Vb

[changes]

in a schem i found i have to connect some points to Vb.what is meant by that???

[Joep]

Usually a reference like that, point to a certain voltage somewhere else in the schem. All these point should be connected together. They are used to make the schems more readable.

An other one you see very often is Vref, pointing to half of the supply voltage, mostly used with opamps.

Let us know which schem you are looking at....

Also look in the DIY FAQ, on the top of this page.

Bye,

Joep

[Mark Hammer]

Audio signals are always fluctuating voltages that occur, in comparison to some reference point.  When you have a +/- power supply, those fluctuations are in comparison to ground, such that signal (line-level, for example) might vary a half volt above or below ground.

So, uh, although they can certainly go positive, how do things go *negative*, relative to ground, when you only have a single-ended supply in the form of a 9v battery providing +9v?  

The answer is that you fake a ground or reference point, as Joep describes, by "pretending" that ground is some point higher than 0 volts, but less than V+.  Standard practice is to have that "pretend ground" be as close to half the supply voltage as possible, so that the signal has as much opportunity to swing positive in comparison to that reference point, as it has to swing negative to it.  The easiest way to provide such a reference point to serve as a virtual or "floating" ground is to simply take the supply voltage and divide it in half using a pair of equal-value resistors.  The voltage provided at the junction of those two resistors can go by several names but they all mean essentially the same thing: Vb, Vbias, Vref, V/2, and a number of other invented names.  It would be tempting to say that the "b" in Vb stands for bias, but I think it has something to do with historically assigned supply voltages for tube-based equipment (Vbb, Vcc, Vdd, etc.) that used alphabetical order to keep the various required multiple supply voltages straight.  

Although Vb *is*, technically, a "bias" voltage, there can be other bias voltages on board a circuit as well for other purposes, so be aware that while Vref usually means only one thing, "Vbias" can mean several different things, depending on the circuit.  A perfect case in point occurs in the John Hollis "Zombie Chorus".  Bucket-brigade chips used for chorusses, flangers, delays, etc need the audio signal to ride on a DC "bias" voltage, which is quite a bit different than the half-of-V+.    So what you'll see in 9v-based chorus and flanger pedals is a Vref used for the audio signal throughout the circuit, and a second bias voltage (derived again from the power supply, via a trimpot to divide the supply voltage down) either added on top of that or else replacing it.  In the case of the Zombie, Hollis used a single Vref as *both* bias voltage for the BBD *and* Vref for the remainder of the circuit, such that the audio signal was now sitting on top of something other than 4.5v.  This single bias voltage permitted the simplified circuit to work, although unfortunately not without some penalty in terms of noise.  It does illustrate, however, that Vb, Vref, and Vbias can sometimes be the same thing, but sometimes need to be different things.

What the ideal Vb/Vref/etc does is perch the audio signal on top of a steady DC voltage (4.5v with a 9v battery).  There are many ways this can be implemented with op-amps, depending on the nature of the circuit.  For instance, in an inverting op-amp configuration (signal goes to negative input through a resistor, feedback from output returns to same negative input pin via resistor, positive input goes to ground), the positive input pin will be routed to Vb/Vref/Vbias.  In a non-inverting op-amp, the signal goes to the positive pin, with output feedback returning to the negative pin.  At the positive pin, however, there will usually be a fixed resistor of a reasonably high value (you rarely see <100k) going to Vref.  

Occasionally, as in the Orange Squeezer Compressor, there will be NO resistor going to Vref.  Rather, the audio signal appears to "pass through the jaws" of the supply voltage, and you'll see a large-value resistor going directly from the input pin of the opamp to V+ and ground.

Not everyone draws Vb sources the same way in their schematics, and sometimes they can be hard to spot when one of the equal value resistors is over here and the other is waaaaaayyyyyyy over there, but eventually you get an eye for it and can spot them easily.

One thing to keep in mind is that Vref, Vb, etc, need to be treated as if they were another supply voltage.  And like any supply voltage, you need to provide some regulation so that noise or spikes on the supply lines from one part of the circuit suddenly drawing current, don't causes glitches or noise on otrher parts of the circuit drawing current from the same limited battery supply.  This is why you will almost always see an electrolytic capacitor going between the junction of the two equal-value resistors, and ground.  That cap is there for the same reasons you'd have a 100uf, for example between your battery and ground - for smoothing out the power and eliminating spikes and dips.  It just happens that this "supply voltage" is half the V+ and used for a slightly different purpose, but it is still helpful to think of it as if it were a power supply.

[Lonehdrider]

Mark,

Just wanted to say thanks for all the info you  put in here, it makes it so easy for newbies like myself to understand. For someone like myself that comes from a more mechanical background, it helps a great deal (I try to invision it in terms of stuff flowing like pipes, guess thats the way my mechanical mind deals with something that on first look seems unmechanical, but your explainations are superb) Thanks again.

Regards,

Lone

[Mark Hammer]

You're most welcome, Lone.

Russian psychologist Lev Vygotsky proposed that most effective learning would occur when children were taking on problems just a bit beyond their current level of expertise; what he called the "zone of proximal development".  The implication of this for instruction was that teaching could be more effective if the teacher could accurately identify this zone for each student, and tweak the examples and tasks provided to build upon what the person already knews.  Much of what we know about the psychology of expertise and how people become experts suggests it is the way in which knowedge is connected through examples and the order in which it is presented that creates the highest skill levels, and great coaches engineer examples in that manner.  

Naturally, people who know the learner better are in a better position to second guess the learner's needs at this moment.  Consequently, one of the reasons why peer tutoring in school is often so effective is because the person who can most accurately identify what a learner does and doesn't know is someone who is just a few steps past that themselves.  As many of you can probably attest to, being taught math or stats or computer sci by someone who is mindbogglingly brilliant in the field is absolutely no guarantee that what they *believe* to be teaching produces learning.  And this is usually because they are so many miles ahead of you they have absolutely no idea of what it is you need to hear right at the moment in order to have things gel in your mind.

I don't know a helluva lot, but I know what it is I used to misunderstand until recently, and try to keep tabs on what others might similarly misunderstand.  That helps in explaining.  So does practicing explaining a lot.  Explanation itself is a skill that takes thousands of hours of practice to develop.  Those who may have joined here recently probably don't have the length of memory to notice it but all the old farts who have been at this for a while have been getting better and better at explaining things over the years.  Jack and RG were no slouches 20 years ago, but they're better at it now.  I hope I am too.