# DC bias voltage vs DC voltage

Discussion in 'General Electronics Chat' started by DexterMccoy, Feb 26, 2014.

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1. ### DexterMccoy Thread Starter New Member

Feb 19, 2014
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What is the difference between a DC bias voltage and a DC voltage?

How do you know when looking at a schematic or when doing measurements on a circuit board that it's a DC bias voltage and not a DC voltage?

What circuits or stages of a circuit have you come across that use DC bias voltages? or needs a DC bias voltage to work?

I can't tell the difference and I don't know when a DC voltage is a bias voltage or it's just a regular DC voltage, how can you tell between the two?

2. ### BReeves Member

Nov 24, 2012
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Dexter... Looking to me like you are asking for a whole basic electronics course to be taught in one thread on an electronics forum. Maybe you need to go back to school then come ask specific questions when you have enough background to understand the answers.

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3. ### DexterMccoy Thread Starter New Member

Feb 19, 2014
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Mr. Chips I'm having trouble posting a new thread, is there a problem with the site? or is it just my account?

4. ### MrChips Moderator

Oct 2, 2009
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Use a short title and then tell us what title you would prefer to have.

5. ### Billy Mayo New Member

Mar 24, 2013
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Title is DC bias voltages vs DC voltages

6. ### MrChips Moderator

Oct 2, 2009
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There is no difference between DC bias voltage and DC voltage.

Every voltage source has an impedance. The impedance will limit the amount of current you can draw from the voltage source.

Last edited: Feb 26, 2014
7. ### DexterMccoy Thread Starter New Member

Feb 19, 2014
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Thanks Mr.chips for making this thread for me

What circuits or stages "need" biasing? Besides amplifiers circuits

what else needs biasing?

biasing to a cutoff is for transistor switching?

Does a bias voltage cause the output of that stage to level shift?

The bias voltage changes the starting point or zero crossing point which is a level shift Change of the input which will level shift the output voltage?

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8. ### Veracohr Well-Known Member

Jan 3, 2011
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In a very general (electronic) sense, 'bias' can mean a voltage or current that is required to cause an active component to operate in an optimal manner. I can't imagine why one would want a bias point to fluctuate, because that would mean the component's parameters might fluctuate. Therefore, bias is pretty much always DC.

9. ### LvW Active Member

Jun 13, 2013
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A dc voltage can serve several different purposes (producing light or heat, running a motor, ...)
One of these possible purposes is to fix a certain operational point (bias point) on a non-linear transfer characteristic of a transistor. The corresponding dc voltage is called "bias voltage" (for BJT transistors app. 0.65...0.75 V).

10. ### DexterMccoy Thread Starter New Member

Feb 19, 2014
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In electronic books they call a reference voltage a "bias voltage" like on voltage or current sensing comparators

The comparator will "compare" the input signal to the Bias voltage ( reference voltage ) , if it exceeds it or not

How is this a bias voltage?

11. ### LvW Active Member

Jun 13, 2013
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It is called "bias voltage" because - in principle - it does the same as the classical transistor bias voltage: It fixes the quiescent dc point (dc voltage or dc current or both) which determines how the circuit will react upon any input signal (switching or amplyfing or...)

12. ### studiot AAC Fanatic!

Nov 9, 2007
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In general a bias voltage is a voltage imposed upon a component by external means. The component would naturally be at a different voltage if the bias was not present.

edit : you can also have bias currents.

You can apply a bias to both active and passive components. You can even apply it to circuits and entire equipment.

For example some signal/function generators have an output control called either bias voltage or offset voltage. This places the signal at a specific voltage above or below zero.

In your comparator one comparator input would not by iself be at the 'bias' voltage. This is set by components/circuit arrangements external to the comparator.

@Veracohr.

You should look up sliding bias amplifers.
(controlled) Variable bias is often used in temperature or other compensation circuits.

Last edited: Feb 28, 2014
13. ### Billy Mayo New Member

Mar 24, 2013
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When is an offset bias used or for? Beside I do see offset bias trim pots for Ic chips.

14. ### Billy Mayo New Member

Mar 24, 2013
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Why does a comparator need to be biased?

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16. ### MrChips Moderator

Oct 2, 2009
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Dexter/Billy, you may be getting hung up on semantics.

Bias, reference, offset, supply voltage can sometimes be used interchangeably.

For the purpose of this discussion, simply ignore any subtle differences and consider them all as voltage or current sources.

(I include current sources because it is simply a matter of paying attention to the impedance of the voltage source.)

17. ### MaxHeadRoom Expert

Jul 18, 2013
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In a comparator example, a reference voltage is an arbitrary voltage value decided by circuit requirements, this can be produced in a few ways, to 'bias', 'set' or otherwise cause the voltage to be biased at one value.
From my COD:
Bias: steady voltage, magnetic field, or other factor applied to a system or device to cause it to operate at a predetermined range or setting.
Max.

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19. ### spinnaker AAC Fanatic!

Oct 29, 2009
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Because Billy and Dexter are the same people and he is so mixed up he can't even keep his aliases straight in the same thread.

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20. ### shortbus AAC Fanatic!

Sep 30, 2009
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He was doing same thing in another thread. May so if one gets banned the other will still be here?

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