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

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.

Thanks,

Zhi Yang

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for_idea
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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.

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

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for a tool enabling you to post your schematic here.

You're welcome.

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--Larry Brasfield
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Larry Brasfield

Slight correction/amendment inserted below.

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.

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--Larry Brasfield
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Larry Brasfield

Dear Larry,

Thanks for your thorough assumptions. I confirm or clarify your assumptions after your words:

Thanks.

You are right.

The ultrasound frequency is 40kHz. The signal is burst pulses(5 --10 pulses in one burst). There is a 0.1uF cap connected between the amplifier supply and the real ground (the battery negative end). From pseudo ground to the real ground, there are one 10uF and one 0.1uF cap. I'm not sure the concept of bypassing the pseudo ground. Could please explain it? thanks.

Yes. Each stage is a simple Rfeedback/Rin configuration. The positive input of the amplifiers are directly connected to the pseudo ground. The stages are coupled by 1000pF caps.

You are right.

you are right.

I didn't do any frequency filtering on the amplifier circuits. Maybe I should do it. Could you give some suggestion about this?

I can't read the language on that website. The schematic is real no more than I described.

Thanks again

Reply to
for_idea

receiver

9v

voltage

However,

How big is the signal at the output of the 3rd stage?

Is it clipping?

Is it random noise you have or an oscillation?

You may need a small pull up or pull down on the output of the 3rd stage.

Mark

Reply to
Mark

Dear Larry,

Here are your two other comments. My words are after yours:

I didn't really take impedance matching into consideration when I made the circuit. The only information about ultrasound receiver is: Part number: 255-400ER18 Center Freq.:40kHz+/-1kHz Bandwith: 1.5kHz(-6dB) Capacitance@1kHz: 2600pF+/-20% Allowable input power: 0.2W input voltage:20v max

I understand the higher the resistance I use, the more thermal noise in circuit, but if I use small resistance, the circuit will consume a lot of power. 10k seems at the trade-off point.

Thanks again

Reply to
for_idea

They were not as thorough as I later thought appropriate. See my later post about this.

....

You appear to have done it without knowing its name.

Good. Depending on your signal bandwidth (likely to be narrow thru air), those caps are about right for the high pass portion of the filtering you should have.

["noise" not oscillation or interference]

With some caps across your feeback resistors, and ajustment of your interstage coupling caps to shave off the low end, you can get most of the noise suppressed that lies outside your signal band. With all those op-amps, you could build a better filter, but I hesitate to go there until we see what kind of detection you are doing. Its selectivity may well obviate the need for a lot of care in the gain stages.

I should have mentioned that a measure of the noise you do see, (together with the bandwidth of the o'scope if that is your measuring tool), would be necessary to determine whether it is excessive or to be expected.

You do English fairly passably. Go here:

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and pull down
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and do the obvious things. For stuff like this, some diagrams can save thousands of words.

You are most welcome.

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--Larry Brasfield
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Reply to
Larry Brasfield

That is normal Usenet convention. No need to say it. (And some complain when anything else shows up!)

Do you have any reasonable way of measuring the impedance, especially the real part of it? From a guestimate based on the capacitance, I think your

10K input resistor may be adding several dB to the noise that comes from the transducer.

Have you considered that the impedance level of the input/feeback network only affects the power consumption while largish signals are present, and does not much affect quiescent power?

I would have to say that if you do not know the source impedance, it is hard to make the tradeoff you mention. And that tradeoff should be made differently for the input stage than the rest.

You're still welcome.

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--Larry Brasfield
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Reply to
Larry Brasfield

Joerg, you do _everything_ with plain transistors.

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Tim Wescott
Wescott Design Services
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Tim Wescott

BW opamp. And per datasheet that's only the typical

transistors.

Considering that the circuit is battery powered, the op-amp approach can make a lot of sense. And unless the actual closed loop gain needs to be stable and predictable, running at a nominal loop gain of 1.5 (as the OP's circuit does) may well be just fine. The sort of device he is conjuring tends to be narrowband without any particular phase response requirement or sensitivity to the distortion that comes up with low loop gain.

That said, the OP may wish to elaborate upon his need, or lack thereof, for precise amplification.

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--Larry Brasfield
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Reply to
Larry Brasfield

wrote

but

40kHz ultrasound burst

9v

voltage

in

second

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.

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.

200k)
a

to

The stages are coupled with 1000pF caps.

Thanks again Zhi

Reply to
for_idea

Hey, how about linear-biased cmos inverters? Lots of bandwidth, only one resitor and one capacitor per stage. Comes in handy Six-Packs.

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Luhan Monat (luhanis 'at' yahoo 'dot' com)
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Reply to
Luhan Monat

Hi, Joerg.

4.5mA for three of them. Probably 6mA because he uses

Good point, and a good reason to choose a different part. There are better devices with respect to GBW per mA of supply current. For example, the LT1492 is about 5 times better with comparable noise performance.

There may be cost issues at work in the OP's choice, and he may be working on something that is on only a fraction of the time, so that old workhorse may be a good choice. Otherwise, it clearly is not.

I look at your discrete design advocacy with some interest, having often been in that camp myself. But there has lately (well, for some years!) been a fact that changes those equations. It used to be that component capacitances were smaller than or comparable to what leads and traces demand. Now, with much smaller devices available within the internal circuitry of IC's, they can deliver speed versus power that is challenging (to say the least) to achieve with discrete circuits, even when fly-turd passives and SOT-23's are used.

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--Larry Brasfield
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Larry Brasfield
[Correcting a sense error in place.]

....

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Larry Brasfield

74C04 has worked for me amplifying 40 k Baud data from IR detector. Current? Yes, but it didn't melt. This is not an original idea; running digital cmos in linear mode.
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Luhan Monat (luhanis 'at' yahoo 'dot' com)
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Reply to
Luhan Monat

I read in sci.electronics.design that for_idea wrote (in ) about 'amplify 40kHz audio signal using TL082: first two stages are fine, but high noise from the third stage', on Wed, 16 Mar 2005:

For what frequency?

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.

What is the peak-to-peak amplitude? With +/- 4.5 V supplies, you can only get about 8 V peak-to-peak.

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.

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Reply to
John Woodgate

Hello John,

He said 40kHz. But you are right, 50 times is pushing it for a 3MHz unity gain BW opamp. And per datasheet that's only the typical value. Quite frankly, for a 40kHz ultrasound amp I'd do it with plain transistors.

If you are lucky and there is literally no load.

Regards, Joerg

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Reply to
Joerg

Hello Tim,

Not everything, only when it is cheaper. At two cents a pop these things are a bargain. But, I did use a 14106 recently. As a current controlled oscillator...

Seriously, in this case it just might make sense to do it with discretes. When you need lots of gain at frequencies above the audio range things get to be expensive with chips.

Regards, Joerg

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Reply to
Joerg

Hello Larry,

Could be but the fairly old TL082 draw around 1.5mA per amp so that would be 4.5mA for three of them. Probably 6mA because he uses duals.

A typical 9V battery would be gone in around 20 hours.

Regards, Joerg

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Reply to
Joerg

Hello Luhan,

Like a 74HCU04? Or a similar CD series? You'd have to stay away from VCC/2 though to avoid lots of quiescent current.

Regards, Joerg

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Reply to
Joerg

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