questions on bias

[Guitar Guy]

Hello forum!
I have three questions on Bias. First, what is bias? How can you tell what part of the circuit controls the bias? And finally, What does changing the bias, IE replacing it with a potentiometer, make the circuit do? Thanks again all.

[CynicalMan]

Bias is adding a DC signal to an AC signal so that an active device can use the signal properly.

The other questions depend on what the active device is and on what kind of bias is being used. What circuit are you referring to?

[Astronaurt]

Bias, at least how the term is used for electronics, means that you are super imposing a DC voltage on top of your AC signal voltage. This is necessary because a Transistor without Bias applied to it acts like a diode - I.E. if you don't Bias the transistor, then only the positive going half of your signal gets through the transistor, while the negative half is cut off. Biasing with a DC voltage raises the functional "zero point" of the signal so that all of the signal gets through. Often with Tranny circuits, there will be a resistor network connected to the Base/gate of the transistor, and the Voltage divider this network makes determines the DC Voltage, the Bias, of the transistor.

If you use a pot to adjust bias, what you're doing is changing the "zero point" of the signal through the transistor. like I said initially, if you don't apply any bias at all then half of the signal is cut off. If you apply ENOUGH bias, then the opposite half of the signal starts to get cut off. What happens when part of the signal is cut off? Distortion. Generally if you want a signal to be amplified cleanly and distortion free, you want to bias the transistor in the middle, between the two extremes. To find out what that middle point is, look up a datasheet for the transistor you're using and learn how to read Characteristic curves and load lines. Like this: http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/loadline.html

[R.G.]

I have three questions on Bias. First, what is bias?

Every amplifying device has a range of possible voltages and currents it can withstand. Designers (should!) pick power supplies and devices which don't result in damage to either one. For pedals, we most often use 9Vdc for historic reasons.

A transistor used as an amplifier cannot conduct less than zero current, and it cannot be turned on to conduct more current than the rest of the circuit, including power supply and external resistors will let it. So it has to work within the range of the available power supply volts and amps. It cannot amplify anything if it sitting either fully off or fully on. It can only amplify in the middle, with part of the power supply voltage across it.

Bias as a word originally came from something like "pushed or leaning to one side; not in the center or at rest". And in electronics, it's used to mean some means of offsetting a device's operation away from where it would otherwise be.

A transistor is biased (i.e. pushed away from rest) so that it does what the designer wants it to. For amplifying, it has to be pushed into the middle of the power supply so it doesn't bang against cut off (zero current) or saturation (full current) with the signals you want amplified. Biasing means to set the operating conditions so that it does what you want it to do with the expected signals.

How can you tell what part of the circuit controls the bias?

You learn what part of the device you're trying to bias affects its operating conditions. For a bipolar transistor, the base-emitter voltage and current set how much current flows, so you bias a bipolar by changing the voltage/current on the base and emitter terminals. For a field effect transistor, you change the electrical field inside it by changing the voltage between the gate and source. In general, bias has to be applied to a control pin with respect to some other pin. To find out which pin does this, you read.

And finally, What does changing the bias, IE replacing it with a potentiometer, make the circuit do?

It makes the circuit turn more completely on or off, depending on the setting of the potentiometer. It lets you manually set the bias at will by turning the pot shaft instead of having to remove and replace fixed resistors.

[Guitar Guy]

Bias is adding a DC signal to an AC signal so that an active device can use the signal properly.

The other questions depend on what the active device is and on what kind of bias is being used. What circuit are you referring to?

I am looking at the Black Cat OD-1 circuit. it does not have a transistor, but it does have this IC; OP275. http://fuzzcentral.ssguitar.com/schematics/blackcatoverdrive.gif
From what you are telling me, I gather that the bias control would be the 1M ohm resistor above U1B. Also i'm thinking that yawl are saying that if I were to replace this resistor with a pot, turning it anywhere but @ 12 o'clock would add distortion of some kind or another?

[Gurner]

The bias in the schematic you linked to, is as supplied by the two 100k resistors....the junction of which (at half the supply voltage) is connected to pins 3 & 5 of the opamp. (I do wish folks would give components actual part numbers...it makes it a whole lot easier to reference specific parts of a schematic)

I liken bias to trying to get a drunk man (your audio signal) to stumble down a narrow alley without him bouncing into either of the two sidewalls** (the walls being your supply voltage +ve & -ve connections). You normally want your drunk 'n disorderly chap to be as far away from either 'wall' as possible...ie the centre of the alleyway (so you normally bias circa half your supply voltage)

** unless you actually seek his face being rearranged (distortion)

[CynicalMan]

The bias is made by the two 100k resistors and the 10u capacitor, all connected to Vbias. They make a bias voltage at half of the supply voltage, or 4.5V. Most op amps are designed to be bipolar devices, which means that they need a positive and negative supply voltage. But the bias voltage uses 4.5V as a virtual ground, which means that it acts like ground for the op amps. Because of this, when 0V and 9V are used to power the op amp they act as +/-4.5V.  Op amps can usually output voltages to a maximum of around 1V within the power rails, or +/-3.5V. If you shift the bias point, the headroom on one side will increase and the headroom on the other side will decrease, leading to a maximum output swing like +5V/-2V. You'll get more distortion, a lower distortion threshold, and asymmetrical distortion. The direction in which you shift it doesn't matter much, so to make a simple control, you could replace one of the resistors with a 100k pot wired as a variable resistor.

[R.G.]

I liken bias to trying to get a drunk man (your audio signal) to stumble down a narrow alley without him bouncing into either of the two sidewalls** (the walls being your supply voltage +ve & -ve connections). You normally want your drunk 'n disorderly chap to be as far away from either 'wall' as possible...ie the centre of the alleyway (so you normally bias circa half your supply voltage)

That's a very apt analogy!

[Guitar Guy]

so, in other words...I would only change out one resistor for a pot as oppossed to both because replacing both would prove redundant. Also all adding the potentiometer to the bias portion of the circuit would do would either add or remove distortion? cool by me since i am a distortion addict. But would i have to worry about blowing the op?

[Gurner]

so, in other words...I would only change out one resistor for a pot as oppossed to both because replacing both would prove redundant. Also all adding the potentiometer to the bias portion of the circuit would do would either add or remove distortion? cool by me since i am a distortion addict. But would i have to worry about blowing the op?

What you talk about is moving the bias to induce distortion, which it will....but it won't be symetrical distortion. Depending on which way you move the bias, either the +ve heading signal portion will solely clip, or the -ve heading portion will solely clip ....which may or may not suit your ears (in my drunk man analogy, you've steered the man from the alleyway 'centre line' over to one side, chances are his left side is going to keep banging into the alley wall & be bruised, but his right side will be just fine...that is, until he gets home ....to be greeted by his angry wife  )

[Guitar Guy]

What you talk about is moving the bias to induce distortion, which it will....but it won't be symetrical distortion. Depending on which way you move the bias, either the +ve heading signal portion will solely clip, or the -ve heading portion will solely clip ....which may or may not suit your ears (in my drunk man analogy, you've steered the man from the alleyway 'centre line' over to one side, chances are his left side is going to keep banging into the alley wall & be bruised, but his right side will be just fine...that is, until he gets home ....to be greeted by his angry wife  )
[/quote]

So in other words I would more than likely be wasting my time by altering the bias... in that case what other parts of this circuit might prove interesting to throw a pot on to give mroe variables to the sound while keeping it useable?