Biasing a transistor

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

1. i_nour Thread Starter New Member

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

2. dl324 Distinguished Member

Mar 30, 2015
3,377
651
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 Active 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
2,097
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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
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how about Ge ones? or the attachment? -ss

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Last edited: Oct 8, 2016
8. MrChips Moderator

Oct 2, 2009
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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 Well-Known Member

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
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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
2,097
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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
2,097
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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
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232
Isn't everything a quantum mechanics device?