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

From: for_idea (zhiyang_at_gmail.com)
Date: 03/17/05 

Date: 16 Mar 2005 16:04:12 -0800

Dear John,

Thanks for offerring ideas. My answers follow your words below.
John Woodgate wrote:
> I read in sci.electronics.design that for_idea <zhiyang@gmail.com>
wrote
> (in <1111004366.450578.264430@z14g2000cwz.googlegroups.com>) about
> 'amplify 40kHz audio signal using TL082: first two stages are fine,
but
> high noise from the third stage', on Wed, 16 Mar 2005:
> >Dear friends,
> >
> >I am making an ultrasound signal receiver.
>
> For what frequency?
40kHz ultrasound burst

>
> >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.
>
> 50 times gain from a TL082 at an'ultrasonic' frequency is pushing its
> capability. It MAY be OK, but a faster op-amp would be better.
As I observed, for first two stages, the amplifier works very well.
Only problem is at the third stage. But choosing a faster op-amp is a
good idea.

> >The signal output in the second stage is very good.
>
> What is the peak-to-peak amplitude? With +/- 4.5 V supplies, you can
> only get about 8 V peak-to-peak.
It's ok for signal get satiated since I just want to know the time
position of the signal. If the signal is contaminated by significant
noise, the follow-up envelop detection circuit will produce bad
waveforms.

> >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.
>
> How are you coupling the stages together? If you have DC-coupled them
> you are amplifying the first op-amp's input offset by 50 x 50 x 20 =
a
> lot. 100 uV of offset at the input becomes 5 V at the output - enough
to
> seriously compromise your results.
The stages are coupled with 1000pF caps.

>
> --
> Regards, John Woodgate, OOO - Own Opinions Only.
> The good news is that nothing is compulsory.
> The bad news is that everything is prohibited.
> http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

Thanks again
Zhi

Relevant Pages

  • Re: E-Meter?
    ... John Woodgate wrote in ... Then what are they, the amplifiers? ... And don't push the hard part of the ... Neurology is one of the most difficult fields of medicine, ...
    (sci.electronics.design)
  • Re: Solution to television reception problem
    ... major in electrical engineering, freelance electrician ... Ο "John Woodgate" έγραψε στο μήνυμα ... > In Japan, it used to be a legal requirement that if a new building ... > The bad news is that everything is prohibited. ...
    (sci.electronics.design)
  • Re: Solution to television reception problem
    ... major in electrical engineering, freelance electrician ... Ο "John Woodgate" έγραψε στο μήνυμα ... > In Japan, it used to be a legal requirement that if a new building ... > The bad news is that everything is prohibited. ...
    (sci.electronics.misc)
  • Re: AM receiver chips?
    ... MasonDG44 at comcast dot net (Just subsitute the appropriate characters in ... "John Woodgate" wrote in message ... > The bad news is that everything is prohibited. ...
    (sci.electronics.design)
  • Re: kol bseder= copacetic
    ... >John Woodgate wrote: ... >> These coinings sometimes arise out of nothing at all, ... >> Black English, it could be a corruption of a word in an African or Carib ... The bad news is that everything is prohibited. ...
    (sci.lang)