? Op amp chip with inbuilt negative voltage gen?  - Page 1
Forum section: electronics / beginners

author: VK5RC
author_rank: Unknown
timestamp: August 02, 2014, 09:40:59 am
content: I am building an rf power amp and want to monitor the power output of the power amp module with an arduino. The whole thing (power module, sequencer, relays etc) is powered by 13v dc, and the PA module has an rf out sniffer circuit that gives 0 to Negative 0.8v.To stick it up the arduino it would be nice to invert (ie make positive) and get to about 3v or so.I have been thinking about using a 741 op amp and icl7660 dc inverter. Is there a op amp/combined dc inverter that can do it from only a positive supply?PS it only needs to go from dc to a few hertz.edit; to make clear voltage sensed is negative wrt ground

author: Simon
author_rank: Unknown
timestamp: August 02, 2014, 09:44:01 am
content: As you are already using the arduino why not use an analogue input with a 1.1V converter reference ? the inversion can be done in software by subtracting your ADC value from 1023

author: mij59
author_rank: Frequent Contributor
timestamp: August 02, 2014, 10:16:27 am
content: Quote from: Simon on August 02, 2014, 09:44:01 amHi,This is not possible, the input voltage needs to be positive, also min. voltage at any pin = -0.5V.

author: mij59
author_rank: Frequent Contributor
timestamp: August 02, 2014, 10:19:29 am
content: Quote from: VK5RC on August 02, 2014, 09:40:59 amHi,Try an inverting opamp, use e.g. LM358 (pos. supply voltage +5V), you won't need a negative rail.

author: Simon
author_rank: Unknown
timestamp: August 02, 2014, 10:20:39 am
content: uh yes I hadn't noticed that the voltage was negative, and neither has the poster before this apparently.

author: VK5RC
author_rank: Unknown
timestamp: August 02, 2014, 10:49:43 am
content: thanks for the comments so far, edited OP to make more clear.

author: David Hess
author_rank: Super Contributor
timestamp: August 02, 2014, 04:57:09 pm
content: An operational amplifier with a common mode range including ground can operate as an inverting amplifier with a negative input.One neat trick for generating a low voltage negative (or positive) bias supply is to use an optocoupler as an isolated photovoltaic generator. Output currents of 200 microamps are feasible this way which are plenty to power a micropower operational amplifier like an OP-90.I know a truly marvellous way to do the same thing at a lower power level using a single resistor and transistor which this post is too short to describe.

author: Andy Watson
author_rank: Super Contributor
timestamp: August 02, 2014, 05:11:03 pm
content: Don't use a 741 - it requires volts of headroom to work properly. Look for op-amps that calm rail to rail operation, and common mode input range that includes the negative rail. Something like LMC6482 (although that's probably a bit old-hat these days!)@David Hess: Can't you write the solution in the margins? There's plenty of unused space there!

author: Bored@Work
author_rank: Super Contributor
timestamp: August 02, 2014, 05:26:40 pm
content: Quote from: Andy Watson on August 02, 2014, 05:11:03 pmI'll guess it is a trick Bob Pease once published.http://electronicdesign.com/analog/bob-s-mailbox-negative-voltage-feeding-dual-channels-and-mysterious-1n430http://electronicdesign.com/site-files/electronicdesign.com/files/archive/electronicdesign.com/files/29/6362/figure_01.gif

author: David Hess
author_rank: Super Contributor
timestamp: August 02, 2014, 05:36:00 pm
content: I tried finding a link to Bob Pease's original article discussing it but failed. I might have it on paper somewhere. An optocoupler is much more efficient which Bob Pease mentions.Some operational amplifiers do include internal charge pumps. I knew about the LTC1152 but the ADA4858-3 is new to me:http://www.linear.com/product/LTC1152http://www.analog.com/en/all-operational-amplifiers-op-amps/operational-amplifiers-op-amps/ada4858-3/products/product.html

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 02, 2014, 10:08:25 pm
content: Use a single supply or R2R amp, e.g. LM358, TLV2371, etc. in the inverting configuration, +in = GND. -in remains near GND except for transients (e.g. slew rate limiting), so add a back-to-back diode clamp on that pin to ensure the voltage input range is good.Tim

author: Rory
author_rank: Frequent Contributor
timestamp: August 03, 2014, 01:57:19 am
content: VK5RC, Do you have a link to the RF amp module?

author: VK5RC
author_rank: Unknown
timestamp: August 03, 2014, 02:14:15 am
content: Thanks for the comments, I ASSUMED (always a bad thing to do) the op amps rails would need to be greater (both negatively and positively than inputs into the circuit. A op amp that can go down to the earth rail , in an inverting configuration should work I think. I need to do some more reading and a little experimentation to get it into my head. Thanks for the input.

author: Rory
author_rank: Frequent Contributor
timestamp: August 03, 2014, 02:26:53 am
content: Reason I ask is I'm wondering if you could simply reverse the polarity of the detector diode in the "rf sniffer" and use a jellybean R2R opamp and single supply.

author: David Hess
author_rank: Super Contributor
timestamp: August 03, 2014, 02:47:18 am
content: All single supply operational amplifiers typically support common mode voltages slightly outside of one supply rail because otherwise variation would mean that some do not support operation *at* the supply rail. There are other limits however like input protection diodes or junction isolation which will limit operation to within about 0.3 volts.The output voltage is a different matter though since they can only sink or source current to a supply rail without outside help and the same protection diode or junction isolation issue comes up.A single transistor with shunt feedback is an inverting amplifier and will also work with negative inputs generating a positive output but of course since it is not balanced, it also has a large offset voltage drift of about 2mV/C.

author: sarahMCML
author_rank: Regular Contributor
timestamp: August 03, 2014, 03:48:11 am
content: Hi,As you're using an Arduino, if you have a spare I/O port (even better a spare timer one), how about using it with two diodes and two capacitors and making it generate your own negative rail for the op-amp! Just a thought.Regards,Sarah.

author: VK5RC
author_rank: Unknown
timestamp: August 03, 2014, 11:43:58 am
content: Thanks for the replies@Rory, I seriously thought about it, but the sniffer circuit rectifying diode, SMT prob 0805 size, is quite close to the RF power device, an XRF 286, which is for good measure mounted on a large chunk of copper heat spreader about 2 inches square and 1cm thick, I think my ability to stuff it up while removing it would be pretty good.@sarah, that's a good idea, I've used it before to generate a low current voltage supply greater than rail (positive), it meets my aim to keep it simple.For interest the RF power amplifier is for Amateur radio and I am having a go at Moon-bounce, (communicating via moon RF reflection) round trip ~10^20dB lossTransmit side uses an Icom 910H 1296MHz 10W out, into W6PQL splitter then two XRF286s with combiner, I should get 150W out. LDF450 coax.Antenna VE4MA feedhorn, (return loss 17dB) circular polarisation, onto 3m satellite dish (steerable 150 degrees Azim, 80 degrees Elev)Receive G4DDK design PHEMT pre-amp, 37dBgain 0.27dB noise factor, back into Icom 910HSequencing of relays, power supply by MiniKits sequencer.Arduino; Current plan is for display of voltages (12 and 28v rails) and current draw, power out (via sniffer), later plan is to use the Arduino also to track the moon.Again Thanks, RobPS photos of PA, looking down feedhorn and pre-amp