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HELP: very low noise pre-amp, 0.01mV, 100KHz

Dear all,

Grateful if you may advise on idea for designing a very low noise
pre-amp for ultrasonic sensor application.

Input 0.01mV
Output 3 volts
Freq 1 kHz to 100KHz with reasonably flat response

How should I choose between:

a) bi-polar transistor
b) Junction FET
c) MOS FET
d) low noise OP-Amp

Would it be right to presume that discrete transitor (bipolar, jfet or
mosfet) amplifier will have lower noise since it can be designed with
about 3 stages with 3 transistors instead of an OP-Amp which contains
tens of transistors?

Any software design tools or books or web pages on the topic?

Thanks in advance

Simon
 
R

Rene Tschaggelar

Jan 1, 1970
0
Dear all,

Grateful if you may advise on idea for designing a very low noise
pre-amp for ultrasonic sensor application.

Input 0.01mV
Output 3 volts
Freq 1 kHz to 100KHz with reasonably flat response

How should I choose between:

a) bi-polar transistor
b) Junction FET
c) MOS FET
d) low noise OP-Amp

Would it be right to presume that discrete transitor (bipolar, jfet or
mosfet) amplifier will have lower noise since it can be designed with
about 3 stages with 3 transistors instead of an OP-Amp which contains
tens of transistors?

Any software design tools or books or web pages on the topic?

Thanks in advance

Simon


You're asking for a gain of 300k at a bandwidth of 100kHz.
This can only be achieved with multiple stages.
To start with, what is the source impedance ?

Rene
 
P

Phil Hobbs

Jan 1, 1970
0
Rene said:
You're asking for a gain of 300k at a bandwidth of 100kHz.
This can only be achieved with multiple stages.
To start with, what is the source impedance ?

Yes, and what's the expected signal-to-noise ratio? Assuming a gain of
300k and an optimistic noise bandwidth of 150 kHz (1-pole rolloff with
fc=100 kHz), a 1 nV/sqrt(Hz) noise source becomes
1e-9*300000*sqrt(150000)=0.116 V rms.

Cheers,

Phil Hobbs
 
B

Ban

Jan 1, 1970
0
Dear all,

Grateful if you may advise on idea for designing a very low noise
pre-amp for ultrasonic sensor application.

Input 0.01mV
Output 3 volts
Freq 1 kHz to 100KHz with reasonably flat response

How should I choose between:

a) bi-polar transistor
b) Junction FET
c) MOS FET
d) low noise OP-Amp

Would it be right to presume that discrete transitor (bipolar, jfet or
mosfet) amplifier will have lower noise since it can be designed with
about 3 stages with 3 transistors instead of an OP-Amp which contains
tens of transistors?
Nope, if you amplify 300000 times with 3 transistors, there will hardly be
any feedback, thus very low bandwidth unlinearity and distortion. The noise
will be high,(0.14Vrms, 0.85Vpeak) with that much of gain in a wideband amp.

To decide which type of transistor/opamp to choose, it is important to have
some data about the sensor. Is it a piezotransducer or a dynamic mic with a
voicecoil? I think the latter, because 1kHz sounds pretty low.

You probably need also a filter, which can improve the S/N ratio. A
transformer can help if the sensor impedance is very low.
Then I would use at least 3 stages of wideband opamps like OP37, better
would be a balanced amplifier (THS4130) 110dB is quite demanding layout wise
as well.

ciao Ban
 
M

martin griffith

Jan 1, 1970
0
On 27 Jul 2005 04:03:43 -0700, in sci.electronics.design
Dear all,

Grateful if you may advise on idea for designing a very low noise
pre-amp for ultrasonic sensor application.

Input 0.01mV
Output 3 volts
Freq 1 kHz to 100KHz with reasonably flat response

How should I choose between:

a) bi-polar transistor
b) Junction FET
c) MOS FET
d) low noise OP-Amp

Would it be right to presume that discrete transitor (bipolar, jfet or
mosfet) amplifier will have lower noise since it can be designed with
about 3 stages with 3 transistors instead of an OP-Amp which contains
tens of transistors?

Any software design tools or books or web pages on the topic?

Thanks in advance

Simon
any chance that a logamp would be more suitable?

AD8307 ISTR, near DC to almost light bandwidth


martin
 
R

Robert Baer

Jan 1, 1970
0
Dear all,

Grateful if you may advise on idea for designing a very low noise
pre-amp for ultrasonic sensor application.

Input 0.01mV
Output 3 volts
Freq 1 kHz to 100KHz with reasonably flat response

How should I choose between:

a) bi-polar transistor
b) Junction FET
c) MOS FET
d) low noise OP-Amp

Would it be right to presume that discrete transitor (bipolar, jfet or
mosfet) amplifier will have lower noise since it can be designed with
about 3 stages with 3 transistors instead of an OP-Amp which contains
tens of transistors?

Any software design tools or books or web pages on the topic?

Thanks in advance

Simon
Noise of an amplifier with a given gain and bandwidth depends on the
source resistance, which you did not specify.
Roughly speaking, a bipolar input stage will have lower noise,
especially at "low" input source impedances.
A MOS FET input stage in general will always give a higher noise at
any source impedance.
A JFET input stage in general is the best choice for lowest noise for
"high" input source impedances.
Use of a low noise op-amp is a good choice because the design
tradeoffs have already been done - but the design should be "matched" to
the source impedance you have.
That usually means a bipolar front end for low impedances, and a JFET
front end for high impedances.
If your source has an unusually low impedance or an unusually high
impedance, a transformer between the source and the amplifier may be
useful to reduce the overall noise.
 
Many thanks for the replies.

The input is piezo transducer. Not 100% sure about their input
impedence. My impression is they are in mega ohms range (please correct
me if I am wrong). Should I use discrete JFET or op-amp with JFET input
stage? Any recommended part numbers?

The transducer is used to measure mechanical parts impact sound. I am
now using an 'ordinary (not low noise) op-amp amplifier' in front of a
PC sound card. The impact sound signal spread from about 1 to 18 kHz.

Have checked www.nsc.com and apparently, the lowest noise op-amp is
about 1nv/sqrt(Hz). So, if I scale down the freq to 20kHz, noise is
1e-9*300000*sqrt(20000) = 0.042 V, giving 20log(0.042/3)= 37 dB signal
to noise ratio. Is that right?

Is there other company making op-amp with noise lower than
1nv/sqrt(Hz)?

Some text book suggest that since the first stage determine the overall
noise perfromance. One can use a discret 'lowest-possible noise' JFET
as front end and then use op-amp to get the needed gain.

Any recommendation and (JFET, MOS or bipolar) transistor part number
for such an approach? What are the ball park figure of signal to noise
ratio that one can expect (this is a medium cost instrument, so, can
afford upper-medium price/quality chips)?

Presume 20kHz bandwidth and gain at 300,000 what type of shielding
and layout may be needed? Assume a three to four stages design, would
it need to make four metal partition, like those used in VHF/UHF
receiver layout?

Many thanks in advance.
 
R

Rene Tschaggelar

Jan 1, 1970
0
Many thanks for the replies.

The input is piezo transducer. Not 100% sure about their input
impedence. My impression is they are in mega ohms range (please correct
me if I am wrong). Should I use discrete JFET or op-amp with JFET input
stage? Any recommended part numbers?

The transducer is used to measure mechanical parts impact sound. I am
now using an 'ordinary (not low noise) op-amp amplifier' in front of a
PC sound card. The impact sound signal spread from about 1 to 18 kHz.

Have checked www.nsc.com and apparently, the lowest noise op-amp is
about 1nv/sqrt(Hz). So, if I scale down the freq to 20kHz, noise is
1e-9*300000*sqrt(20000) = 0.042 V, giving 20log(0.042/3)= 37 dB signal
to noise ratio. Is that right?

Basically yes, but...
Also note that those ultralownoise amps having that
little noise are mainly wideband amplifiers. Artificially
limiting the amplifier with an RC in the feedback for
the noise doesn't work either. You have to fully use
the gain bandwidth. Sad enough, the ultralow noise amps
rely on low source impedance eg the LT1028 is having these
0.9nV/rtHz with a 100 Ohms. Have a look at their datasheets.


Rene
 
P

Phil Hobbs

Jan 1, 1970
0
Many thanks for the replies.

The input is piezo transducer. Not 100% sure about their input
impedence. My impression is they are in mega ohms range (please correct
me if I am wrong). Should I use discrete JFET or op-amp with JFET input
stage? Any recommended part numbers?

The transducer is used to measure mechanical parts impact sound. I am
now using an 'ordinary (not low noise) op-amp amplifier' in front of a
PC sound card. The impact sound signal spread from about 1 to 18 kHz.

Impacts of hard materials on hard materials go *much* higher in
frequency than that. You could probably get a huge SNR improvement by
going to a faster transducer and increasing the BW. This sounds
paradoxical, but the peak power in a pulse goes as the BW**2, whereas
noise power goes as BW, so you win by widening the BW until the pulse
stops getting narrower. Small-object collisions can easily get up into
the megahertz range--and impacts of micron-size particles go up to ~ 1 GHz.

Cheers,

Phil Hobbs
 
F

Fred Bartoli

Jan 1, 1970
0
Phil Hobbs said:
[email protected] wrote:

stops getting narrower. Small-object collisions can easily get up into
the megahertz range--and impacts of micron-size particles go up to ~ 1 GHz.

I first was stuned reading that GHz story, but further thinking (speed in
the 1000s m/s and micron size) finally get us in the GHz region.
I wouldn't have thought.
 
P

Phil Hobbs

Jan 1, 1970
0
Fred said:
GHz.


I first was stuned reading that GHz story, but further thinking (speed in
the 1000s m/s and micron size) finally get us in the GHz region.
I wouldn't have thought.
Yes, the typical width of the pulse is the particle diameter divided by the
speed of sound in the material. It gets up into big numbers for small particles.

Cheers,

Phil Hobbs
 
P

Pooh Bear

Jan 1, 1970
0
Dear all,

Grateful if you may advise on idea for designing a very low noise
pre-amp for ultrasonic sensor application.

Input 0.01mV
Output 3 volts
Freq 1 kHz to 100KHz with reasonably flat response

How should I choose between:

a) bi-polar transistor
b) Junction FET
c) MOS FET
d) low noise OP-Amp

Would it be right to presume that discrete transitor (bipolar, jfet or
mosfet) amplifier will have lower noise since it can be designed with
about 3 stages with 3 transistors instead of an OP-Amp which contains
tens of transistors?

Any software design tools or books or web pages on the topic?

What's the source impedance from your transducer ?

Can't advise on best method without that knowledge.

Is the input required to be differential too ?

Graham
 
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