OPamp current source - Page 1
Forum section: electronics / beginners

author: csheldon
author_rank: Contributor
timestamp: December 20, 2013, 09:50:23 pm
content: Hello to everyone !I will be frank and I'll ask a question right away.I've designed a circuit consisting of a few stages with single task -> to put a 1 amp peak current of exponential function (trise=10uS and tfall=340us) every 30secs-2min (depending on the user adjustable potentiometers and trimmers) . Every stage works fine , except this one last stage , where i used a opamp as shown , to sink current I=(Vin/1ohm) (1amp peak). My idea was to put a generated exponential function through a voltage follower -> resistor divider(1V peak) , and then to this FET/bipolar current source. I measured a voltage drop across 1 ohm resistor , and it was an oscillatory response within the envelope of a distorted input signal. what can i do to reduce the oscllation aand make an opamp stable and non oscilatory output ? I am using tl072.Thanks in advance to all skilled engineers out there who will know the solutions , please helpEdit: the 10k potentiometor from 50 Vdc to collector is actually a 30-50 ohm load. sory for the mistake.

author: prenato
author_rank: Regular Contributor
timestamp: December 20, 2013, 10:05:32 pm
content: Stability issues with this arrangement aren't uncommon. I've run into them in my constant current load project and describe the solution I found there:http://paulorenato.com/joomla/index.php?option=com_content&view=article&id=91:constant-current-load&catid=4:projects&Itemid=4I don't think you will be able to use the "heavy-handed" dominant-pole compensation using the 470Ohm and 0.1 uF because that will make the circuit way too slow but you may want to try the RC compensation in the feedback path...Google Ron Mancini's apnotes on opamp stability from Ti for more info...10/340 us exponential sounds familiar... Is this some sort of surge voltage pulse you are trying to generate?

author: csheldon
author_rank: Contributor
timestamp: December 20, 2013, 10:10:08 pm
content: Thank you for your response. It is actually a generator intended for certain scientific measurements . It simulates the lightning strike on a grounding electrode.I don't think i could use a 470 ohm and 100nF , because the circuit will indeed become too slow , but honestly i dont have an idea how to solve it

author: prenato
author_rank: Regular Contributor
timestamp: December 20, 2013, 10:24:08 pm
content: Quote from: csheldon on December 20, 2013, 10:10:08 pmYes, I used to work on telecom systems that were surge (lighting) protected and the 10/340 us exponential is a standard test pulse fro one of the compliance specs.If you look in my circuit, there are two compensation networks: the 470R + 0.1uF that, as we both noted, you could not use because it would be to slow, *and* a second parallel RC network (10n/1K) in the voltage feedback path. The later you might be able to use though you may need to adjust the component values...Regards,

author: csheldon
author_rank: Contributor
timestamp: December 20, 2013, 10:34:09 pm
content: Can you please be so kind and draw it on my schematic how do i need to rearrange it and connect it and how do I choose a component values. I would be most gratefull , since i have no experience with this problem whatsoever

author: plesa
author_rank: Frequent Contributor
timestamp: December 20, 2013, 10:57:21 pm
content: What about using hi-current opamp like OPA548?

author: prenato
author_rank: Regular Contributor
timestamp: December 20, 2013, 11:00:11 pm
content: Quote from: csheldon on December 20, 2013, 10:34:09 pmSee attached. As I mentioned, these values may need to be adjusted for your application. Unfortunately, there is some math involved in understanding OPAMP compensation and you may have to do some reading-up on it to optimize the values. These should be a good starting point though.

author: csheldon
author_rank: Contributor
timestamp: December 20, 2013, 11:13:27 pm
content: Thanks. Maths is not a problem , only problem is , where can i find a literature that is less theory , and more of a practicall approach. I googled it , but almost everything i find is more of a theoretical approach

author: prenato
author_rank: Regular Contributor
timestamp: December 20, 2013, 11:59:16 pm
content: Here's a good resource:http://www.ti.com/lit/an/sloa020a/sloa020a.pdf

author: mrkev
author_rank: Regular Contributor
timestamp: December 21, 2013, 03:17:06 am
content: This is quite a common problem. The easy destription is that the high freq. of signal is delayed by opamp and the transistors enough to make a positive feedback (instead of negative, usually around the maximum freq it can do). So all you need to do is to block that high freq. by more direct feedback.Easiest way how to do that is to take some cap around few pF and try to put it from output to negative input, or between the pos and neg inputs. (directly on the board as close to OpAmp as you can). That usually works...

author: Rerouter
author_rank: Super Contributor
timestamp: December 21, 2013, 03:42:06 am
content: Try a 100 ohm resistance on the output and a few hundred picofarad between the output and inverting input pin, it'll slow it down a bit, but it will be more stablethe 100 ohm resistance is because op amps dont like directly driving capacitive loads (which a fet gate is)

author: mrkev
author_rank: Regular Contributor
timestamp: December 21, 2013, 03:47:17 am
content: Quote from: Rerouter on December 21, 2013, 03:42:06 amI don't think that those few pF this fet has is such a big dealOfc. it won't hurt

author: csheldon
author_rank: Contributor
timestamp: December 21, 2013, 10:53:14 am
content: Thank you all very much for your replies , I will try to implement the methods i have learned about this topic and will post the results and waveform asap.

author: csheldon
author_rank: Contributor
timestamp: December 21, 2013, 08:53:52 pm
content: Thank you very much for your reply.I am using BS107 FET (200Vdc max Uds) , my external supply is delivering 50 volts , that's correct.I wired the thing up today , I put a 100 ohm resistor from opamp output to MOSFET BS 107 gate , 33nF (actually 15nF would be quite right , but i didn't had one lyin around at the time) from output to inverting input of the opamp , and 1k from the emiter 1ohm resistor to the inverting input.It seems to work extraordinary (test circuit built on breadboard). 33nF kills the oscillation completely , but i noticed I no longer have trise=10uS , but instead 20 us (maybe because this 33nf cap cuts the peak ) , even though , the input signal is just as it needs to be , but i think , 15nF will work out just fine.

author: Rerouter
author_rank: Super Contributor
timestamp: December 21, 2013, 09:07:30 pm
content: yes the 33nf would really slow things down, you can trade this off by trying smaller capacitance's,

author: csheldon
author_rank: Contributor
timestamp: December 21, 2013, 09:48:45 pm
content: That's right , i also had a 1nF capacitor and i tried with it but it just wasn't enough , although the oscillations were a litlle bit more dumped , it was insufficient capacitance

author: kg4arn
author_rank: Unknown
timestamp: December 22, 2013, 02:57:42 pm
content: @csheldonDid you place the compensation capacitor the way prenato (emitter resistor to inverting input) shows it or Achmed99 shows it (op amp output to inverting input)?

author: csheldon
author_rank: Contributor
timestamp: December 22, 2013, 04:22:47 pm
content: As i said in the earlier post , i have connected the compensation capacitor from the output to the inverting input. I also put a 100 ohm resistor from output to gate , and instead of directly connecting the inverting input to the emiterr 1 ohm resistor , i've connected it through 1k resistor . and it seems to work very well , but I must lover the capacitance to make things fast enough , because trise is now 20 us , while tfall is left intact , i will experiment with different values and see what goes best

author: csheldon
author_rank: Contributor
timestamp: December 22, 2013, 04:33:55 pm
content: Here is the schematic of the improvement (instead of 15nF , i am having 33nF)

author: kg4arn
author_rank: Unknown
timestamp: December 22, 2013, 09:40:51 pm
content: Thanks to all of you.I was working on a very similar project and having stability problems.A 2A step response has a rise time of 7us.The circuit appears stable unless the load voltage drops below 2.5V and the cb juctions of the TIP142s become forward biased.At that point there is some ringing.

author: mrkev
author_rank: Regular Contributor
timestamp: December 23, 2013, 02:19:28 am
content: Quote from: kg4arn on December 22, 2013, 09:40:51 pmTIP transistors are power darlingtons. So even that bipolar transistors can somethimes go a bit below 0,6V at C-E, it wont be the case here. There is C-E pass of the first transistor and then B-E pass of the second, you won't be able to run it below about 1V (when 1A running trough) or 2V (10A) across that tip142.