Voltage divider resistor values

[drummer4gc]

Hey all,

I'm looking at a booster/preamp circuit which uses a simple voltage divider to provide 4.5V to the non-inverting input of an opamp. Most of the time, I see folks use 100k resistors to form this divider, but this pedal uses 22M resistors. Both methods create 4.5V, but is there a difference between using higher or lower value resistors?

Thanks,
Matt

[ashcat_lt]

Schematics are nice. 

We don't know what you're trying to bias or how, and the answer completely depends on those details.  I could start spitting, but it would be nice to have something to look at and point to.

[drummer4gc]

The schematic is a bit of a sloppy work in progress, which is why I didn't post it yet. I think I drew it upside down. I'm tracing a circuit that was covered in goop and is supposedly fairly rare, so I'm not sure what the rules are here for posting a schematic. I suppose I could post it without naming the circuit....

But like I said, it's meant to be a preamp/booster, using a non-inverting op amp. Signal goes into pin 3 of a TL071 along with 4.5V.

[ashcat_lt]

Crap.

Do the 2.2M resistors meet at the input, or does the 4.5V go through another resistor to get there?

One of the differences between the resistor sizes is the amount of bias current available at the input.  JFET opamps don't need hardly any, so huge resistors are fine, except that they tend to give more noise in the process.

If those resistors meet at the input, then their parallel resistance (in parallel with any  pulldown resistors between there and the input) pretty much sets the input impedance.  Smaller resistors give less noise, but quickly start to "load down" the passive guitar and kill treble.

Which is why there's that "noiseless biasing" thing, where smaller resistors are used in the divider, decoupled with a capacitor to remove a lot of the noise, and then "carried" to the input via a large resistor to give a happy healthy in-Z.

One of the smart guys around here wrote a good article on it, but I can't seem to find it right now.

[electrosonic]

There's a page on the geofx site titled "Designing Bias Supply (Vbias or Vb) Networks for Effects" that gives a good explanation.

You'll have to search for it - I can't figure out how to link to pages on that site.

Andrew.

[davent]

There's a page on the geofx site titled "Designing Bias Supply (Vbias or Vb) Networks for Effects" that gives a good explanation.

You'll have to search for it - I can't figure out how to link to pages on that site.

Andrew.

For Geofex just google the article title and it will provide the direct link.

http://www.geofex.com/circuits/biasnet.htm

[electrosonic]

Ok. If I navigate to the ariticle from within geofx, I just see www.geofex.com.

Andrew.

[davent]

Yup, me too, have to let google drag up the link for us.

[R.G.]

=> geofex.com => circuit sweepings => Designing bias networks to provide Vbias in effects

[Thecomedian]

have a look at http://www.youtube.com/watch?v=ryCr4Y7ypLA It's helped me when I've been unclear or confused about something.

it gives the fundamentals about how biasing affects input and output from point of view of a voltage controlled object (tubes), and you can just swap the same principles for Current controlled objects (transistors). it visually illustrates the way the voltages change and what that means for the gain factor, and shows how you set up the bias network, etc.  It's also in the soothing sounds of black and white 50's or 60's television narrator voice.

[merlinb]

I'm looking at a booster/preamp circuit which uses a simple voltage divider to provide 4.5V to the non-inverting input of an opamp. Most of the time, I see folks use 100k resistors to form this divider, but this pedal uses 22M resistors. Both methods create 4.5V, but is there a difference between using higher or lower value resistors?

Usually you want to minimise the current that is wasted in the resistors. A pair of 100k resistors in series sucks 45uA from a 9V supply, which is negligible enough for most people. Heck, in many designs you might not care about 0.45mA (which would imply a pair of 10k resistors) if the rest of the circuit sucked tens of milliamps.

If the opamp is a JFET type like the TL072 then it doesn't really matter; you can use values up to the meg-ohm range. Often a designer will use whatever resistor value he has already used somewhere else in the design; it keeps the bill of materials simpler.

But if the opamp is a BJT type like the 4558 then high-value resistors are not a good choice, because the input current of the opamp will change the bias voltage to something other than 4.5V. To avoid this the designer would probably choose something smaller than 100k.

[R.G.]

As noted in the article on bias networks, the value of the bias resistors is intimately related to the amount of current drawn by the device being biased.

The rule of thumb is that the current THROUGH both resistors from power to ground should be at least ten times the current taken FROM the resistors through the bias voltage point.

The reason for this is that if the current taken out of the bias point is less than 1/10 of the total current through the two resistors, you can largely ignore the current take out of the bias voltage point in calculating the resistors. That is => It gives you independence: the bias resistors are not critical values if they're at least that small.

The resistors can be smaller, and allow more than 10X the current taken from the middle to flow through them, making them more independent of the bias current. This wastes power and power supply current.

So to actually pick the resistors, you have to know, at least roughly, what the amount of bias current is, so you can pick the bias divider resistors appropriately.

Oh! But wait! I just re-wrote the article!

[merlinb]

Oh! But wait! I just re-wrote the article!

Add pictures. People are more likely to read stuff with pictures.

[tubegeek]

Add pictures of Scarlett Johansson. People are more likely to read stuff with pictures of Scarlett Johansson.

There: fixed that for you.

[R.G.]

Oh! But wait! I just re-wrote the article!

Add pictures. People are more likely to read stuff with pictures.

Doesn't help all that much.

I've been reminding people to read geofex for about 17 years now. With pictures, without, doesn't seem to help. There is always a group that will spend time typing questions on forums and not doing research. There is even a small contingent of effects builders that will be referred to Geofex, and read only that one thing, not bothering to go read the rest.

I suppose I should make youtube videos of the geofex content, make a geofex facebook page, put up several geofex blogs, make a daily geofex cartoon, whatever.

[tubegeek]

Well, to be fair, though, I don't recall seeing a single picture of Scarlett Johansson on GEOFEX in all the time I've been reading it. You're hardly holding up your end of the bargain, RG.

[ashcat_lt]

One of the smart guys around here wrote a good article on it, but I can't seem to find it right now.

[psychedelicfish]

I suppose I should make youtube videos of the geofex content, make a geofex facebook page, put up several geofex blogs, make a daily geofex cartoon, whatever.

I think more people would read GEOFEX if the website were to be updated. In an ideal world nobody would care about the aesthetics of the website if the content was good, but we don't live in an ideal world. Aesthetics have a HUGE impact on any website's effectiveness.

There are several aspects of GEOFEX that would be considered problematic by today's web developers. They may have been acceptable when the website was first made, but people expect more from websites these days. If a website isn't good enough, they'll just go somewhere else, which is why they post questions on this forum when all the answers they might need are on your site.

To start with, the navigation of the site is, at the best of times, confusing. Take the example of this very thread, people had great difficulty finding and linking to one of your articles. There should be a clear menu bar, visible from every page. This menu bar should fit all the website's content into several categories, for example:
Articles
Schematics
Guides
FAQ
There may need to be more categories, however you should keep the menu bar as simple as possible, dividing each of those sections into subsections, and perhaps even subsections of those subsections. The order of this navigation should be as logical as possible, and should make the viewers think as little as possible. Everything you do when designing a website should reduce how much the viewer needs to think, as this allows people to focus more on the content.

Secondly, there are far too many 404s and such on your website. So far, I have found one. One 404 is too many. You need to go through your website and prune out or update any missing content. I'm sure you know firsthand just how frustrating 404s can be.

Thirdly, there are many small, aesthetically irritating details on your website. It doesn't have an overarching colour scheme, and while this may not seem important, it graetly enhances the readability of your site. Having too litle colour makes a website boring to look at, while clashing colours make a website hard on the eyes. The <title> should be appropriate for the page; New Page 1 is not an acceptable title for every page, in fact it's not an acceptable title for any page. There's cut off text just below the GEOEX banner, I think it says "Now".

There are many other aspects of your website which require rethinking, but there's too many to list in one post. In short, your website, while excellent on the content side of things, needs some work done to it to make it more reader friendly. Once again, if it takes too much effort from the reader, they'll just go somewhere else.

[B Tremblay]

Start advertising the site with a little cartoon banner ad that says "Want to banish hum in effects? Try this one weird tip"

[GibsonGM]

Naaaah.....if someone won't go read that priceless gem of information gleaned over DECADES because it doesn't fit into the 'modern look' for websites...then to heck with 'em!!   Guess they're not motivated enough to learn.  This is sort of a problem overall now, it appears...if it doesn't reach out and grab you, sing you a song and do a dance, ppl seem to pass it by.  Not good.  Not at all.  Why feed into it and make it worse?    Fixing 404s is one thing.  Re-doing the whole thing to 'make it pop' is another, IMHO.

When I was starting out, there WAS no internet...and NO BOOKS ON THIS SUBJECT.  Only on Ham Radio.  We had to translate from that over to audio applications.   It was a lot of WORK, and you can still see the holes in my knowledge. And you can see it in R.G.'s work, compiling and collecting, analyzing.    The wealth of data RIGHT THERE on the net today about this stuff is AMAZING, as is GEOFEX.  I wouldn't change a thing.

If someone won't even use the search on THIS site, what makes you think they'll 'do the legwork' anywhere else?  *shrug*  Call me old-fashioned...if you want something, which is free and for your personal edification, you should put the effort in to find it rather than have it spoon-fed.  Anything worth having is made better by the whole chase/search/acquisition...