This one's bugging me bad; thanks in advance for reading!

I have a simple op amp circuit using a TI LM324N quad op amp package. I am using a dual supply and a signal generator to provide +/- 10VDC power rails and a sine wave referenced to chassis. My op amp has been stripped down to just one channel in a voltage-follower feedback configuration; the other 3 amps are voltage-follower with their inputs hooked to chassis.

The input to the amp is a 40kHz sine wave. When the amplitude is less than 1V peak, the output looks beautiful; overlapping input with output on my scope confirms there is no noticable distortion.

When the input amplitude is more than 1V peak, the output distorts, but only on the negative half of the wave (as far as I can tell by looking at the scope). There are notable discontinuities, and a 600kHz (?) sawtooth pattern that appears--but again, only on the bottom half of the waveform, and with the input clean as a whistle.

Am I doing something really silly here?

Thanks again!

 

Hello,

Can you post a schematic of the circuit you are using?
You can attach the image using the manage attachments button.
We prefer to use the .png format over here.

Bertus

 

I think you are running into slew rate limits. Look at the specsheet gain-bandwidth product for +/- 10V

I know. +/- 10v isn't labeled. extrapolate.

 

Thanks for the replies. The slew rate is spec'd at 0.5V/us at a light load; I thought that'd be plenty, but I could be wrong. I did note that when I added a 10k load it got much worse. The output is only 40kHz @ 1V peak (though I'd like to amplify up to 10V peak eventually). Think I need a different amp?

I've attached pics; let me know if that clears my problem up. Sorry, I used jpgs; png file sizes bloated really badly for some reason.

Thanks again!

 

Hello,

Are you using decoupling capacitors at the opamp?
There should be two used.
One 0.1 uF from +10 Volts to ground and
one 0.1 uF from -10 Volts to ground.

See this thread for more info:
Decoupling or Bypass Capacitors, Why?

Bertus

 

I'm guessing about 200kHz at a gain of one. Figure 5

 

I tend to agree. A LM324 is a improved 741, but it is an old design. Look at something like a TL084.

Radio Shack has a dual member of a similar family. It is much faster, but has other characteristics that may not be suitable for your application.

http://www.radioshack.com/product/index.jsp?productId=2062594

This is why we need to see your schematic. There are many, many different types of op amps out there.

High Speed Op Amp Query

 

The TL082 has a GBW (4MHz) that is much higher than the LM324, and a slew rate of 13v/uS with a light load.
It's not quite as good as a TL072 (the latter has better noise specifications) but it's basically the same. The TL072, TL082, and LF353 are all very similar.

The big caveat with these three opamps is that they can't get within about 3v of the negative rail, or within about 1.5v of the positive rail, depending on output load.

As long as you keep that in mind, you'll probably have much better results than with the LM324.

Rather than trying to get a gain of 10 from a single channel, use two channels with a gain of ~3.162 per channel; the gain of the channels will then be multiplied. The noise will increase, but you won't lose nearly as much bandwidth.

Be certain to use bypass capacitors. 0.1uF and 1.0uF or larger from +V to GND and -V to GND; the 0.1uF caps' leads should be as short as possible.

 

Excellent, thanks everyone. I guess I'll try some of those other amps you suggested. The gain isn't an issue right now; right now I only have a gain of 1, but with a large enough input (1V) the output will distort. Since I'll want an input of ~3V and gain up to ~10V (in one or more stages), maybe this just isn't the right amp for the job.

I didn't draw the bypass caps; I have healthy bypass all over between +10V and earth and -10V and earth. Higher voltage supplies are not a problem at this point, either, so I can live with amps that have near-rail issues.

 

I mentioned the TL082 because if you are in the States, your local Radio Shack store should carry it. That, the LM741 and the LM324 are the only opamps that they carry anymore; and the LM741 should be avoided wherever possible. It was the greatest thing since sliced bread about 43 years ago, but it didn't take long for it to get left in the dust by newer opamps - the TL07x, 8x family being one of those.

There are far better opamps available nowadays, but you'd have to order them.

 

The datasheets for the LM324 and LM358 show that they get into trouble when their output is at a fairly high level above only 2kHz because their slew rate is too low.
Better opamps go to 100kHz at full output level.

 

Thanks again all; I'm going to try some TL084s next. I have a feelling I'll be much better off.

One of my problems was that the TI datasheet for the LM324s didn't have a lot of charts; if I'd done better homework and looked at other datasheets, it might have been more obvious what I was dealing with.

Thanks for the enlightenment! Although I've done quite a lot with circuitry, I've done very little hands-on work with amps. This clearly demonstrated some of the drawbacks of a non-ideal amp.

 

The TL084 is a quad opamp. You could use one of those, but unless you really need three or all four channels, you'll probably be better off going with an '82. Otherwise, you'll have to wire the unused channels as emitter followers to keep them from oscillating.

 

The original manufacturer of an IC usually has a very detailed datasheet. Manufacturers who copy an IC have a brief datasheet that is missing many important details.

National Semiconductor invented the LM324 quad and LM358 dual low power opamps. they are fairly old so today their performance is poor.
Texas Instruments and others simply copy them.