Improved OpAmp absolute value amplifier  - Page 1
Forum section: electronics / beginners

author: Unixon
author_rank: Frequent Contributor
timestamp: February 05, 2014, 08:43:46 pm
content: Hi! Need a little advice on the topic.I have a circuit that outputs a signal withing range 0V .. Vcc around the quiescent value Vref = Vcc/2 and I need to convert this to absolute value in the same range (0 .. Vcc) as the original signal. Most of schematics for absolute value amplifiers I found uses two opamps and suggests that output range is only a half Vcc/2 .. Vcc of a full power supply range. I tried to figure out how to redesign these circuits to output at full range using just thesame two opamps, but couldn't come to a solution in short time. The PCB area in this project is very limited and I can't go on with adding another IC package.Can anyone suggest a modification of either of following circuits or a an alternative schematic?http://www.cirvirlab.com/index.php/tutorials/77-op-amp-absolute-value-amplifier-with-one-diode.htmlhttp://www.linear.com/solutions/1608I know there are lot other arrangements for such circuit, these two are provided just for the reference.

author: Unixon
author_rank: Frequent Contributor
timestamp: February 07, 2014, 09:13:39 am
content: Up

author: Rerouter
author_rank: Super Contributor
timestamp: February 07, 2014, 10:14:52 am
content: Your not clear on what the transformation needs to be, it sounds like your signal is +-Vcc/2 offset by Vcc/2, and you want to represent that as +-Vcc?sas such am left wondering why a differential amplifier with a varied gain would not work?

author: AlfBaz
author_rank: Super Contributor
timestamp: February 07, 2014, 10:19:04 am
content: Isn't what you are suggesting just rectifcation?If your output is only 0 to Vcc/2 why not just add a x2 gain stage

author: Unixon
author_rank: Frequent Contributor
timestamp: February 07, 2014, 11:00:23 am
content: Yes, it is a rectification, but I want the signal to be rectified and amplified at the same time.Vinvaries from0VtoVccand when there's no meaningful signalVinsettles atVref=Vcc/2.Voutvaries fromVcc/2toVccin the original scheme and I wantVoutto vary from0VtoVcc.The problem is that I can't just drop in another amplification stage which would be perfectly easy way to go.I need first two opamps to produce this x2 gain along with rectification.So I'm looking for a configuration of feedback circuit that would allow to do this rectification+amplification with only two opamps.

author: Smokey
author_rank: Super Contributor
timestamp: February 07, 2014, 11:36:29 am
content: This is one of those trick circuit app notes that I always remember but haven't had an actual use for yet. Check it out.http://www.analog.com/library/analogdialogue/archives/44-04/absolute.html

author: Unixon
author_rank: Frequent Contributor
timestamp: February 07, 2014, 06:06:03 pm
content: Yes, I've seen this before. A little different arrangement than what I had provided for the reference, but just the same result.I found required configurations for positive and negative signals (relative toVref) separately, but not for complete signal.

author: AlfBaz
author_rank: Super Contributor
timestamp: February 08, 2014, 01:07:44 am
content: I know there a lots of sneaky little circuit tricks around but I'm struggling to see what you want achieved without an extra opampanda negative supply.If you do find a solution please post it here

author: Unixon
author_rank: Frequent Contributor
timestamp: February 08, 2014, 05:46:38 am
content: Adding an extra opamp is easy, but I want to squeeze this easy circuit into only two opamps.The reason is available PCB area. I can afford one SO8 or a couple of SOT23-5 packages,but adding an extra opamp will force me to switch to SO14/SO16 or add an extra SOT23 package.I basically have no space for this including feedback resistors that will eat free space even faster.As for the signal itself,I want to achieve twice as better scale in further processing by rectifying it with x2 gain.All circuits are unipolar, working voltage is 5V (3.3V should be also OK).Maybe I should look for even tinier packages like SC-70 and switch to 0402 for resistors (from 0603)...Fortunately, I will be able to afford adding an extra opamp... not sure if I should proceed this way.