# Biasing a transistor

Discussion in 'General Electronics Chat' started by i_nour, Oct 1, 2016.

1. ### i_nour Thread Starter Member

Nov 9, 2015
31
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Why do we -usually- have to use a DC voltage (power supply) to bias the transistor?

2. ### dl324 AAC Fanatic!

Mar 30, 2015
7,080
1,626
Because the difference is bias point is what we're more interested in. If bias point was unstable, results would be invalid.

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3. ### upand_at_them Senior Member

May 15, 2010
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It's more fun that way.

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4. ### crutschow Expert

Mar 14, 2008
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You can't use AC to bias a transistor, so DC is all that's left.

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5. ### atferrari AAC Fanatic!

Jan 6, 2004
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Usually you say? You are wasting everybody's time.

6. ### hp1729 Well-Known Member

Nov 23, 2015
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(100 words or less ...)

The objective of setting a bias point in a transistor circuit is to put the transistor into conduction but not saturation. An input AC signal increases or decreases the conduction to produce a full waveform out.

Setting this bias point determines the class of the amplifier. What input voltage will produce what kind of output.

7. ### ci139 Member

Jul 11, 2016
341
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how about Ge ones? or the attachment? -ss

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• ###### SolarDev EE_00_00_01.asc
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Last edited: Oct 8, 2016
8. ### MrChips Moderator

Oct 2, 2009
16,857
5,174
Imagine I have a 4-metre high wall to paint.

I can only reach the lower 2 metres while standing on the ground.
Hence I have to bias my height upwards by standing on a 2-metre step ladder.
Now I can paint from the centre of the wall and cover the whole height.

Hope this helps.

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9. ### BR-549 AAC Fanatic!

Sep 22, 2013
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Because a transistor is a one way DC device. It's a pilot check valve. We can keep the check valve open a little bit (if we want to) with DC bias.

10. ### merts New Member

Apr 1, 2016
8
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If you look at the specs.for a transistor it starts at zero and goes into saturation.
Normally biasing is used to allow it to operate in a linear manner for regular use.
However you can bias it to operate anywhere along the graph.Since it is a non linear device
square,log,cube and all sorts of outputs can be achieved.Therefore you bias it to get an output
that is suited for the project being design.

11. ### hp1729 Well-Known Member

Nov 23, 2015
2,306
283
Why do you think they would be different?
Re: attachments
Enough base current to turn them on.

12. ### hp1729 Well-Known Member

Nov 23, 2015
2,306
283
Good, and in 100 words or less.

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13. ### ci139 Member

Jul 11, 2016
341
39
as amazing as it may sound i don't think - i watch and stay baffled about what the spice is showing

however the tunnel diodes have negative impedance - those things are quantum mechanics devices - i won't go predicting what is or is not possible here - if you tell me how to measure the circuits without signifficantly affecting (pushing) their operation - id likely made it clear

Last edited: Oct 8, 2016
14. ### hp1729 Well-Known Member

Nov 23, 2015
2,306
283
Isn't everything a quantum mechanics device?