Definition of "Bias" from the web..........."Bias is a natural leaning or preference for or against something"
As I fumble through learning electronics repair by restoring old amplifiers and searching for guidance in the diyAudio forums, I encounter the term "Bias" often. "Measuring the Bias" and "setting the Bias" is something I can do but I am still having a hard time wrapping my head around the concept. I have several electronics books and none of them give a definition of "Bias".
Is there another word or phrase that could be used in the place of "Bias" in electronics?
Does "setting the Bias" always work like a balance control between two points?
Is "Bias" always DC voltage?
As I fumble through learning electronics repair by restoring old amplifiers and searching for guidance in the diyAudio forums, I encounter the term "Bias" often. "Measuring the Bias" and "setting the Bias" is something I can do but I am still having a hard time wrapping my head around the concept. I have several electronics books and none of them give a definition of "Bias".
Is there another word or phrase that could be used in the place of "Bias" in electronics?
Does "setting the Bias" always work like a balance control between two points?
Is "Bias" always DC voltage?
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The internet calls it "biasing" ...
https://en.wikipedia.org/wiki/Bipolar_transistor_biasing
https://www.electronics-tutorials.ws/amplifier/transistor-biasing.html
https://en.wikipedia.org/wiki/Bipolar_transistor_biasing
https://www.electronics-tutorials.ws/amplifier/transistor-biasing.html
Bias in a transistor or tube is a DC current that continuously flows in the device, when there is no signal.
The bias current allows an undistorted signal to be made, and for it to be able to change in both directions.
If there were no bias current, the signal would be very distorted (rectified).
The bias current can be interpreted in terms of a DC voltage, for example a resistor that the DC bias current flows through
develops a DC voltage across it. Measuring this DC bias voltage is a proxy for measuring the DC bias current.
Or in a tube, the DC bias voltage at the grid can set the DC bias cathode current (as in a fixed bias tube output stage).
As an analogy, the static air pressure can be thought of as a bias. Sounds cause this static air pressure bias to fluctuate
more positive and more negative than the ambient pressure.
The bias current allows an undistorted signal to be made, and for it to be able to change in both directions.
If there were no bias current, the signal would be very distorted (rectified).
The bias current can be interpreted in terms of a DC voltage, for example a resistor that the DC bias current flows through
develops a DC voltage across it. Measuring this DC bias voltage is a proxy for measuring the DC bias current.
Or in a tube, the DC bias voltage at the grid can set the DC bias cathode current (as in a fixed bias tube output stage).
As an analogy, the static air pressure can be thought of as a bias. Sounds cause this static air pressure bias to fluctuate
more positive and more negative than the ambient pressure.
Thanks fubar3 and rayma, this gives me something more tangible to think about.
rayma, if I'm understanding your explanation correctly, in an audio amplifier for instance, since bias is DC current, I'm assuming it originates from the power supply
rayma, if I'm understanding your explanation correctly, in an audio amplifier for instance, since bias is DC current, I'm assuming it originates from the power supply
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Maybe you'd like "idle current" better. It's the current flowing through the output stage when doing nothing, IOW, idling.
Setting the bias in power amplifiers can often be the same as adjusting the idle current or check the quiscent current in the output transistors.
But it can also be to adjust the DC offset at the speaker terminals.
.
But it can also be to adjust the DC offset at the speaker terminals.
.
Sure, all the power used in the amp comes from the power supply (unless there's a battery ).
Vacuum tubes need to conduct a small amount of current when no signal is passing through them, sort of like an internal combustion engine that needs to keep idling when the car is not moving while driving. So, the term bias just refers to this idling current when the tube is passing no signal. If the tube were to completely stop conducting during no signal conditions, then large amount of distortion would occur as the tube goes into and out of cutoff, or no current condition. Drawing a continuous small amount of plate current prevents that from happening.
I think it’s also called a bias because it pushes the devices (hopefully) into their linear operating regions.
Originally the word meant slanted or oblique, and it still has this meaning in woven fabric. It developed into meaning away-from-centre, away-from-neutral or offset. Thus its the offset from 0V (or 0A) needed to place an active device into the least-non-linear region so it can pass a signal relatively undistorted. So in electronics it means an offset. Simple.
Lot of different devices and operating points, so hard to cover the whole concept quickly.
With Class A or B or A/B
Very simplified example is class B amplifier with common BJT transistor.
And bias usually easily understood with a simplified description of "crossover" distortion.
Assuming a push pull amp, or separate transistor for the negative and positive points of the AC waveform.
.7 volts considered generic voltage to where the BJT transistor actually turns on or starts conducting.
Changes with temperature of course, but ignore for now for simplicity.
Below .7 volts = off
Apply a AC signal
Typically the classic pure waveform sinewave is used for examples.
Upper sinewave in the picture.
.7 volts would be left out of the signal. Since it hasn't turned on yet.
Hence Crossover distortion the dead zone or signal gap and numerous other descriptions.
.7 volts is missing from the positive/negative portions because the transistor has not turned on.
Hence the noticeable kink or distortion seen in the waveform.
Lower sinewave in the picture.
How do you fix it? = bias
Apply .7 volts DC and the transistor is already on.
So no portion of the AC signal is now lost.
The transistor is already on and ready to go.
Basically the zero crossing of the waveform or 0 volt portion going from negative to positive AC
is where crossover distortion will occur. And would continue to .7 volts or 700mV till it finally turns on.
low level signals it would be a real problem and more noticeable.
if the signal peak was 500mV your utterly F'ed it wont even turn on till 700mV
So bias is your friend.
If the signal was 20 volt peak seems like no big deal if .7 volts goes missing.
Distortion is Distortion crossover distortion has a scratchy crackle to it in large signal.
Small signal say a 1 volt peak or 1000mV would definitely sound like krap with more than half missing
or 700mV and if your signal is 500mV then = nothing the transistor dont even turn on till 700mV
So you apply DC bias so it is already on. And no AC signal goes missing.
Hence if your apply .7 volts DC you would have DC current now flowing.
It is a tradeoff to not loose AC signal. The transistor is always slightly on
Why people say 30ma or 100ma of bias current. Just a tradeoff of always
being on so we dont loose AC signal, or music or sound. Since it is a AC waveform
With Class A or B or A/B
Very simplified example is class B amplifier with common BJT transistor.
And bias usually easily understood with a simplified description of "crossover" distortion.
Assuming a push pull amp, or separate transistor for the negative and positive points of the AC waveform.
.7 volts considered generic voltage to where the BJT transistor actually turns on or starts conducting.
Changes with temperature of course, but ignore for now for simplicity.
Below .7 volts = off
Apply a AC signal
Typically the classic pure waveform sinewave is used for examples.
Upper sinewave in the picture.
.7 volts would be left out of the signal. Since it hasn't turned on yet.
Hence Crossover distortion the dead zone or signal gap and numerous other descriptions.
.7 volts is missing from the positive/negative portions because the transistor has not turned on.
Hence the noticeable kink or distortion seen in the waveform.
Lower sinewave in the picture.
How do you fix it? = bias
Apply .7 volts DC and the transistor is already on.
So no portion of the AC signal is now lost.
The transistor is already on and ready to go.
Basically the zero crossing of the waveform or 0 volt portion going from negative to positive AC
is where crossover distortion will occur. And would continue to .7 volts or 700mV till it finally turns on.
low level signals it would be a real problem and more noticeable.
if the signal peak was 500mV your utterly F'ed it wont even turn on till 700mV
So bias is your friend.
If the signal was 20 volt peak seems like no big deal if .7 volts goes missing.
Distortion is Distortion crossover distortion has a scratchy crackle to it in large signal.
Small signal say a 1 volt peak or 1000mV would definitely sound like krap with more than half missing
or 700mV and if your signal is 500mV then = nothing the transistor dont even turn on till 700mV
So you apply DC bias so it is already on. And no AC signal goes missing.
Hence if your apply .7 volts DC you would have DC current now flowing.
It is a tradeoff to not loose AC signal. The transistor is always slightly on
Why people say 30ma or 100ma of bias current. Just a tradeoff of always
being on so we dont loose AC signal, or music or sound. Since it is a AC waveform
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No, that’s adjusting the offset. Nothing to do with setting the bias.
Most amplifying devices (tubes, transistors) are controlled by a negative or positive voltage at their gate or grid. Generally it's either a negative or positive voltage, not both.
So, for single-ended operation, where one device amplifies both negative and positive parts of the signal, the device needs to be turned on (biased) even when no signal is present so both negative and positive phases of the input can control the output. Otherwise, only one phase (positive or negative) affects the output.
So, for single-ended operation, where one device amplifies both negative and positive parts of the signal, the device needs to be turned on (biased) even when no signal is present so both negative and positive phases of the input can control the output. Otherwise, only one phase (positive or negative) affects the output.
