Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Solid State Talk all about solid state amplification.

bypassing for opamp circuits
bypassing for opamp circuits
Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Thread Tools Search this Thread
Old 11th January 2012, 04:40 AM   #1
diy didi is offline diy didi  South Africa
diyAudio Member
Join Date: Sep 2009
Location: Pretoria
Default bypassing for opamp circuits

Ok. So i've got a circuit using opamps. its a balanced line driver. My power supply board is seperate and generating +&- 15V.
The supply rails are well filtered on the power supply side with 100uf caps.
Should 100uf caps be added again on the opamp board from each supply rail to ground?
Also, should I bypass these electrolytics with 100nf caps too?
I did see somewhere that a single 100nf cap from +15V rail to -15V rail was used in another design.
SOOO, what exactly is the general rule here??
  Reply With Quote
Old 11th January 2012, 05:57 AM   #2
xeclipse is offline xeclipse  Kenya
diyAudio Member
Join Date: May 2010
Location: Where the world's best tea come from:Kericho Kenya
100nf cap between the rails as close as possible to the op-amp.10uf (or larger) from the rails to ground not necessarily close to the op-amp but on the same pcb.
if there are no sparks,it probably doesn't work!
  Reply With Quote
Old 11th January 2012, 01:21 PM   #3
diy didi is offline diy didi  South Africa
diyAudio Member
Join Date: Sep 2009
Location: Pretoria
  Reply With Quote
Old 11th January 2012, 05:40 PM   #4
AndrewT is offline AndrewT  Scotland
Join Date: Jul 2004
Location: Scottish Borders
Does the driver datasheet give the guidance you are seeking?
regards Andrew T.
  Reply With Quote
Old 11th January 2012, 06:29 PM   #5
raul_77 is offline raul_77  Europe
diyAudio Member
Join Date: May 2001
Location: Spain
From Op Amp for Everyone 3:

[The Number 1 Design Mistake
I saved the very best and most common mistake for last. And it doesn’t even involve an op amp. It involves support components: the decoupling capacitors!
In Chapter 1, I mentioned some part numbers that are etched in the memory of every design engineer, at least those involved in analog design. There is one other: 0.1 mF.
Need to decouple? OK, everybody knows you put a 0.1 mF capacitor on every power supply input and the job is done, right? I can disprove that truism very easily with two words: cell phone.
Put your cell phone near your prototype circuit, which is bypassed with 0.1 mF, and make a call while monitoring the output on a high bandwidth oscilloscope. You will see horrendous 2.4 GHz leakage!
An alternative version of this problem came from some cellular telephone base
station installers who called in a panic, “We have 90 MHz noise running all over our system—and can’t figure out where it is coming from.” A suspicion on my part asked them to tell me the exact coordinates where they were installing the system, and they provided the exact latitude and longitude. A quick check of the FCC database revealed the problem. I asked them, “Are you anywhere near the tower for W____ 90.5 FM, a 100,000 W NPR station listed at those coordinates?” They told me on the phone that they could see the transmitter 5 ft away—they were colocating with the station!
The point of this is that their board was bypassed with 0.1 mF capacitors. While that worked fine for the digital portions of the board, the analog portions were being clobbered by radiation of the powerful 90.5 MHz FM station. Conventional thinking
is that the lower the value of capacitance, the lower the frequencies it will filter.
So, 0.1uF should get rid of just about everything because it is a very large value (relatively speaking). This conventional wisdom is wrong! The actual case is the exact opposite.
Where did the value 0.1 mF come from, anyway? A high technology store near me used to have antiquated computer boards as a wall decoration. Backlit with white light, the translucent green boards made a pretty sight. But, they were also populated with 0.1 mF decoupling capacitors. A quick survey of the circuitry revealed that the clock rate of the old computer had been 1 MHz.
So, the 0.1 mF capacitor value seems to have come from bypassing TTL logic in the 1960s! Isn’t it time to rethink the issue a bit, in light of op amps and other analog components that can operate to frequencies of 3 GHz, especially when almost every engineer carries a 2 W, 2.4 GHz transmitter into the lab (cell phone)?
The reality of the situation is that a good 0.1 mF capacitor with an X7R dielectric exhibits a resonance in the 10 MHz region. This is due to parasitic inductance creating an LC circuit. Below 10 MHz, its impedance is capacitive, decreasing almost linearly on a logarithmic plot until it reaches the resonant frequency. Above the resonant frequency, the impedance is inductive. Since inductor resists the flow of high frequencies and passes only low frequencies, the decoupling capacitor is useless above its resonant frequency.
Looking at representative plots from capacitor manufacturers, at 100 MHz, the venerable 0.1 mF bypass capacitor has become an inductor with an XL of at least 1 Ohm.
By 2.4 GHz, XL has risen to above 10 Ohm.
A good rule of thumb for effective bypassing is to put several capacitors in parallel. The standard 0.1 mF capacitor does quite nicely for frequencies up to 10 MHz, a 1000 pF NPO dielectric does nicely up to 100 MHz, and 33 pF NPO eliminates frequencies in the 2.4 GHz region. Bulk decoupling of the power supply as it enters the board eliminates low frequency ripple.
Here is a truism to replace the older one: When poor decoupling is suspected, decrease (do not increase) the value of the capacitance.]

Also see the table 2

General Layout Guidelines for RF and Mixed-Signal PCBs - Maxim

Happy days, Raúl
  Reply With Quote
Old 11th January 2012, 10:13 PM   #6
nigelwright7557 is offline nigelwright7557  United Kingdom
diyAudio Member
nigelwright7557's Avatar
Join Date: Apr 2008
Location: Carlisle, England
I always put 100nf from b+ to gnd and b- to gnd.
PCBCAD51 pcb design software. 2018 version out now with lower prices >> http://www.murtonpikesystems.co.uk
  Reply With Quote
Old 12th January 2012, 07:17 AM   #7
qusp is offline qusp  Australia
diyAudio Member
qusp's Avatar
Join Date: Oct 2009
Location: Brisbane, Australia
ive taken to using 10nf lately from each rail to ground, no specific reason other than that i'm running higher clock speeds/bandwidth than i used to. i cant claim a master stroke, i tend to use either a larger 4.7uf x7r or pps film, or the 10nf and sometimes both. they are also slightly cheaper
  Reply With Quote
Old 12th January 2012, 07:34 AM   #8
abraxalito is offline abraxalito  United Kingdom
diyAudio Member
abraxalito's Avatar
Join Date: Sep 2007
Location: Hangzhou - Marco Polo's 'most beautiful city'. 700yrs is a long time though...
Send a message via Yahoo to abraxalito
There's no general rule about decoupling - you'd ideally decouple according to the application. Here you say its a balanced line driver - in such cases there's little point in substantial decoupling to ground, provided that the load is indeed balanced. When the load is balanced, no current flows to ground, so why put a cap to it? However I've experienced oscillations when caps to gnd are totally absent on opamp circuits. So use relatively smaller caps to gnd (say 10nF X7R 0805) with series resistors (1-10R determined empiirically - use the highest value which gives consistent freedom from oscillation).
'The total potential here must be nothing less than astronomical.'
'Nothing less. The number 10 raised almost literally to the power of infinity.'
  Reply With Quote
Old 10th August 2016, 08:15 AM   #9
ginetto61 is offline ginetto61  Italy
diyAudio Member
Join Date: Sep 2009
Location: Milano
Hi !
keeping the low uF bypassing I still have the feeling that especially for opamps asked to output more current,
like the ones used in an headphone amp output stage, more uF very close to the op amps can be beneficial.
When there is an instantaneous peak in the music signal the opamp sucks current from the rail.
The fastest this "over-current" is delivered the best.
So a let's say 1000uF of low esr for each rail on each opamp would be a good trick.
I read here something on the same lines but I cannot find again the post.
I see caps like shock absorbers on a car. When the road gets bumpy you need absorbers with a big stroke. If not they will be bottoming out.
The feeling of a flat and dry sound could depend on this bottoming out of the opamp.
However I do not have a clue about how much the current drawn by an opamp varies with the variation of the output current .
I should place a voltmeter on the opamp rails and see.
Maybe even 470uF could be enough.
But for sure the energy release from an opamp is istantaneuos.
I will try something soon.

I wonder if anyone has carried out any experiment with different uF values.

Regards, gino
Thank you very much indeed
Kind regards,

Last edited by ginetto61; 10th August 2016 at 08:44 AM.
  Reply With Quote
Old 10th August 2016, 08:38 AM   #10
TBTL is offline TBTL  Germany
diyAudio Member
Join Date: Oct 2013
Originally Posted by ginetto61 View Post
keeping the low uF bypassing I still have the feeling that especially for opamps asked to output a little more current, like the ones used in an headphone amp output stage, more uF very close to the op amps can be beneficial.
When there is an instantaneous peak the opamp sucks current. The fastest the current is provided the best.
For audio purposes it does not have to be instantaneously, bandwidth is limited to 20 kHz which is quite slow in electronics. Decoupling is done for stability, so that the circuit does not oscillate (in the MHz region?). Of course it does not hurt to add extra capacitors.

Last edited by TBTL; 10th August 2016 at 08:45 AM.
  Reply With Quote


bypassing for opamp circuitsHide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Bypassing opamp and using transformers...can it be easily done on this kit? jmillerdoc Digital Line Level 1 9th April 2011 04:01 PM
Scythe Kama Bay SD-1100 input section opamp replacement or bypassing? rickedd46 Class D 0 26th June 2010 11:49 PM
Research on tubes and circuits simple circuits Cazcotty Tubes / Valves 6 16th June 2006 12:50 PM
85 powersupply circuits 58 charge circuits gev Power Supplies 0 31st July 2005 11:27 AM
What value of cap for opamp PS bypassing? overmind Solid State 4 13th February 2002 06:47 PM

New To Site? Need Help?

All times are GMT. The time now is 11:34 AM.

Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 15.79%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Copyright ©1999-2018 diyAudio