| jondavis |
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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| Eva |
| Be careful, noise in bipolar transistors actually increases for higher biasing currents, altough linearity is also usually improved |
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| jondavis |
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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| amplifierguru |
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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| 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 |
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| mauropenasa |
| To do you an idea of the distribution of the noise in the bjts... |
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| jondavis |
| 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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| rayfutrell |
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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| rayfutrell |
There is a schematic for a MC amp at http://www.st-and.demon.co.uk/Audio/700/700page5.html . In this case the transistors are biased at abut 1.3 mA. Years ago I saw a design for a MC amp where the input transistor was biased at 10 mA. If I recall correctly they used a TIP30 as the first stage because of the low base spreading resistance of around 10 ohms.
Regards,
Ray |
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| jondavis |
the circuit from zetex is usually called discrete opamp. I've made that and currently used as mc frontend with addition of current sources. but i'm still not contented with the performance, it still has noise, about -50db i think with a 30db gain. it might be because my mc fontend has still no case.
the mc frontend circuit from armstrong looks quite like an mc headamp. i haven't tried that. |
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| mhelin |
| quote: | Originally posted by jondavis
the circuit from zetex is usually called discrete opamp. I've made that and currently used as mc frontend with addition of current sources. but i'm still not contented with the performance, it still has noise, about -50db i think with a 30db gain. it might be because my mc fontend has still no case.
the mc frontend circuit from armstrong looks quite like an mc headamp. i haven't tried that. |
Any schematics for your circuit? |
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