Re: amplify 40kHz audio signal using TL082: first two stages are fine, but high noise from the third stage

From: Larry Brasfield (donotspam_larry_brasfield_at_hotmail.com)
Date: 03/16/05 

Date: Wed, 16 Mar 2005 12:55:50 -0800

Slight correction/amendment inserted below.

"Larry Brasfield" <donotspam_larry_brasfield@hotmail.com>
 wrote in message news:pY0_d.19$fG.1032@news.uswest.net...
> "for_idea" <zhiyang@gmail.com> wrote in message
> news:1111004366.450578.264430@z14g2000cwz.googlegroups.com...
>> Dear friends,
>>
>> I am making an ultrasound signal receiver. The transmitter and receiver
>> is apart from each other about 6 meters. The circuit is powered by a 9v
>> battery. I used three amplifiers from two TL082. The reference voltage
>> (about 4.5v) is generated from voltage divider (two 100k resisters in
>> series). All amplifiers are in inverting input mode. First and second
>> stages are configured as: 10k input resister and 500k feedback
>> resister. The signal output in the second stage is very good. However,
>> the signal from the third stage (input res.= 10k, output res. = 200k)
>> is significantly corrupted by noise. Please give me some advice to
>> clean up the amplified signal.
>
> If my stated assumptions are incorrect, you can
> take them as a form of advice.
>
> I assume your single-supply circuit is referenced to
> a "pseudo ground" developed by that divider.
>
> I assume the amplifier supplies are well bypassed
> to that pseudo ground at the frequencies you care
> about, (and beyond for stability's sake).
>
> I assume that your amplifiers stages are the simple,
> Rfeedback/Rin configuration and nothing really
> boneheaded is hidden by the invisible schematic.
>
> I assume that you would not say "noise" when you
> meant "oscillation".
>
> I assume that you would not say "noise" when you
> mean "interference getting into the input because of
> inadequate shielding or shield connection".
>
> I assume that you know about frequency selective
> filtering to remove noise from those parts of the
> spectrum you do not intend to capture as signal.

I assume that the choice of TL082 is appropriate
for the source impedance of your transducer. If
you are not sure of this, you should mention what
it is and what you know about that impedance.

I assume that your choice of a 10k input resistor
is made with knowledge of thermal noise, and is
not the source of input referred noise higher than
your application can tolerate. This again depends
on the transducer characteristics.

> Now, given all that, you should expect some noise,
> and you should expect the most in the last gain stage.
> So I do not see what can be done unless the noise
> is excessive, and even then I would need to see a
> schematic. See http://www.tech-chat.de/aacircuit.html
> for a tool enabling you to post your schematic here.
>
>> Thanks,
>
> You're welcome. 

-- 
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.

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