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Low Noise Preamp Design Questiuon

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BarNash

Jan 1, 1970
0
Hi all

I want to design an ultra low noise dynami c mic preamp .
It will be based on 2 or 3 bipolar transistors .

My question is how to choose the transistors that will denote minimm noise
to the final design ?

Also which kind of resistors would serve best this low noise preamp in mind
?

Thanks in advance
BarNash
 
P

Phil Allison

Jan 1, 1970
0
"Barmitzvah Nashes his Teeth "

I want to design an ultra low noise dynamic mic preamp.


** Is this dumb **** drunk ????

It will be based on 2 or 3 bipolar transistors .


** That many ???????

My question is how to choose the transistors that will denote minimm noise
to the final design ?

** This ought to be a REAL BLAST !!!

Also which kind of resistors would serve best this low noise preamp in
mind ?

** Naked, virginal ones - of course........

Thanks in advance.................


** This wog cunthead is way in advance of any thanks he will ever get.

For being alive.



..... Phil
 
A

Adrian Tuddenham

Jan 1, 1970
0
BarNash said:
Hi all

I want to design an ultra low noise dynami c mic preamp .
It will be based on 2 or 3 bipolar transistors .

Be aware that most microphones generate more thermal noise in themselves
then a good pre-amp does; so you will find you are hitting an
irreducible noise floor which no amount of expensive components or
clever designs will overcome.
 
P

Paul Keinanen

Jan 1, 1970
0
Hi all

I want to design an ultra low noise dynami c mic preamp .
It will be based on 2 or 3 bipolar transistors .

Be aware that most microphones generate more thermal noise in themselves
then a good pre-amp does; so you will find you are hitting an
irreducible noise floor which no amount of expensive components or
clever designs will overcome.[/QUOTE]

The noise output from a transducer at room temperature is about -174
dBm/Hz, so with a 20 kHz bandwidth, the noise power would be about
-141 dBm.

In modern RF amplifiers, (F<3 dB) the _additional_ noise is well below
this, however, at audio frequencies, the 1/f noise may invalidate most
of such calculations :-(.

Paul
 
I

Ian Bell

Jan 1, 1970
0
Paul said:
Be aware that most microphones generate more thermal noise in themselves
then a good pre-amp does; so you will find you are hitting an
irreducible noise floor which no amount of expensive components or
clever designs will overcome.

The noise output from a transducer at room temperature is about -174
dBm/Hz, so with a 20 kHz bandwidth, the noise power would be about
-141 dBm.
[/QUOTE]

No, it is about -131dBm (log10(root(20KHz)) = 43dB)
In modern RF amplifiers, (F<3 dB) the _additional_ noise is well below
this, however, at audio frequencies, the 1/f noise may invalidate most
of such calculations :-(.

Noise factors of less than 3dB are commonly achieved but 1/f noise can
still be a problem.

CHeers

Ian
 
P

Phil Allison

Jan 1, 1970
0
"Paul Keinanen from Pluto " <[email protected]

The noise output from a transducer at room temperature is about -174
dBm/Hz, so with a 20 kHz bandwidth, the noise power would be about
-141 dBm.

** Err - how about -131 dBm ??

And this is the irreducible thermal noise of the resistor made from the
voice coil wire of a dynamic mic.

Condenser mics are a whole nuther ball game.

In modern RF amplifiers, (F<3 dB) the _additional_ noise is well below
this, however, at audio frequencies, the 1/f noise may invalidate most
of such calculations :-(.

** Neither resistors nor dynamic mics have 1/f noise.

Fool.


...... Phil
 
P

Phil Allison

Jan 1, 1970
0
"Phil Hobbs"
Thick film resistors have a lot of 1/f noise due to conductivity
fluctuations at the grain boundaries, and of course carbon comp is even
worse. See for instance
http://www.ligo.caltech.edu/docs/T/T070016-A/T070016-A.pdf.

** There is no DC bias voltage involved in the situation being discussed -
ie the voice coils of microphones.

So there is no excess noise.

Pay attention to the context - pal.



...... Phil
 
P

Phil Allison

Jan 1, 1970
0
"Phil Hobbs is a Nut Case "
Building an amplifier with no bias supply is a wonderful advance, sir. My
hat's off to you. Would you show us the schematic?


** What illegal drugs are you on - fuckhead??

Take a whole box full and drop dead.




...... Phil
 
P

Phil Allison

Jan 1, 1970
0
"Bill Slowman Mental Defective"

"Phil Hobbs is a Nut Case "

Wrong.

** So says an ever bigger autistic lunatic.

A criminal too.

He's one of our reliable gurus.


** ROTFLMAO !!




....... Phil
 
P

Phil Allison

Jan 1, 1970
0
"Phil Hobbs is a Nut Case "

Building an amplifier with no bias supply is a wonderful advance, sir. My
hat's off to you. Would you show us the schematic?


** What illegal drugs are you on - fuckhead ??

Take a whole bag full and drop dead.




...... Phil
 
A

Adrian Tuddenham

Jan 1, 1970
0
Paul Keinanen said:
Be aware that most microphones generate more thermal noise in themselves
then a good pre-amp does; so you will find you are hitting an
irreducible noise floor which no amount of expensive components or
clever designs will overcome.

The noise output from a transducer at room temperature is about -174
dBm/Hz, so with a 20 kHz bandwidth, the noise power would be about
-141 dBm.[/QUOTE]

The noise level depends on the resistance of the transducer, the figure
you have given appears to correspond to 15Kohms. Most microphones are
in the range 50 to 600 ohms with thermal noise levels of -167 to -156
dBm (BW=20Kc/s, Tamb=20C)
In modern RF amplifiers, (F<3 dB) the _additional_ noise is well below
this, however, at audio frequencies, the 1/f noise may invalidate most
of such calculations :-(.

The LF noise is the biggest problem as far as measurement is concerned,
but it is not as audibly intrusive as a corresponding level of MF or HF
noise would be.
 
P

Phil Allison

Jan 1, 1970
0
"Adrian Tuddenham"
The noise level depends on the resistance of the transducer, the figure
you have given appears to correspond to 15Kohms.

** Where is your proof ??

Most microphones are
in the range 50 to 600 ohms with thermal noise levels of -167 to -156
dBm (BW=20Kc/s, Tamb=20C)


** Totally WRONG !!!!

The LF noise is the biggest problem as far as measurement is concerned,


** No it is NOT - you fuckwit dope.

but it is not as audibly intrusive as a corresponding level of MF or HF
noise would be.


** HF noise dominates entirely for resistive sources

- cos it is WHITE NOISE !!

DICK HEAD !!!!




..... Phil
 
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Paul Keinanen

Jan 1, 1970
0
The noise output from a transducer at room temperature is about -174
dBm/Hz, so with a 20 kHz bandwidth, the noise power would be about
-141 dBm.
[/QUOTE]

Actually -131 dBm. I guess that i should a calculator for such simple
calculations as -174+43 :).
The noise level depends on the resistance of the transducer, the figure
you have given appears to correspond to 15Kohms. Most microphones are
in the range 50 to 600 ohms with thermal noise levels of -167 to -156
dBm (BW=20Kc/s, Tamb=20C)

Both the (loaded) noise voltage and noise current depends on the
source resistance, but the noise power is simply P=kTB regardless of
source impedance.
The LF noise is the biggest problem as far as measurement is concerned,
but it is not as audibly intrusive as a corresponding level of MF or HF
noise would be.

At least in older op-amps, the 1/f noise level dropped to the white
noise (constant noise density) levels somewhere between 1-10 kHz.
Fortunately the human audio sensitivity at low levels (below 50 dB)
drops quite quickly when going below about 100-200 Hz, so the 1/f
noise should drop to the white noise level at frequencies well below
100 Hz.

Paul
 
A

Adrian Tuddenham

Jan 1, 1970
0
Actually -131 dBm. I guess that i should a calculator for such simple
calculations as -174+43 :).
The noise level depends on the resistance of the transducer, the figure
you have given appears to correspond to 15Kohms. Most microphones are
in the range 50 to 600 ohms with thermal noise levels of -167 to -156
dBm (BW=20Kc/s, Tamb=20C)

Both the (loaded) noise voltage and noise current depends on the
source resistance, but the noise power is simply P=kTB regardless of
source impedance.[/QUOTE]

Thats OK if you express the noise level in dB relative to 1 milliwatt,
which gives the same figure regardless of the resistance. If you use a
50-ohm microphone and express the noise level in dBm, the calculation
becomes much more complicated because dBm refers to 1 milliwatt into 600
ohms and is a voltage-based measurement.

I think you have to calculate the voltage across 50 ohms which
corresponds to the noise power, then work out what power that would give
if it were across a 600-ohm resistor. Then that power is compared to 1
milliwatt using a decibel scale.

The LF noise is the biggest problem as far as measurement is concerned,
[...]
At least in older op-amps, the 1/f noise level dropped to the white
noise (constant noise density) levels somewhere between 1-10 kHz.

I don't know if it is still true, but it used to be said that Nuvistors
were better than any semiconductor device for low LF noise. They were
particularly useful for the front ends of electroencephalograph machines
and for low jitter V.F.O.s. at R.F.
 
A

Adrian Tuddenham

Jan 1, 1970
0
Jim Thompson said:
Thats OK if you express the noise level in dB relative to 1 milliwatt,
which gives the same figure regardless of the resistance. If you use a
50-ohm microphone and express the noise level in dBm, the calculation
becomes much more complicated because dBm refers to 1 milliwatt into 600
ohms and is a voltage-based measurement.
[/QUOTE]

Sorry, this is rubbish - I was thinking "dBv" and typing "dBm"
[...]
Here's a little tool my son Aaron wrote for me when I was doing a WiFi
repeater chip design and the RF guys were driving me nuts talking
eveything in dBm...

http://www.analog-innovations.com/SED/NODBM.zip

You can enter dBm & impedance or Vp-p & impedance.

Pity it doesn't work on a Mac. ...but I've written a calculator in
ClarisWorks4 spreadsheet, for my own use, which does noise calculations
for any combination of temperature, bandwidth and resistance.

If any Mac users are interested, I have put a copy at:
http://www.poppyrecords.co.uk/other/UsefulCalcs.sit>
 
A

Adrian Tuddenham

Jan 1, 1970
0
Jim Thompson said:
On Fri, 28 Aug 2009 20:25:06 +0100,
[...]
Here's a little tool my son Aaron wrote for me when I was doing a WiFi
repeater chip design and the RF guys were driving me nuts talking
eveything in dBm...

http://www.analog-innovations.com/SED/NODBM.zip

You can enter dBm & impedance or Vp-p & impedance.

Pity it doesn't work on a Mac. ...but I've written a calculator in
ClarisWorks4 spreadsheet, for my own use, which does noise calculations
for any combination of temperature, bandwidth and resistance.

If any Mac users are interested, I have put a copy at:
http://www.poppyrecords.co.uk/other/UsefulCalcs.sit>

I have a macro in PSpice that does that, so I can display noise figure
directly ;-)

It's interesting to find out how many tools like that have been written
by people for their own use. I have another one I use for time
constants, which saves me no end of work (bundled in the above '.sit'
file).

One problem I have never been able to crack is a method of automatically
selecting series and parallel combinations of preferred-value resistors
to give an accurate non-standard value. I have approached it with a
spreadsheet showing a grid of the various possible combinations, but
some sort of iterative program would be better.
 
A

Adrian Tuddenham

Jan 1, 1970
0
Jim Thompson said:
[...]
Here's a little tool my son Aaron wrote for me when I was doing a WiFi
repeater chip design and the RF guys were driving me nuts talking
eveything in dBm...

http://www.analog-innovations.com/SED/NODBM.zip

You can enter dBm & impedance or Vp-p & impedance.

Pity it doesn't work on a Mac. ...but I've written a calculator in
ClarisWorks4 spreadsheet, for my own use, which does noise calculations
for any combination of temperature, bandwidth and resistance.

If any Mac users are interested, I have put a copy at:
http://www.poppyrecords.co.uk/other/UsefulCalcs.sit>

I have a macro in PSpice that does that, so I can display noise figure
directly ;-)

It's interesting to find out how many tools like that have been written
by people for their own use. I have another one I use for time
constants, which saves me no end of work (bundled in the above '.sit'
file).

Golly 'dern... it's binary :-([/QUOTE]

It's a Stuffit file containing two Claris Works spreadsheets. Some PC
un-Zipping programs will also handle Stuffit expansion and there is a PC
version of Claris Works which will read the Mac versions if they are
given a '.cwk' file extension. Better still just bung them into a Mac
running Classic OS and CW4.

I tried saving them as Excel spreadsheets, but the differences are too
great to be bridged by the translator.

Mine...

[MACROS]
BETA(A)=I(A:c)/I(A:b)
....

Golly 'dern... it's ... it's ...well whatever it is, I can't
run it.

I don't have that problem... if I need a 137K resistor I just
calculate the silicon length for the width I've chosen... also
automated, I just build the foundries equations into my template
generator ;-)

All my jobs were one-offs on tagboard ...until recently, when the
manufacturers stopped making my preferred type of tagboard. Now they
are built on Veroboard (perforated stripboard), take five time as long
to debug because of all the hidden short-circuits and are far less
reliable.
 
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