what is VBias?

[jamontoast]

see this picture here? I want to make this circuit, but i don't know where the Vbias is ment to go? Can anyone help me?

Thanks,

Jam

[Ansil]

voltage bias is where you take the supply voltage.. in most cases a 9v  batterie or wall wart and half the power via a resistor and capacitor network and supply 4.5v as the voltage bias..  most opamps need either this trick or a bipolar power supply, since they like the +/- voltage.. ie

+9v   and -9v..  so we kind of trick the ic into doing what it is supposed to do this way.

check out like the mxr dist in the schematic section and you will see two 1meg resistors coming off of the 9v supply coupled with a cap going through another 1m into the positive input.

hope that helps..  if not and you only need a single opamp use a lm386  they don't need no stinking voltage bias

[Peter Snowberg]

Welcome Jam,

Think of it as a way to "park" the unused opamp input in the middle of the input range.

A very typical way to do this is with equal resistors from Vbias to ground and Vbias to power. You should also add a capacitor across the resistor going to ground to stabilize the bias voltage.

With a 9 volt supply, as Ansil mentioned, the Vbias is typically 4.5 volts.

For component values, a good range is 10K to 47K for the resistors (but use the same value for both), and a 10uF cap.

Take care,
-Peter

[jamontoast]

thanks heaps you guys!

i'm building a acoustic guitar preamp and i'm mixing this circuit with a simple buffer preamp, hopefully it will sound pretty good!

Thanks again, rock on!

jam

[Mark Hammer]

AC voltages go up and down, right?  But up and down relative to what?  In a device with a bipolar supply, they go up and down relative to that neutral ground mid-point.  But what if, for example, you only use a single-ended power supply with a 9v battery?  What becomes the "reference" point?

The answer is "the reference voltage".  That can go by many nams.  Sometimes it is called the "bias voltage (Vb)", sometimes the "reference voltage (Vref)", and sometimes other names.  Either way, it provides a steady DC voltage that the AC portion "rides on", so that if you have an AC signal of, say, 100mv and a Vref of 4.5v, the AC signal is actually fluctuations from around 4.4 to 4.6 volts.  In most instances, to provide equivalent room to accommodate transient peaks on either negative or positive half-cycles, the Vref is usually the midpoint of the supply voltage.  So, with a 9v battery, Vref will normally be 4.5v.  The normal way it is derived is to feed the supply voltage to an equal-component-value voltage divider, (e.g., a pair of 10k resistors) which will chop whatever voltage is applied in half, and take the Vref from their junction.  

In some instances, the "ideal" Vref is not +V/2 but some other voltage.  For instance, the Dallas Rangemaster uses a bias well off the midpoint (around 6-7v with a 9v supply) to provide asymmetrical clipping (remember, the exact midpoint is normally adopted to minimize clipping on either half-cycle).  In the case of bucket-brigade delay chips used for chorus and flanger appications, these need their own bias voltage well off the supply mid-point or else they won't even *pass* signal, let alone pass it cleanly.