gate of NMOS pulled down to source via 1k. Drain of NMOS drives load.

I have used this configuration to gate-switch large GaAs FETs and it works well.

Posted  11/12/2012

Your RF Cafe
Progenitor & Webmaster

Click here to read about RF CafeKirt Blattenberger… single-handedly redefining what an engineering website should be.

View the YouTube RF Cafe Intro Video Carpe Diem!
(Seize the Day!)

5th MOB: My USAF radar shop

Airplanes and Rockets: My personal hobby website

Equine Kingdom: My daughter Sally's horse riding website