Hi guys,
I'm working on a pedal/schematic that has a number of Vbias points. A friend of mine suggested including an IC as a Vbias Buffer.
As a noob, please could someone explain the thinking behind this and perhaps the execution? ie What IC would I look to use, how would I use the pinout on Eagle, and exactly what I might be achieving over not including a Vbias Buffer.
As a side to this (which may require posting in the Eagle forum), I wish I could have a plane setup in Eagle to connect all my Vbias points, like my GND & 9v, presumably this is where 4 layer points start to come in if tracing space/ board size becomes an issue?
Many thanks
Si
Often times, designers use an otherwise unused half of an opamp as a Vb buffer. They do this because it is unused and also because leaving an unconnected opamp half is not best practices.
You certainly could use a plane for vb if you really wanted, but it's often not done with 4 layer boards just due to the sheer cost (very expensive).
For 99% of all PCB layouts for pedals, I personally don't think a separate plane for vb is needed. With careful component placement and layout choice, most circuits can be done on a double or (even single sided board) without much trouble.
Thanks for your reply.
So what would be the benfits of buffering your vbias over not?
How is the buffer setup with regards to an ICs pinout? Schematic examples welcome :)
Many thanks
Si
It's one of those "better, but not always necessary" things - more helpful if you need more current from the bias voltage.
RG has an article on the subject: (http://www.geofex.com/circuits/biasnet.htm)
"There is another way to do it.
Many commercial opamp circuits with more than a few IC's need a more solid reference than the sloppy 10% we assumed. (Worse, actually. Did you notice that I didn't even calculate the variation in the reference voltage that happens because the reference resistors have a +/-5% tolerance, let alone end of life and temperature drift?)
In that case, they will make a resistor divider for ONE opamp, tie it to the + input, hook the output to the - input, and use the output for the reference voltage. Now, the opamp sources and sinks current to the rest of the circuit. The resistor reference has a low and constant load, and things have the potential to be much more accurate."
Here's a snippet from one of my schematics:
(http://cl.ly/image/2B0Y0L0x1f2y/Screen%20Shot%202014-01-09%20at%2011.08.39%20AM.png)
If you are working on a circuit that is sensitive to having a good buffered bias voltage and you don't have an extra op-amp half or even that isn't good enough for the design, check out using the TLE2426. They are available in a 3 pin transistor-like package that doesn't need any external resistors or anything. It can actually simplify the circuit and provide a more stable and efficient reference voltage.
Pricey little fellows. $1.77 each for ten from Mouser...
Jacob
Quote from: jkokura on January 09, 2014, 07:23:51 PM
Pricey little fellows. $1.77 each for ten from Mouser...
Jacob
Yeah, they are. You can get them through Avnetexpress for less, sometimes down to about a dollar -- still more than a couple of resistors and a spare op-amp.
R.G has always suggested an LM386 for such a job as its output automatically biases to half supply.
You can also use a spare transistor such as if you have unused darlington in an OTA.
Quote from: Scruffie on January 09, 2014, 07:57:16 PM
R.G has always suggested an LM386 for such a job as its output automatically biases to half supply.
You can also use a spare transistor such as if you have unused darlington in an OTA.
That's a good thought. I'll have to try that out. When I was playing with the LM3900 circuits, I did it with an unused section of that for it too. It worked well.
Thanks for the input guys. I'll see if I can post a picture of what I have so far for the added buffer in the schem when I get a chance.
It's an old (vintage?!) pedal schem I'm messing with, and it's a step above the Overdrive/Fuzz/Boost that I've cut my teeth on over the last 6months or so :). In the circuit itself, there are a number of Vbias and Vref points, could someone explain how they relate to each other, as well as how they relate to the proposed buffer?
I've googled, but there doesn't seem to be much on Vbias etc wirth regards to pedals, I know I should probably understand it from a general Electronics point of view, however coming to building from a 'user' point of view rather than from an electronics back-ground, it's easier if I have the 'pedal' context :)
All the best
Si