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Old 4th June 2005, 10:09 AM   #1
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Default transistor biasing

how much current is enough to prevent noise in transistors?
does it need more or less in the range of mA or uA?
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Old 5th June 2005, 04:38 AM   #2
Eva is offline Eva  Spain
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Be careful, noise in bipolar transistors actually increases for higher biasing currents, altough linearity is also usually improved
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Old 6th June 2005, 07:51 AM   #3
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ok. so it should have low current bias so it won't create noise too much. i'm planning to make an MC gain stage so it should be really silent enough.

thanks.
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Old 6th June 2005, 08:01 AM   #4
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sorry Eva,

It's a little more complicated than that. Let's look at those excellent for MC transistors produced by Rohm - the 2SB737 and 2SD786 - we loved them , made heaps of MC front ends out of them , always biassed at 2+mA! for low noise with low source impedances! Then there's the electret condenser microphone preamp having to deal with sources of Mohms not tens of Ohms. Very different.


Horses for courses.

Greg
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Old 6th June 2005, 11:56 AM   #5
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before anything else
it is also customary to know which topology are you refering
for.....

because talking about noises it varies with amplifier topologies used...

for better noise rejections used the MIRROR IMAGE topology.


rgrds,

macweb
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Old 6th June 2005, 01:52 PM   #6
jcx is offline jcx  United States
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you will find low noise phono and mc pre design with noise analysis here:

http://www.national.com/an/AN/AN-222.pdf#page=1


more info from national on noise calculation:

http://www.national.com/an/AN/AN-104.pdf#page=1
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Old 6th June 2005, 02:30 PM   #7
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To do you an idea of the distribution of the noise in the bjts...
Attached Images
File Type: jpg bc560noise.jpg (96.8 KB, 437 views)
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Old 8th June 2005, 07:22 AM   #8
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Quote:
Originally posted by amplifierguru


It's a little more complicated than that. Let's look at those excellent for MC transistors produced by Rohm - the 2SB737 and 2SD786 - we loved them , made heaps of MC front ends out of them , always biassed at 2+mA! for low noise with low source impedances! Then there's the electret condenser microphone preamp having to deal with sources of Mohms not tens of Ohms. Very different.


Greg

Quote:
Originally posted by hienrich
before anything else
it is also customary to know which topology are you refering
for.....

because talking about noises it varies with amplifier topologies used...

for better noise rejections used the MIRROR IMAGE topology.


rgrds,

macweb
can you give some sample schematics of your preferred circuit topology for an mc stage?

thanks for helping guys.
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Old 8th June 2005, 02:10 PM   #9
brouce is offline brouce  Libya
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yes its k
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Old 8th June 2005, 06:42 PM   #10
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Suggest you also look at the ZETEX Semiconductor design notes at http://www.zetex.com/3.0/3-8-2a.asp . Look at DN11 for a MC amp. They don't give specific information biasing, just a design idea.
Regards,
Ray
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