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

From: John Woodgate (jmw_at_jmwa.demon.contraspam.yuk)
Date: 03/16/05 

Date: Wed, 16 Mar 2005 21:56:06 +0000

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?

>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.

>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.

>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. 

-- 
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
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