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op-amp amplifier oscillating

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Avi

Sat Aug 25 2007, 10:12AM

Registered Member #580 Joined: Mon Mar 12 2007, 03:17PM
Location: Melbourne, Australia
Posts: 410

I have constructed this attached circuit which was planed to be a non inverting amplifier which can handle DC to 200khz and have at least enough gain to convert 2mv(p-p) to 3.3v(p-p). All op amps are TL074CN (quad) and the 2 not in use have been connected as a voltage follower with their nin inverting inputs to negative supply rail. I do have vcc/2 from the output of the first op amp, however the second op amp under certain conditions (which i am not 100% sure of) will cause the op amp to go into oscillations. Could someone advise or point me in the right direction to preventing this.

Steve Conner

Sat Aug 25 2007, 10:40AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

There are any number of reasons why it could be oscillating. Here are a few things you can try:

Small electrolytic cap across the power supply rails (pins 4 and 11) and also another one across R2 (to ensure the supply rails and virtual ground are both free of noise)

220 ohm resistor in series with U1B output, if you're driving a length of screened cable with it (the capacitance of the cable can sometimes make the amp unstable)

Small capacitor across U1B pins 6 and 7 to limit the high frequency response (if you're thinking WTF at this, see note below)

1M resistor across the input, to set the DC condition when it's not connected to anything else

More voltage than 6V: try 9, 12 or 18 (the TL074 isn't really so good for low supply voltages)

Note: you wouldn't have got 200kHz BW at the maximum gain anyway, as the TL074 only has a gain-bandwidth product of 3MHz, which means it can only achieve 200kHz BW with a gain of 15 or less. To meet your spec, you need to use another op-amp with more GBW, or use several stages of TL074s. Two identical stages controlled by a 500k dual gang pot - with the R1's increased in value to spread the gain out more - might be a good start.

steve

Dave47

Sat Aug 25 2007, 11:25PM

Registered Member #84 Joined: Thu Feb 09 2006, 01:06PM
Location: Dallas, TX
Posts: 47

Most op-amps do not like to see any parasitic capacitance at the inverting input. I'd be suspicious of your pot arrangement. Since you are going for variable gain with the pot, you probably have some length of wire running to a big pot that you plan on mounting on the front panel of the box. You may be all right with just putting 10 ohms in series with the pot terminals at both ends as close to the device as possible.

David

Avi

Sun Aug 26 2007, 02:21AM

Registered Member #580 Joined: Mon Mar 12 2007, 03:17PM
Location: Melbourne, Australia
Posts: 410

oops, i forgot to draw in that there is a 10nf capacitor across the supply rails of the chip and another one on the bottom half resistor of the voltage divider. There is no long cables that make up the circuit, i have soldered it to the ic socket directly. conner has suggested though that i use a larger voltage (i will clip it at the recieving side with a series resistor and 2 diodes), and larger then 1k resistors and several stages with a small amount of gain each such that the gain bandwidth product is still within required amount.
So i will try that and see what happens i supose.

I may attempt to put it in a box with a panel at a later stage if it works out.

Sulaiman

Sun Aug 26 2007, 07:41AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Just a little sidenote;

All op-amps have input offset voltage errors (e.g. TL071 = 3 mV typical, 10 mV maximum)
AND
Input offset voltage change with temperature (e.g. 10 uV/Celcius)
SO
If your signal is dc coupled
the input offset voltages will be greater than your input signal
and the output will change by 1% of full scale per degree Celcius

You need to
EITHER
Have a system to null the offset voltage
OR
Have no dc gain - only ac gain.

Avi

Mon Aug 27 2007, 11:11AM

Registered Member #580 Joined: Mon Mar 12 2007, 03:17PM
Location: Melbourne, Australia
Posts: 410

at the momnet, the DC offset, although noticeable, is not causing much of a problem. The reason i am not filtering out DC is because i want to measure a mixed signal with frequencies from the very low to the very high.

Test results:
I tested 2 states with each having a gain of 2 and another test with gains of 10.1
Both of these tests worked perfectly and as expected when the input was 100khz sine from a signal generator, however when i connected my actual signal source it seems to cause it to go into wild oscillations and i cant quite figure out why. (well actually when i has gains of 2 i think the first stage output was alright, but amplitude was still so low to tell if was doing anything yet, but the second stage went haywire)

thedatastream

Mon Aug 27 2007, 02:29PM

Registered Member #505 Joined: Sun Nov 19 2006, 06:42PM
Location: Yorkshire!
Posts: 329

Avi wrote ...

at the momnet, the DC offset, although noticeable, is not causing much of a problem. The reason i am not filtering out DC is because i want to measure a mixed signal with frequencies from the very low to the very high.

AC coupling the stages with a mixture of capacitor technology would work - perhaps a 100uF electrolytic in parallel with a 100nF ceramic would do the trick?

Avi

Mon Aug 27 2007, 03:39PM

Registered Member #580 Joined: Mon Mar 12 2007, 03:17PM
Location: Melbourne, Australia
Posts: 410

but i don't want to use stages of gains of 2, otherwise i will have so many stages the signal to noise ratio will be horrendous.
What i cant understand is why my signal generator shows it to work, but a real signal (with multiple frequencies) causes it to miss-function.

Sulaiman

Mon Aug 27 2007, 04:41PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

In general it's only the signal-to-noise ratio of the first amplification stage that's significant

For good gain-bandwidth and low noise I use CLC425 op-amps.

I just checked the National Semiconductor website and they're obsolete
replaced by LMH6624

P.S. they do free samples

Dave47

Mon Aug 27 2007, 05:14PM

Registered Member #84 Joined: Thu Feb 09 2006, 01:06PM
Location: Dallas, TX
Posts: 47

I'm going to go back and harp on the capacitance of the feedback some more. Have you tried to replace the pot with a fixed resistor? I know that defeats the purpose of variable gain, but I think it should be good for an experiment.

As for why it works with 100KHz from a generator and not in real life... Op-amp frequency responses often have peaking at some frequency just before they roll off. It is easy to only put in a frequency less than this that will not give more than the expected gain. The peaking for a voltage feedback device increases with increasing feedback resistor (the opposite is true for current feedback op-amps). The peaking increases even more with stray capacitance at the inverting node.

This would be really easy to figure out with a network analyzer, but you should be able to sweep your function generator frequency and record the gain and figure out if the response is peaking. You should get a lot of gain at the oscillation frequency.

David

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