#2
Just say you have something powered from 100 Volts. Your signal can range from 0 to 100 Volts.

Now imagine you insert a sinusoidal wave, this wave can be 0 to 100V in amplitude but no more and no less. The bias is the origin that the sinusoid oscillates about.

So just say you have a 30V peak to peak sine wave. It's biased at 75V, so the upper peak will be 100V and the lower peak will be (75-30) = 45V. Notice how the positive peak doesn't hit 105V? It clips because your power supply is limited to 100V.

Now if you biased the transistor / op amp / or whatever you are biasing correctly, if you biased it at 50V and inserted a 30V peak to peak sinusoidal wave, the waveform wouldn't clip.

Basically you just want to bias the amplifier in the center of the power supply voltage so that you get an exact replica of the signal without it clipping.
#3
Oh, and the reason you bias something is because of this:

Just say you are playing your guitar and you send that signal to the amplifier. Say the amplifier runs from 0 to 100V (just randomly picked numbers). Just say you have a 9V pickup and say that the sinusoid oscillates about the half way point of 4.5V. If you send a sinusoid centered at 4.5V to a 0 to 100V amplifier, it's going to be biased way too low.

So what you do is you put a capacitor in series with the output of the pickup. Putting a capacitor in series makes it so the sinusoid oscillates about 0V (called AC coupling). If you stick a signal that oscillates about 0V into a 0 to 100V amplifier, then the positive portion of the signal will be retained but the negative portion won't be retained.

So you bias the 0 to 100V input of the amplifier to 50V after you AC couple the signal from the pickup and you will get a good signal at the output of the amplifier.
#6
Quote by manubro1
Wondering what it means.

Why? Do you need to bias your amp? What amp do you have? Not all amps are created equal. Think of it as fine tuning your amp. Like adjusting the tuner on a radio.

Google 'amp biasing' and I have a Blog that explains how I do it for my amp. I'm not an amp tech so my Blog is laid out more for the average joe.
#7
setting the operating point of your amplifier.

in more complex amplifier designs it also lets you balance and match your tubes yourself.
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#8
Here's a simple explanation:

You start a car and it idles at a particular RPM, right? The bias point of a tube can be thought of as an idle point. The amount of current flowing through the tube when no "throttle" is applied.
#9
(Jumping in on this since I'm kinda clueless about the subject)

So does biasing your amp mean setting how loud cranking the volume to eleven is?
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#10
i thought it was how hard the tubes are turned on standard setting. so biasing it high smokes your tubes and thus also shortens life-span. same for low.
correct me if wrong though i've just read stuff on the interwebz
#12
Well, TS wanted a basic explanation. Here's some elaboration on that:

Bias point is idle current. So for instance, we might bias a tube to where 25mA of current flows through the tube at idle--or in other words with no signal being run into it. Or we might bias a tube to where 75mA flows through it at idle. We're simply setting idle current flow.

Now, tubes have a spec rating called maximum plate dissipation. This is a rating of how much heat the tube's plate can give off and is specified in watts. The higher our bias current is, the more heat the plates will dissipate. It's one of the reasons we say "hotter" or "colder" bias. But plate dissipation in watts is not to be confused with output power.

Where tubes should be biased is entirely dependent on the voltages applied to their plates and screen grids. These conditions create a set of curves by which the tube operates. This also determines the max plate dissipation curve. With plate voltage and transformer primary impedance one derives what is called a load line. Based on where this falls given the aforementioned data, a designer can determine where to safely bias a tube.

So beyond my initial description in my previous post, it's not so simple anymore.
#13
You just want your bias to be in the center of the max and min voltage. If your center is too high, then your signal clips at the max voltage. If your bias is too low, your signal clips at the low end.

In electronics they used to use 0 as the midway point and would use a positive voltage rail and a negative voltage rail. For example, they would have +15V and -15V, and 0 would be your center. If you use a single power supply, you can only be 0 to 15V or 0 to -15V, so your center needs to be 7.5V.
#14
Quote by farmosh203
You just want your bias to be in the center of the max and min voltage. If your center is too high, then your signal clips at the max voltage. If your bias is too low, your signal clips at the low end.

In electronics they used to use 0 as the midway point and would use a positive voltage rail and a negative voltage rail. For example, they would have +15V and -15V, and 0 would be your center. If you use a single power supply, you can only be 0 to 15V or 0 to -15V, so your center needs to be 7.5V.


I'm honestly not sure what you are talking about. It's true that sometimes you want to center bias a class A amplifier but this is not the way you go about doing it for tubes. It certainly isn't how one would bias a class AB amp. Power supply voltage is not a factor in the way you are describing.

Bias voltage is a negative number. To be precise, it's how negative the control grid is in respect to the cathode. The range of adjustment for a power pentode or tetrode is dependent on screen grid voltage. But it has nothing to do with being half of the power supply voltage or whatever you were suggesting here.
#15
Have an amp that uses EL34s? Want to use EL84s or 6L6's instead? You gotta bias the amp.
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#16
Quote by CECamps
I'm honestly not sure what you are talking about. It's true that sometimes you want to center bias a class A amplifier but this is not the way you go about doing it for tubes. It certainly isn't how one would bias a class AB amp. Power supply voltage is not a factor in the way you are describing.

Bias voltage is a negative number. To be precise, it's how negative the control grid is in respect to the cathode. The range of adjustment for a power pentode or tetrode is dependent on screen grid voltage. But it has nothing to do with being half of the power supply voltage or whatever you were suggesting here.


I don't know jack about tube amplifiers but I'm talking about biasing amplifiers in general. Biasing has everything to do with the steady state voltage being in the middle of the power supply voltage, what's the point of biasing an amplifier if you don't agree?

http://www.media.mit.edu/resenv/classes/MAS836/bias.pdf

From the article:

Bias, in an electronic circuit, describes the steady state operating characteristics with no signal being applied. In an op-amp circuit, the operating characteristic we are concerned with is the output voltage of our op-amp. If an op-amp is said to be biased to 2.5V, this means that, for no incoming signal or no sensor excitation, the output voltage will rest at 2.5V. Bias is, therefore, strictly a DC value. Once an AC signal is applied, the output will then begin to move about the bias point. We bias an amplifier to a particular value to keep the op-amp from saturating (amplifying a signal beyond the supply voltage limitations) and to allow the signal to have as large of a range as possible.
#17
Quote by farmosh203
I don't know jack about tube amplifiers but I'm talking about biasing amplifiers in general. Biasing has everything to do with the steady state voltage being in the middle of the power supply voltage, what's the point of biasing an amplifier if you don't agree?

http://www.media.mit.edu/resenv/classes/MAS836/bias.pdf

From the article:



For tube amplifiers you don't care as much about center biasing them. In an amplifier that you don't want any clipping introduced you would bias in the center of the total maximum voltage swing. In a tube screamer you apply a bias voltage in order to keep the opamp from introducing clipping, but its not the same in a guitar amp output stage. In most tube amplifiers not only can you cause your tubes to die from over dissipation, but some times its not even possible to do such a thing.

Bias is not just the voltage applied to the grid of the tubes, but rather the combination of quiescent conditions that the tube sees.

So here the TS is most likely referring to a tube amplifier. In that case you don't normally take the power supply voltage into account for determining signal swing since it is inductively coupled to the plates and can swing higher than the power supply voltage.

For an op amp or transistor based amplifier you do take the power supply voltage into account, and you don't want them to clip as it is unplesant.
Last edited by XgamerGt04 at Apr 22, 2011,
#18
Quote by farmosh203
I don't know jack about tube amplifiers but I'm talking about biasing amplifiers in general. Biasing has everything to do with the steady state voltage being in the middle of the power supply voltage, what's the point of biasing an amplifier if you don't agree?

http://www.media.mit.edu/resenv/classes/MAS836/bias.pdf

From the article:


That's not how tubes are biased. You're pulling info that is about as far out of context with tube amplification as possible. Sure, it shares the common term "bias," but it doesn't translate in the tube world.

Tube bias (voltage) is a negative number. It's control grid voltage to be exact. At 0 volts on the control grid a tube is in full saturation and cannot draw any more current.

By your explanation, if we have a tube with 400 volts on the plates, you're suggesting biasing the control grid at 200 volts. This makes no sense.

The range of control grid voltage available for bias varies from tube to tube, and is entirely dependent on screen grid voltage. So 1 tube with 400v on the plates and 400v on the screens may have a control grid voltage range from 0v to -65v from saturation to cutoff respectively. Another tube with the same voltages applied may only have a range from 0v to -40v.

Half the plate *or* half the screen grid voltage is not an option, ever. This is not how tubes are biased and is never a design consideration when creating an amp.

Nor is center biasing something that is universally striven for in tube amps. ESPECIALLY in class AB amps where each tube is amplifying only 1 half of the wave form.

So that's why I'm saying, where you're getting your information does not apply to biasing tube amps and should not be stated here as such.
#19
So what is the output bias point for a tube amp if it's not half the supply voltage?

And yup I agree that bias is just the state when no signal is applied.
#20
Quote by farmosh203
So what is the output bias point for a tube amp if it's not half the supply voltage?

And yup I agree that bias is just the state when no signal is applied.


It depends really on the design. Usually you have to use composite curves for the tubes and draw load lines. Very similar to the way you can do it with transistors. You find an operating point you want, which is denoted by a given grid voltage, and you either apply a negative voltage at the grid so that you have that point, or have a cathode bias resistor. You want tubes to clip though.

That being said... bias is not just the voltage and should not be given as just the voltage either... Bias is the combination of the grid voltage, plate voltage, and plate current. The manufacturers that say "Bias the amp to -xx volts" are the ones that I see coming in the most often for tube changes.
#21
That's not how tubes are biased. You're pulling info that is about as far out of context with tube amplification as possible. Sure, it shares the common term "bias," but it doesn't translate in the tube world.


An amplifier is an amplifier, whether it be tube or solid state. My example is 100% related, it's an amplifier.

Tube bias (voltage) is a negative number. It's control grid voltage to be exact. At 0 volts on the control grid a tube is in full saturation and cannot draw any more current.


That's fine, I was talking about op amps in my situation. I could have mentioned a PFET bias and that would be the same as what you're talking about here.

By your explanation, if we have a tube with 400 volts on the plates, you're suggesting biasing the control grid at 200 volts. This makes no sense.


No, I'm just talking about single supply amplifiers.

Half the plate *or* half the screen grid voltage is not an option, ever. This is not how tubes are biased and is never a design consideration when creating an amp.

Nor is center biasing something that is universally striven for in tube amps. ESPECIALLY in class AB amps where each tube is amplifying only 1 half of the wave form.


I'm talking about the output bias, not the control bias.


I'm talking in broad terms about amplifiers, you are getting into the specifics of tube amplifiers. It's still the same thing, you want your signal to get max dynamic range without clipping, that's the point of biasing (see the images I posted).
#22
That being said... bias is not just the voltage and should not be given as just the voltage either... Bias is the combination of the grid voltage, plate voltage, and plate current. The manufacturers that say "Bias the amp to -xx volts" are the ones that I see coming in the most often for tube changes.


I think we're getting confused here. The no signal output voltage you always want to be the same but voltage used to control will be different for each tube/transistor.
#23
I highly doubt that in the GG&A section of UG, the TS is wondering about how to bias op amps.
#25
Bias means the same thing for op amps, transistors or tubes. The specific control voltages/currents are different for each of these but I think the TS is more interested in what bias means and not the circuit analysis.
#26
^^ ^^^ farmosh- you are arguing with people that build amps for a living


And they are arguing with someone that builds electronics for a living.

I know what they're saying, I think it's just miscommunication.
#27
Farmosh,

What you are saying would work if the output stage of a guitar amp was restively coupled, but as it isn't the whole regular ball game goes out the window. Like I said, since it is inductively coupled you can get two or three times your HV supply. So instead you pick an operating point that gives you a mix of safe tube operation and signal swing with the transformer that you are going to use. Clipping isn't frowned upon in a guitar amp, but in a HiFi amp you would want to avoid clipping at all cost.

In a preamp section you might bias the tube so that it is at 1/2 of the HV supply at idle, or you might not. Since we are talking about guitar amps clipping isn't necessarily a bad thing.

As a note... this is coming from a guy that builds guitar amps as well as other electronic devices for my day job. Inductive coupling is what is messing you up. The signal in a guitar amp power section can swing above the HV.
Last edited by XgamerGt04 at Apr 22, 2011,
#28
Quote by farmosh203
Bias means the same thing for op amps, transistors or tubes. The specific control voltages/currents are different for each of these but I think the TS is more interested in what bias means and not the circuit analysis.


I'm actually guessing that the TS has an amp that requires self biasing and he has know idea what that means (to him) or how to go about doing it. Until we know what amp he has, my guess is we are going overboard
#29
What you are saying would work if the output stage of a guitar amp was restively coupled, but as it isn't the whole regular ball game goes out the window. Like I said, since it is inductively coupled you can get two or three times your HV supply.


That's true, I was thinking it was just a 1:1 transformer.
#30
bias is to adjust the device's operating point. in some devices, characteristics will fluctuate depending on bias.

theres the universal explanation...
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#31
Quote by ECistheBest
bias is to adjust the device's operating point. in some devices, characteristics will fluctuate depending on bias.

theres the universal explanation...


So wait.... can I bias my dog?
#32
Quote by XgamerGt04
So wait.... can I bias my dog?


Perhaps with a shock collar you may be able to get somewhat of a bias temperament. Just gotta get the voltage right.
#33
They should all have a knob on the butt.
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