various opamp questions - diyAudio
Go Back   Home > Forums > Design & Build > Parts

Parts Where to get, and how to make the best bits. PCB's, caps, transformers, etc.

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
Reply
 
Thread Tools Search this Thread
Old 20th September 2006, 06:44 PM   #1
traw is offline traw  United States
diyAudio Member
 
Join Date: Sep 2005
Default various opamp questions

have some more specific questions about passive components around opamps. learned things from snippets here and there, but haven't hit anything that is like all things opamps for audio circuits...

1. input cap followed by resistor to ground, 1/2piRC low rolloff and dc block. still need 1k resistor going to + on NI config?

2. does single 1k, for example, straight to + input just make sure there's voltage? is that same as input straight to + no inline resistor but a resistor to ground purpose? basically explanation for when that resistor is inline from signal to + and other times not.

3. input to resistor, resistor to ground. voltage divider. any similarity to resitor to ground, then input to resistor prior to going to + input (kind of how a pot out in front would look)? kind of a fuzzy question

actually think i maybe generalizing too much... mostly in reference to non inverting config.

4. capacitor parallel with feedback, 47pf for example, that to cut HF noise?

5. output side. use of 100-300 ohm to help deal with capacitive lines / loads. required if wish to use cap then resistor to ground ? what happens if just capacitor and no resistors (worst case). curious...

6. whats the cap across the + to - for again? stability?...

just try to gather up questions that don't have clear answers in my head. often resources cover basics, ideal opamp theory, jump into various applications (integrators, filters etc.).... have looked over 'opamps for everyone'.... but maybe someone can chime in with explanations for which way when/why
  Reply With Quote
Old 20th September 2006, 09:00 PM   #2
diyAudio Member
 
jan.didden's Avatar
 
Join Date: May 2002
Location: Great City of Turnhout, Belgium
Blog Entries: 7
Default Re: various opamp questions

Quote:
Originally posted by traw
have some more specific questions about passive components around opamps. learned things from snippets here and there, but haven't hit anything that is like all things opamps for audio circuits...

1. input cap followed by resistor to ground, 1/2piRC low rolloff and dc block. still need 1k resistor going to + on NI config?

2. does single 1k, for example, straight to + input just make sure there's voltage? is that same as input straight to + no inline resistor but a resistor to ground purpose? basically explanation for when that resistor is inline from signal to + and other times not.

3. input to resistor, resistor to ground. voltage divider. any similarity to resitor to ground, then input to resistor prior to going to + input (kind of how a pot out in front would look)? kind of a fuzzy question

actually think i maybe generalizing too much... mostly in reference to non inverting config.

4. capacitor parallel with feedback, 47pf for example, that to cut HF noise?

5. output side. use of 100-300 ohm to help deal with capacitive lines / loads. required if wish to use cap then resistor to ground ? what happens if just capacitor and no resistors (worst case). curious...

6. whats the cap across the + to - for again? stability?...

just try to gather up questions that don't have clear answers in my head. often resources cover basics, ideal opamp theory, jump into various applications (integrators, filters etc.).... have looked over 'opamps for everyone'.... but maybe someone can chime in with explanations for which way when/why

1 each input must see a path to gnd for DC, because those inputs have tiny currents, which have to go somewhere. Whether it is 1k depends on the topology. Generally, you want to have equal DC impedance from each input to ground, to balance the tiny input DC currents to avoid DC on the output (aka DC offset). But the feedback resistors also determine the DC impedance so it is more involved (but not difficult).

2 not sur I understand but if you do DC coupling on the input (can do that if your source is at 0V DC) you automatically provide a path for the tiny DC input currents and the inline resistor may just be there to limit input current in case of overload which might damage the chip.

3 I wouldn't do this, seems like a waste of input signal. have you actually seem this? May be done to decrease the input signal if there is a danger of overload, but it is better to have the circuit being able to handle the expected input level.

4 No, is sometimes necessary for stability with heavy feedback. Value is normally less than 47pF, sometimes just 10pF. Depends on amp chracteristics, often done by trail and error.

5 series R is indeed to decouple cap load (or cap cable) from output for stability. Don't know about the R and C, unless you refer to power amps where this is sometimes done (called a zobel) to maintain amp load with long speaker cables. Those speaker cables are inductive and have a climbing impedance for increasing freq and therefore make the load (speaker) at the end of that cable decrease. If the amp became unloaded that could give stability problems. But, if this zobel is used, it should be directly at the output before the series output resistor.

6 you mean + and - power supply? That's done to improve the smoothness of the supply, to prevent ripple and noise on the supply that could appear at the output as noise or distortion. Cap across + and - inputs is to improve stability by increasing what is called the noise gain. It is a bad way to improve stability and generally is bad design practise.

Hope this helps.

Jan Didden
__________________
I won't make the tactical error to try to dislodge with rational arguments a conviction that is beyond reason - Daniel Dennett
Check out Linear Audio Vol 7!
  Reply With Quote
Old 20th September 2006, 09:16 PM   #3
traw is offline traw  United States
diyAudio Member
 
Join Date: Sep 2005
that helps...

3 - seems most clones go 1k then 22k to ground. then additional throw a pot in front of that. in some educational material, they often omit resistors on input. sometimes show a resistor to ground saying to ensure a voltage. also seen resistor in series for same reason (hope i'm not mixing up NI and I config). some more reading seems to indicate on inverting gain config, input impedence essentially that resistor value.

5 - more less dc blocking thing.... similar to how cap in series, then resistor to ground. seen on rod elliot p88 ~ 2uf then 100k to ground at output. although i imagine the next stages input impedence trumps that 100k but suppose 100k is have least impact when parallel with that load

4, 6 - saw this on commercial pro amp schematic and other places around . maybe brute force accomodate for the worst...
  Reply With Quote
Old 21st September 2006, 11:22 AM   #4
diyAudio Member
 
jan.didden's Avatar
 
Join Date: May 2002
Location: Great City of Turnhout, Belgium
Blog Entries: 7
Quote:
Originally posted by traw
that helps...

3 - seems most clones go 1k then 22k to ground. then additional throw a pot in front of that. in some educational material, they often omit resistors on input. sometimes show a resistor to ground saying to ensure a voltage. also seen resistor in series for same reason (hope i'm not mixing up NI and I config). some more reading seems to indicate on inverting gain config, input impedence essentially that resistor value.

5 - more less dc blocking thing.... similar to how cap in series, then resistor to ground. seen on rod elliot p88 ~ 2uf then 100k to ground at output. although i imagine the next stages input impedence trumps that 100k but suppose 100k is have least impact when parallel with that load

4, 6 - saw this on commercial pro amp schematic and other places around . maybe brute force accomodate for the worst...
3. The major problem here is to be able to separate the BS from the sensible. Everybody jumps on the chip amp bandwagon and some info is plain disastrously wrong. There is no clear solution to this and is a disadvantage of the way the 'net works. read the datasheet, while not iffalible, at least it doesn't tell you plain BS. The data sheet has tested designs, and READ the text. You'll learn!

5 I may have misunderstood you previous question. If you do a cap in series with the output, you may include the R to ground after the cap to make sure that whatever you will drive with it sees a path to gnd for the reasons discussed on the inputs. Also you need to provide a ground path for the series cap to charge (at switch-on and discharge at switch off. If no R to gnd you have the risk of two caps in series (the output cap and the input cap of the next equipment) and that may give rise to slicks and pops at switch-on.

4,6 yes, its possible.

Jan Didden
__________________
I won't make the tactical error to try to dislodge with rational arguments a conviction that is beyond reason - Daniel Dennett
Check out Linear Audio Vol 7!
  Reply With Quote
Old 10th October 2006, 12:42 AM   #5
traw is offline traw  United States
diyAudio Member
 
Join Date: Sep 2005
another question...

on the input side. say device is said to have 10k input impedance. want to put cap for dc blocking. if i put 2.2uf, then 22k to ground... is it 22k ||10k for the 1/(2piRC) calc for cutoff? where would be 10.5 hz versus 3.28 if figuring the 22k alone? or is there a different way to look at it?
  Reply With Quote
Old 13th October 2006, 10:25 PM   #6
mikee55 is offline mikee55  United Kingdom
diyAudio Member
 
mikee55's Avatar
 
Join Date: Dec 2005
Location: West Howe
Lightbulb Been here?

http://sound.westhost.com/dwopa.htm:)
__________________
If it don't work, I'll fix it in the mix! Or visit http://lsdp.proboards.com/index.cgi
  Reply With Quote

Reply


Hide 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
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
A couple basic questions on opamp I/V NeoY2k Digital Source 6 13th November 2008 11:55 AM
Opamp line preamp questions Tolu Solid State 27 3rd November 2008 03:36 PM
dvd 2900 opamp questions... Htguy Digital Source 3 4th July 2005 02:19 AM
opamp questions audiokid25 Digital Source 3 26th April 2005 01:31 AM
DRV134 Input Resistor & Opamp Output Questions Devil_H@ck Chip Amps 10 16th September 2004 10:24 AM


New To Site? Need Help?

All times are GMT. The time now is 05:18 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2